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. 2022 Jul 27;28(5):135. doi: 10.1007/s10989-022-10437-7

The Updated Review on Plant Peptides and Their Applications in Human Health

Saiprahalad Mani 1, Smruti B Bhatt 1, Vinduja Vasudevan 1, Dhamodharan Prabhu 2, Sundararaj Rajamanikandan 2, Palaniyandi Velusamy 2, Palaniappan Ramasamy 2, Pachaiappan Raman 1,
PMCID: PMC9326430  PMID: 35911180

Abstract

Biologically active plant peptides, consisting of secondary metabolites, are compounds (amino acids) utilized by plants in their defense arsenal. Enzymatic processes and metabolic pathways secrete these plant peptides. They are also known for their medicinal value and have been incorporated in therapeutics of major human diseases. Nevertheless, its limitations (low bioavailability, high cytotoxicity, poor absorption, low abundance, improper metabolism, etc.) have demanded a need to explore further and discover other new plant compounds that overcome these limitations. Keeping this in mind, therapeutic plant proteins can be excellent remedial substitutes for bodily affliction. A multitude of these peptides demonstrates anti-carcinogenic, anti-microbial, anti-HIV, and neuro-regulating properties. This article's main aim is to list out and report the status of various therapeutic plant peptides and their prospective status as peptide-based drugs for multiple diseases (infectious and non-infectious). The feasibility of these compounds in the imminent future has also been discussed.

Keywords: Therapeutic plant peptides, Peptide-based drugs, Anti-carcinogenic, Anti-HIV, Antifungal, Ribosomal-Inactivating Proteins (RIPs)

Introduction

Plants can be exploited as a bioreactor for many therapeutic proteins, the majority of which are secondary metabolites and their derivatives. Nephroblastoma lymphoma and acute lymphoblastic leukemia are treated with paclitaxel and vincristine which are derived from Taxus brevifolia Nutt and Catharanthus roseus, respectively (Seca and Pinto 2018). Furthermore, ingenol mebutate and curcumin extracted from Euphorbia peplus L and Curcuma longa L. being were tested in clinical trials for pancreatic, colorectal (Pan et al. 2012), and non-melanoma skin cancers (Seca and Pinto 2018). Notwithstanding, these peptide-based drugs are accompanied by unquestionable impediments, including toxicity, low abundance, complex multi-step synthesis, developmental stage-specific production, improper metabolism, poor absorption, poor systemic bioavailability, development of multi-drug resistance, and associated adverse health issues (Seca and Pinto 2018). These preordained constraints have compelled the scientific community to explore plants for other medicinal peptides.

In contrast to metabolite-based drugs, protein-based drugs have high therapeutic efficiency due to: (i) High specificity ergo fewer chances of interference with biological processes, thereby alleviating the toxicity, (ii) Performance of complex functions, (iii) High tolerance (Leader et al. 2008) and (iv) Varying charging of proteins/peptides due to existence of numerous functional groups thereby targeting different tissues of our body with varying pH (Reddy and Yang 2011). Although a multitude of therapeutic plant peptides has been identified, only a small number of them have found their way into databases i.e., research on the characterization of plant peptides has been left halfway or indeterminate (Leader et al. 2008 and references therein). This is due to: (i) the absence of high-throughput techniques, (ii) expensive and arduous, (iii) problems associated with protein stability. Nevertheless, therapeutic plant peptides appear propitious in peptide-based drugs for many diseases and are brought to the scientific community. This review article encapsulates therapeutic plant proteins and their implementation focusing on infectious and non-infectious diseases in light of this situation.

Infectious diseases encompass diseases caused by organisms (bacteria, viruses, fungi, parasites, nematodes). In contrast, non-infectious diseases constitute metabolic disorders (diabetes, obesity, cancer, cardiovascular, genetic disorders, neuroregulatory, and much more). This review might guide to development of peptide drugs for the treatment of various diseases and disorders (Fig. 1).

Fig. 1.

Fig. 1

Schematic representation of plants peptides displaying various therapeutic properties

Infectious Diseases

Anti-microbial activity of plant peptides/proteins

Microbes are one of the leading causes of various infectious diseases like common cough, cold, influenza, etc. Owing to their ubiquitous nature, infectious diseases can be transmitted easily from anywhere. To protect our bodies from such conditions, antibiotics have been used. However, the consumption of many antibiotics has given rise to the problem of anti-microbial resistance and has rendered present anti-microbial drugs fruitless (Kaur et al. 2012). Therefore, the scientific community has taken a keen interest in identifying prospective anti-microbial agents from native sources, particularly plants. Plants produce an extensive range of Anti-microbial Peptides/Proteins (AMP) since these peptides act as the first line of defense against pathogens, hence dubbed as Pathogen Response/Pathogenesis-Related (P.R.) Proteins. Plant AMPs are tissue-specific and are expressed constitutively, having both polar and non-polar groups and positive charges. They are cysteine-rich residues, and they generate multiple (2–6) disulfide bonds, thereby granting them stability and resistance against proteases and chemicals (Hernández-Ledesma et al. 2009; Hernández-Ledesma and Hsieh 2017). In addition to this, plant AMPs are small, have high target specificity, simple configuration (structure), various modes of administration, quick modifications can be performed, and negligible antigenicity (Yadav and Batra 2015). Considering the characteristics mentioned above and advantages, plant AMPs have been used to develop novel, highly efficient drugs to resolve multi-drug resistance infections. The main drawback is that only a few (not more than thousands) have been structurally and functionally characterized. In this article, we will  consider three main classes of microbes: Bacteria, Fungi and Viruses. This review also deals with the Pathogenesis—Related (PR) proteins from plants and their therapeutic applications.

Anti-bacterial activity

Plant ABPs (Anti-Bacterial Proteins) had emerged as potential alternative for a new class of antibiotics, tackling the obstacle of multi-drug resistance pathogens. Purothionin, the introductory ABP, was extracted from Triticum aestivum to inhibit a multitude of bacteria, including Xanthomonas campestris, Corynebacterium michiganens, and Pseudomonas solanacearum (Naider and Anglister 2009). The majority of these plant ABPs are positively charged (Kaur et al. 2012). They are highly antagonistic against a multitude of bacteria, even in lower concentrations. In contrast, some of them are highly specific. Nevertheless, though promising, only a few have been identified and characterized structurally and functionally (Naider and Anglister 2009). The amino acid sequence, location and number of cysteine residues are the key classification criteria for ABPs. There are several families such as defensins, thionins, lipid transfer proteins (LTR), snakins, cyclotides, thaumatins, etc.

Talking about the mechanism of action of ABPs, the most established notion is that ABPs will cause the breakage of bacteria when they come in contact with the negatively charged membrane (Pan et al. 2012). The strong selectivity of ABPs towards bacterial cells is due to the intrinsic negative charge of the bacterial cell membrane which protects the host cell against infection. Once the ABPs associate with the cell membrane, the ABP concentration builds until it reaches its threshold value (Girish et al. 2006). Upon attaining the threshold value ABP oligomers were generated to enter the membrane perpendicularly forming micelle-like structures (Barrel-Stave Model). Owing to the electrostatic interactions, ABPs assemble on bacterial membrane, manifesting like a carpet generating tension in the lipid membrane and subsequent phospholipids rearrangement. This results in varied membrane fluidity and membrane disruption (Carpet Model); ABPs, upon interacting with the polar head groups of the phospholipids, manifest into a transmembrane pore that provokes bends in the membrane, causing the adjacent layers of the pore to merge. Pore formation causes ion and metabolite efflux, membrane depolarization, deranging the respiratory mechanism, preventing cell wall formation, disrupting the membrane, ultimately leading to cell death (Toroidal Pore Model) (Girish et al. 2006). One of the primary purposes for producing ABPs is to overcome the challenge of antibiotic resistance. ABP drugs are multifarious and have a high potential of forming a new class of antibiotics with lower odds of bacterial resistance. Many proteins have been extracted from plants with high antibacterial activity having low IC50 value (Half Maximal Inhibitory Concentration) and Minimal Inhibitory Concentration (MIC). Shepherin I and II, two glycine-histidine-containing peptides isolated from Capsella bursa-pastoris inhibits several gram-negative bacteria. Circulin A and B, macrocyclic peptides (Cyclotides) extracted from Chassalia parviflora inhibits a multitude of gram-positive and gram-negative bacteria, with Circulin B inhibiting both (Park et al. 2004). In Chromobacterium violaceum, the amino acid lysine had anti-QS and anti-biofilm properties. It was documented that at a concentration of 0.684 mM, lysine decreased biofilm development by 16%, chitinolytic activity by 88.3%, and EPS production by 12.5% after 24 hours. It might also be used as a key component in the synthesis of peptides/proteins and tested for use in the treatment of bacterial infections, perhaps lowering the need for traditional antibiotics (Champalal et al. 2018).

The chitin-binding peptides isolated from Tulipa gesneriana Tu-AMP-1 and Tu-AMP-2, affect a wide variety of bacteria, including Agrobacterium rhizogenes, Curtobacterium flaccumfaciens, Erwinia carotovora, and Agrobacterium radiobacter, having an IC50 value of 11–20 μg/ml (Walsh et al. 2013). The ABPs mentioned above are some of the examples that have been structurally and functionally defined. A variety of ABPs with superior specificity and other novel properties is yet to be explored. Additionally, research should be focused on identifying novel ABPs having low toxicity, rapid mode of action and reported antibacterial peptides as shown in Table 1.

Table 1.

List of anti-bacterial and anti-microbial peptides/proteins from plants

S. No Plant and its part Protein Nature M. Wt (kDa) N-terminal sequence Bacterial species (Tested) *IC50 References
1 Vigna sesqui- pedalis (Seeds) Sesquin Peptide 7 KTCENLADTY M. phlei 87 ± 5 µM Wong and Ng (2005b)
B. megaterium 105 ± 5 µM
B. subtilis 98 ± 2 µM
P. vulgaris 75 ± 6 µM
2 Phaseolus lunatus L. (Seeds) Lunatusin Peptide 7 KTCENLADTFRGPCFATSNC M. phlei 96 ± 9 µM Wong and Ng (2005a)
B. megaterium 115 ± 6 µM
B. subtilis 98 ± 5 µM
P. vulgaris 81 ± 6 µM
3 Cycas revoluta (Seeds) Cy-AM P1 Peptide 4.58 KGAPCAKKPCCGPLGHYKVD C. michiganensi 7.3 µg/ml Yokoyama et al. (2008)
C. flaccumfaciens 8.9 µg/ml
A. radiobacter 8.3 µg/ml
A. rhizogenes 8.5 µg/ml
E. carobora 8.0 µg/ml
Cy-AMP2 Peptide 4.57 KGAPCAKKPCCGPLGHYKVD C. michiganensi 7.6 µg/ml
C. flaccumfaciens 8.3 µg/ml
A. radiobacter 7.8 µg/ml
A. rhizogenes 8.2 µg/ml
E. carobora 8.1 µg/ml
Cy-AMP3 Peptide 9.27 AVTCNTVTSSLAPCVPFFA C. michiganensi 235 µg/ml
C. flaccumfaciens 195 µg/ml
A. radiobacter 260 µg/ml
A. rhizogenes 235 µg/ml
E. carobora 230 µg/ml
4 Phytolacca americana (Seeds) Pa-AMP-1 Protein 3.94 B. megaterium 8 µg/ml Liu et al. (2000)
Staphylococcus. sp. 11 µg/ml
E. coli  > 300 µg/ml
5 Impatiens balsamina (Seeds) Ib-AMP1 Peptide 2.46 QWGRRCCGWGPGRRYCVRWC B. subtilis 10 µg/ml Tailor et al. (1997)
M. luteus 10 µg/ml
S. aureus 30 µg/ml
S. faecalis 6 µg/ml
Ib-AMP4 Peptide 2.52 QYGRRCCNWGPGRRYCKRWC B. subtilis 5 µg/ml
M. luteus 5 µg/ml
S. aureus 20 µg/ml
S. faecalis 5 µg/ml
X. campestris 6 µg/ml
X. oryzae 15 µg/ml
6 Capsella bursapastoris (Roots) Shepherin I Peptide 2.36 E. coli  < 2.5 µg/ml Park et al. (2000)
P. putida  < 2.5 µg/ml
P. syringae  < 2.5 µg/ml
S. typhimurium  < 2.5 µg/ml
Serratia sp. 8 µg/ml
7 Mirabilis jalapa (Seeds) Mj-AMP1 Homodimeric peptide 8 B. megaterium 6 µg/ml Cammue et al. (1992)
S. lutea 100 µg/ml
Mj-AMP2 Homodimeric peptide 7 B. megaterium 2 µg/ml
S. lutea 50 µg/ml
8 Psidium guajava (Seeds) Pg-AMP1 Peptide 6.0 Klebsiella sp. ND Pelegrini et al. (2008)
E. coli
Proteus sp.
9 Withania somnifera (Root tubers) WSG Glycoprotein 28 B. subtilis ND Girish et al. (2006)
P. fluorescens
C. michiganensis sub. sp, michiganensis
X. oryzae pv. oryzae
X. axanopodis pv. malvacearum
10 Ficus glomerata (Leaves) NA Protein 35 S. entrica ND Thapliyal et al. (2016)
P. aeruginosa
E. coli
B. subtilis
11 Foeniculum vulgare Mill. (Seeds) Elute1 Protein mixture S. aureus 27.64 µg/ml al Akeel et al. (2017)
E. coli 67.56 µg/ml
P. aeruginosa 28.01 µg/ml
P. vulgaris 59.68 µg/ml
Elute2 Protein mixture 34.4–48 S. aureus 25.91 µg/ml
E. coli 64.12 µg/ml
P. aeruginosa 68.33 µg/ml
P. vulgaris 57.83 µg/ml
Elute3 Protein mixture S. aureus 21.27 µg/ml
E. coli 60.52 µg/ml
P. aeruginosa 25.02 µg/ml
P. vulgaris 41.24 µg/ml
Elute4 Protein mixture S. aureus 20.8 µg/ml
E. coli 41.06 µg/ml
P. aeruginosa 26.67 µg/ml
P. vulgaris 35.67 µg/ml
12 Murraya koenigii L. (Leaves) APC Protein 35 S. aureus ND Ningappa et al. (2010)
B. subtilis
E. coli
S. typhi
V. cholerae
K. pneumoniae
S. paratyphi
13 Chassalia parviflora (Whole Plant) Circulin A Macrocyclic peptides 3.17 S. aureus ND Tam et al. (1999)
C. kefyr
C. tropicalis
Circulin B Macrocyclic peptides 3.30 E. coli ND
P. vulgaris
K. oxytoca
S. aureus
14 Spinacia oleracea (Leaves) So-D1 Peptide 2.29 C. michiganensis 1 µM Segura et al. (1998)
R. solanacearum 15 µM
So-D2 5.80 C. michiganensis 1 µM
R. solanacearum 2 µM
So-D6 2.55 C. michiganensis 1 µM
R. solanacearum 6 µM
So-D7 4.23 C. michiganensis 0.1 µM
R. solanacearum 1 µM
15 Oldenlandia affinis (Whole Plant) Kalata B2 Macrocyclic peptides 2.9 S. aureus (DA7127) ND Pränting et al. (2010)
E. coli (DA4201)
S. enterica (DA6192)
Kalata B1 2.89 S. aureus (DA7127) ND
E. coli (DA4201)
S. enterica (DA6192)
16 Vigna unguiculata (Seeds) Cp-thionin II Peptide 5.2 S. aureus (ATTC 25923) ND Franco et al. (2006)
E. coli (ATTC25922)
P. syringae
17 Pharbitis nil (Seeds) Pn-AMP1 Peptide 4.3 B. subtilis 38 µg/ml Koo et al. (1998)
Pn-AMP2 Peptide 4.2 B. subtilis 20 µg/ml
18 Vigna angularis (Seeds) VaD1 Peptide 5.0 S. epidermidis 36.6 µg/ml Chen et al. (2005b)
X. campestris pv. vesicatoria 40.8 µg/ml
S. typhimurium 143.4 µg/ml
B. cereus  > 500 µg/ml
E. coli  > 500 µg/ml
E. carotovora pv.carotovora 1000 µg/ml
P. vulgaris  > 1000 µg/ml
S. enteritidis  > 1000 µg/ml
P. syringae pv. syringae  > 1000 µg/ml
19 Phaseolus vulgaris (Seeds) Vulgarinin Seeds 7 M. phlei 87 ± 5 µM Wong and Ng (2005c)
B. megaterium 105 ± 5 µM
B. subtilis 98 ± 2 µM
P. vulgaris 75 ± 6 M µM
20 Tulipa gesneriana (Tulip Bulbs) Tu-AMP-1 Peptide 4.9 E. carotovora 11 µg/ml Fujimura et al. (2004)
A. radiobacter 15 µg/ml
A. rhizogenes 20 µg/ml
C. michiganensis 14 µg/ml
C. flaccumfaciens 13 µg/ml
Tu-AMP 2 Heterodimeric Peptide 5 E. carotovora 15 µg/ml
A. radiobacter 17 µg/ml
A. rhizogenes 20 µg/ml
C. michiganensis 17 µg/ml
C. flaccumfaciens 15 µg/ml
21 Solanum tuberosum (Tubers) Snakin-1 Peptide 6.9 M C. michiganensis 4 µM Berrocal-Lobo et al. (2002)
Snakin-2 Peptide 7.0 MAISKALFAS LLLSLLLLEQ C. michiganensis 1 µM
R. meliloti 8 µM
22 Triticum aestivum L. (Endosperm) α-Purothionin Polypeptide 6 MKSCCRSTLG RNCYNLCRAR P. solanacearum ND de Caleya et al. (1972)
X. phaseoli
β-Purothionin Polypeptide 6 MGSKGLKGVM VCLLILGLVL P. solanacearum ND
X. phaseoli
23 Viola odorata (Whole Plant) Cycloviolacin O2 Macrocyclic peptides 3.14 GIPCGESCVW IPCISSAIGC SCKSKVCYRN S. enterica (DA6192) ND
E. coli (DA4201)
S. aureus (DA7127)
24 Viola abyssinica (Whole Plant) Vaby A Macrocyclic peptides 2.86 S. enterica (DA6192) ND Pränting et al. (2010)
E. coli (DA4201)
S. aureus (DA7127)
Vaby D Macrocyclic peptides 3.06 S. enterica (DA6192) ND
E. coli (DA4201)
S. aureus (DA7127)
25 Beta vulgaris (Leaves) AX 1 Peptides 5.0 AICKKPSKFF KGACGRDADC EKACDQENWP GGVCVPFLRC ECQRSC C. beticola 0.4–0.8 µM Kragh et al. (1995)
AX2 Peptides 5.1 ATCRKPSMYF SGACFSDTNC QKACNREDWP NGKCLVGFKC ECQRPC
26 Mirabilis expansa (Roots) ME1 Protein 27 METMRLLFLL LTIWTTVVGS P. syringae B ND Vivanco et al. (1999)
A. tumefaciens C58
A. rhizogenes ATCC15834
B. subtilis G13R
F. carotovora ATCC15713
X. campestris pv vesicatoria
R. leguminosarum
S. marcescens
ME2 Protein 27.5 P. syringae
A. tumefaciens
A. rhizogenes (ATCC15834)
B. subtilis G13R
F. carotovora
X. campestris pv vesicatoria
R. leguminosarum
S. marcescens
27 Benincasa hispida (Seeds) Hispidalin Peptide 5.7 E. coli ND Sharma et al. (2014)
P. aeruginosa
S. enterica
S. aureus
28 Zizyphus jujuba (Fruit) Snakin-Z Peptide 3.3 E. coli ND Daneshmand et al. (2013)
K. pneumoniae
B. subtilis
S. aureus
29 Fagopyrum esculentum Moench. (Seeds) Fa-AMP1 Peptide 3.8 AQCGAQGGGA TCPGGLCCSQ WGWCGSTPKY CGAGCQSNCK E. carotovora 11 µg/ml Fujimura et al. (2003)
A. radiobacter 24 µg/ml
A. rhizogenes 20 µg/ml
C. michiganensis 14 µg/ml
C. flaccumfaciens 13 µg/ml
Fa-AMP2 Peptide 3.9 AQCGAQGGGA TCPGGLCCSQ WGWCGSTPKY CGAGCQSNCR E. carotovora 15 µg/ml
A. radiobacter 17 µg/ml
A. rhizogenes 24 µg/ml
C. michiganensis 17 µg/ml
C. flaccumfaciens 15 µg/ml
30 Allium sativum (Bulbs) Alliumin Protein 13 DDFLCAGGCL P. fluorescens ND Xia and Ng (2005)
31 Vicia faba (Flower) Fabatin-1 Peptide 5.2 LLGRCKVKSN RFHGPCLTDT HCSTVCRGEG YKGGDCHGLR RRCMCLC E. coli ND Zhang and Lewis (1997)
P. aeruginosa
E. hirae
Fabatin-2 Peptide 5.20 LLGRCKVKSN RFNGPCLTDT HCSTVCRGEG YKGGDCHGLR RRCMCLC E. coli ND
P. aeruginosa
E. hirae
32 Moringa Oleifera (Seeds) MoCP Dimeric protein 13 E. coli ND Shebek et al. (2015)
33 Zea mays (Kernel) MBP-1 Peptide 4.1 RSGRGECRRQ CLRRHEGQPW C. michiganense ssp. Nebraskense ND Duvick et al. (1992)
34 Vigna radiate (Seeds) VrD1 Peptide 5.1 MERKTFSFLF LLLLVLASDV E. coli ND Lin et al. (2007)

*IC50  Concentration of protein required for 50% growth inhibition, NA  Not available, ND Not determined, Cy-AMP  Cycad antimicrobial peptide, Pa-AMP-1 Phytolacca americana antimicrobial protein, Ib-AMP1 Impatiens balsamina antimicrobial peptides, Pg-AMP  Psidium guajava-antimicrobial peptide, WSG Withania somnifera glycoprotein, APC  antioxidant protein from curry leaves, VaD1 Vigna angularis defensing, M.E. Mirabilis expansa, MoCP Moringa oleifera cationic protein, MBP-1  Maize Basic Peptide 1, VrD1  Vigna radiate defensing-1, Mj-AMP Mirabilis jalapa antimicrobial peptide

Anti-fungal activity of plant peptides/proteins

There have been high incidences of patients with threatening fungal infections, particularly those with a compromised immune system like AIDS, organ transplants, cancer, etc. The prolonged use of medicines they take for their therapy makes them vulnerable to potent fungal infections that can ultimately lead to death. The main challenge is that not many drugs are available for many conditions and, worst case, the absence of drugs for the treatment. Furthermore, another obstacle of drug resistance originates from extended drug utilization, rendering the current drug unusable. Correspondingly, we have to hunt for novel drugs, especially from natural sources like plants. Antifungal Proteins/Peptides (AFP) are low molecular weight compounds that act as the first line of defense against fungal pathogens. These proteins include defensins, thionins, lipid-transfer proteins (LTR), chitinase-like proteins, lectins, etc. (Lee-Huang et al. 1991a). The majority of AFPs work by lysis of fungal cell wall or by targeting components like sphingolipids and chitin, thereupon inhibiting cell wall synthesis. One of the instances is that certain AFPs result in pore formation or membrane polarization upon binding of chitin on its conserve domain, causing an efflux of K+ and influx of Ca2+, ultimately cell lysis (Lee-Huang et al. 1991b). Some other examples of AFPs mechanism of action are of defensins. They follow receptor-mediated activation (Leader et al. 2008). Subsequent binding to this receptor causes ion permeability and pore formation. Other AFPs cause various modifications in host cell signaling processes, leading to ROS generation (Reactive Oxygen Species), eventually leading to apoptosis. AX1 and AX2, thionin-like peptides that are cationic, interact with anionic phospholipids causing fungal membrane permeabilization (Lee-Huang et al. 1991a; b). Thaumatin-like proteins, a class of AFPs, inhibit the fungal spore formation, leading to lysis. Pn-AMP-1 and Pn-AMP-2 (extracted from Pharbitis nil) hinder the hyphal growth, causing the tips to be shattered upon insertion of hyphae, ultimately leading to rupture of fungal membrane and cytoplasmic leakage (Leader et al. 2008). Like other plant peptides, AFPs are diverse, having inert anti-cancer and anti-HIV activity. Mungin, sesquin, lunatusin, and PHP (Peganum harmala protein) are examples (Lee-Huang et al. 1991a; b; Liu et al. 2000; Mazalovska and Kouokam 2018).

The non-specific lipid transfer protein (nLTP) PHP, isolated from Peganum harmala have been shown to inhibit various fungal species with an IC50 value ranging 1.5–12.19 μM (Yokoyama et al. 2008). Hypotin (extracted from Arachis hypogaea) has been shown to inhibit the activity of species like Pythium aphanidermatum, Fusarium solani, Physalospora piricola, Alternaria alternata, Botrytis cinerea, Fusarium oxysporum, and Pythium aphanidermatum (Stirpe et al. 1986). Vulgin inhibits the fungal activity of a wide variety of species, combined with potent anti-HIV activity by inhibiting HIV reverse transcriptase (Ye and Ng 2003). It was reported that a proteinaceous α-amylase inhibitor extracted from rhizome of Cheilocostus specious and purified employing anion exchange chromatography and column gel filtration had an activity on fungal α -amylase. The fungal activity was reduced by this 31.18 kDa protein from C. speciosus by 71% using ion-exchange chromatography and 96% using gel filtration (Balasubramanian et al. 2018). It was documented that Ferula asafoetida root was used to extract three major proteins with molecular weights of 14 kDa, 27 kDa, and 39 kDa. The 39-kDa protein significantly improved chymotrypsin activity, while the 14-kDa protein had antibacterial action towards Pseudomonas aeruginosa. All three pure proteins were also reported to have significantly increased antioxidant activity (Chandran et al. 2017). Quorum-sensing inhibitors from Solanaceae family were also reported to possess anti-bacterial action against Pseudomonas aeruginosa (Singh et al. 2015).

Until now, hundreds of AFPs have been identified as having negligible toxicity. Tu-AMP-1 and Tu-AMP-2 are highly potent AFPs inhibiting Fusarium oxysporum and Geotrichum candidum (Wong and Ng 2005). Ginkbilobin (extracted from Ginkgo biloba) strongly affects the activity of B. cinerea (Wang and Ng 2000). Sesquin (extracted from Vigna sesquipedalis) is a highly active AFP with an IC50 value of 0.15 μM and 1.4 μM for Mycosphaerella arachidicola and F. oxysporium, respectively (Wani et al. 2020). Despite all of these studies showing the therapeutic effects of AFPs, not many have reached clinical trials. Most of these peptides have been ignored due to a lack of proper classification and structural and functional diversity. Efforts in this direction are required so that the therapeutic potential of AFPs can be used to a full extent and the available AFPs are tabulated (Table 2).

Table 2.

List of anti-fungal peptides/proteins from different parts of plants

S. No Plant and its part Protein Nature M.Wt. (kDa) Peptide sequence Fungal species (Tested) *IC50 References
1 Momordica charantia (Leaves) MCha-Pr Protein 25.5 VEYTITGNAGNTPGG A. brassicae 33 µM Zhang et al. (2015)
C. personata 42 µM
F. oxysporum 37 µM
Mucor sp., 40 µM
R. solani 48 µM
2 Arachis hypogaea (Seeds) Hypotin Protein 30.4 CDVGSVISASLFEALQKHRN P. aphanidermatum 18.9 µM Wang et al. (2007)
B. cinerea NA
A. alternate
S. rolfsii
F. oxysporum
F. solani
3 Phaseolus coccineus cv. ‘Major’ (Seeds) Coccinin Peptide 7 KQTENLADTY M. arachidicola 75 ± 5 µM Ngai and Ng (2004)
F. oxysporum 81 ± 7 µM
P. piricola 89 ± 4 µM
B. cinerea, 109 ± 5 µM
C. comatus 122 ± 7 µM
R. solani 134 ± 2 µM
4 Phaseolus vulgaris (Seeds) Vulgin Polypeptide 5 VDVGTVLTATFIEQFFKHRNDQAPEGKGFYTYNAFISAAR B. cinerea 7 µM Ye and Ng (2003)
M. arachidicola NA
C. comatus
F. oxysporum,
Fraction PTA2c Peptide 5 KTCENLVDTYRGPCFT M. arachidicola NA Ye and Ng (2001)
B. cinerea 1 µM
F. oxysporum NA
5 Chrysanthemum coronarium (Seeds) Chrysancorin Protein 13.4 RVDQKAQNLKCCQQHRFNCHCERVCVFQDQ B. cinerea 11 µM Wang et al. (2001)
M. arachidicola 17.4 µM
P. piricola 14.6 µM
6 Phaseolus lunatus L. (Seeds) Lunatusin Peptide 7 KTCENLADTFRGPCFATSNC F. oxysporum 1.9 µM Wong and Ng (2005a)
B. cinerea 2.6 µM
M. arachidicola 0.32 µM
7 Brassica junceavar.integrifolia (Seeds) Juncin Protein 18.9 GVEVTRELRSERPSGKIVTI F. oxysporum 13.5 µM Ye and Ng (2009)
H. maydis 27 µM
M. arachidicola 10 µM
8 Vigna angularis (Seeds) Angularin Peptide 8 B. cinerea 14.3 µM Ye and Ng (2002b)
M. arachidicola NA
9 Ginkgo biloba Ginkbilobin (Seeds) Protein 13 B. cinerea 0.25 µM Wang and Ng (2000)
M. arachidicola 6.5 µM
F. oxysporum 3.6 µM
R. solani 8.7 µM
C. comatus 3.4 µM
GAFP (Leaves) Peptide 4.24 P. sasakii Ito ND Huang et al. (2000)
A. alternate (Fries) Keissler
10 Dendrocalamus latiflora Munro (Shoot) Dendrocin Protein 20 B. cinerea 1.8 µM Wang and Ng (2003)
F. oxysporium 1.4 µM
M. arachidicola 5.1 µM
11 Vigna sesquipedalis (Seeds) Sesquin Peptide 7 B. cinerea 2.5 µM Wong and Ng (2005b)
F. oxysporum 1.4 µM
M. arachidicola 0.15 µM
12 Withania somnifera (Root tubers) WSG Glycoprotein 28 A. flavus ND Girish et al. (2006)
A. niger
A. nidulans
A. flaviceps
A. alternate
A. carthami
F. oxysporum
F. verticilloides
13 Allium sativum (Bulbs) Alliumin Protein 13 M. arachidicola 1.3 µM Xia and Ng (2005)
14 Pharbitis nil (Seeds) Pn-AMP1 Peptides 4.29 B. cinerea 16 µg/ml Koo et al. (1998)
C. langenarium 10 µg/ml
S. sclerotiorum 11 µg/ml
F. oxysporum 10 µg/ml
R. solani 26 µg/ml
P. capsici 5 µg/ml
P. parasitica 3 µg/ml
Pythium spp. N.A
S. cerevisiae 14 µg/ml
Pn-AMP2 Peptides 4.21 B. cinerea 2 µg/ml
C. langenarium 4 µg/ml
S. sclerotiorum 3 µg/ml
F. oxysporum 2.5 µg/ml
R. solani 75 µg/ml
P. capsici 0.6 µg/ml
P. parasitica 2 µg/ml
Pythium spp. 2.5 µg/ml
S. cerevisiae 8 µg/ml
15 Beta vulgaris L. (Leaves) IWF4 Dimeric protein 4.5 C. beticola  ≤ 2 µg/ml (0.7 µM) Nielsen et al. (1997)
16 Eucommia ulmoides Oliv (Bark) EAFP1 Peptides 4.20 A. lycopersici 155 µg/ml Huang et al. (2002)
F. moniliforme 56 µg/ml
F. oxysporum 46 µg/ml
C. gossypii 35 µg/ml
EAFP2 Peptides 4.15 A. lycopersici 109 µg/ml
F. moniliforme 18 µg/ml
F. oxysporum 94 µg/ml
C. gossypii 56 µg/ml
17 Capsella bursa-pastoris (Roots) Shepherin I Peptide 2.36 C. albicans 8 µg/ml Park et al. (2000)
C. neoformans  < 2.5 µg/ml
S. cerevisiae 7 µg/ml
A. alternate 7 µg/ml
A. flavus 65 µg/ml
A. fumigatus  > 100 µg/ml
F. culmorum 72 µg/ml
Shepherin II Peptide 3.26 C. albicans 5 µg/ml
C. neoformans  < 2.5 µg/ml
S. cerevisiae 3 µg/ml
A. alternate  > 100 µg/ml
A. flavus 60 µg/ml
A. fumigatus  > 100 µg/ml
F. culmorum 68 µg/ml
18 Hevea brasiliensis (Latex) Hevein Protein 4.7 B. cinerea 500 µg/ml van Parijs et al. (1991)
F. culmorum 600 µg/ml
F. oxysporum 1.25 mg/ml
P. blakesleeanus 300 µg/ml
P. triticirepentis 350 µg/ml
P. oryzae 500 µg/ml
S. nodorum 500 µg/ml
T. hamatum 90 µg/ml
19 Gentiana triflora (Leaves) GtAFP1 Protein 20 A. alternate 51 μg /ml Kiba et al. (2005)
B. cinerea 61 μg /ml
F. solani 99 μg /ml
20 Acacia confusa (Seeds) Acaconin Protein 32 R. solani 30 ± 4 µM Lam and Ng (2010)
21 Tulipa gesnerian (Tulip Bulbs) Tu-AMP1 Peptide 4.9 F. oxysporum 2 μg /ml Fujimura et al. (2004)
G. candidum 2 μg /ml
Tu-AMP2 Dimeric peptide 2.259 F. oxysporum 2 μg /ml
G. candidum 2 μg /ml
22 Cicer arietinum (Seeds) CLAP Protein 18 M. arachidicola 5.5 μM Ye and Ng (2002b)
B. cinerea 1.3 μM
C-25 Lectin protein 25 C. krusei, C. tropicalis, C. parapsilosis 1.56–12.5 µg/ml Kumar et al. (2014)
23 Gymnocladus chinensis Baill (Beans) Gymnin Peptide 6.5 F. oxysporum 2 µM Wong and Ng (2003a)
M. arachidicola 10 µM
24 Adzuckia angularia (Seeds) Fraction AB2 Peptide 5 B. cinerea 3.5 µM Ye and Ng (2001)
M. arachidicola NA
F. oxysporum
25 Macadamia integrifolia (Seeds) MiAMP1 Peptide 5.9 C. michiganensis 50 µg/ml Marcus et al. (1999)
26 Vigna angularis (Seeds) VaD1 Peptide 5.0 F. oxysporum 30 µg/ml Chen et al. (2005b)
F. oxysporumf. sp. pisi 53.2 µg/ml
T. rubrum  > 500 µg/ml
27 Phaseolus vulgaris (Seeds) Vulgarinin Peptide 7 KTCENLADTYKGP CFTSGGD B. cinerea 2.9 µM Wong and Ng (2005c)
F. oxysporum 1.7 µM
P. piricola 2.2 µM
M. arachidicola 0.21 µM
C. albicans cc
P. azadirachtae
P. ultimum
G. candidum 25 µg/ml
28 Spinacia oleracea (Leaves) So- D2 Peptide 5.80 F. culmorum 0.2 µM Segura et al. (1998)
F. solani 11 µM
So-D6 Peptide 2.55 F. culmorum NA
F. solani 11 µM
So-D7 Peptide 4.23 F. culmorum N.A
F. solani 9 µM
29 Actinidia chinensis (Fruit) Kiwi TLP Protein 21 B. cinerea 0.43 µM Wang and Ng (2002)
M. arachidicola 8 µM
P. piricola NA
30 Benincasa hispida (Seeds) Hispidalin Peptide 5.7 A. flavus ND Sharma et al. (2014)
F. solani
C. geniculata
P. chrysogenum
C. gloeosporioides
31 Peganum harmala (Seeds) PHP Homodimeric protein 18 A. alternate 1.5 µM Ma et al. (2013)
P. degitatum 7.5 µM
R. stuolonifer 8.44 µM
M. grisea 2.19 µM
32 Cycas revoluta (Seeds) Cy-AMP1 Peptide 4.58 F. oxysporum 6.0 µg/ml Yokoyama et al. (2008)
G. candidum 7.4 µg/ml
Cy-AMP2 Peptide 4.56 F. oxysporum 7.1 µg/ml
G. candidum 7.0 µg/ml
Cy-AMP3 Peptide 9.27 F. oxysporum 250 µg/ml
G. candidum 200 µg/ml
33 Allium tuberosum (Shoot) Fraction MS3 Protein 36 R. solani NA Lam et al. (2000)
F. oxysporum
C. comatus
M. arachidicola
B. cinerea 0.2 µM
34 Dolichos lablab (Seeds) Dolichin Protein 28 F. oxysporum ND Ye et al. (2000)
R. solani
C. comatus
35 Panax ginseng (Roots) Panaxagin Homodimeric protein 53 F. oxysporum ND Ng and Wang (2001)
C. comatus
R. solani
36 Phaseolus mungo (Seeds) Mungin Protein 18 R. solani ND Ye and Ng (2000)
C. comatus
M. arachidicola
B. cinerea
F. oxysporum
37 Zea mays (Kernels) MBP-1 Peptide 4.13 F. graminearum ND Duvick et al. (1992)
F. moniliforme
A. flavus
F. oxysporum  
A. solani
T. reesei
T. harzianum
38 Raphanus sativus (Seeds) RsAFP1 Tetrameric polypeptide 20 A. brassicola ND Terras et al. (1992)
Ascochyta pis
B. cinerea
C. beticola
C. lindemuthianum
F. culmorum
T. hamatum
P. oryzae
RsAFP2 Trimeric polypeptide 15 A. brassicola ND
Ascochyta pis
B. cinerea
C. beticola
C. lindemuthianum
F. culmorum
T. hamatum
P. oryzae 0.08 µM
39 Zingiber officinalis (Rhizome) G-24 Protein 24 F. oxysporium 4.6 µM Terras et al. (1992)
C. albicans 8.0 µM
40 Trichosanthes dioica (Seeds) TDSC Glycoprotein 39 ± 1 EING GGA A. niger and Trichoderma sp. ND Kabir et al. (2016)

*IC50 Concentration of protein required for 50% growth inhibition, ND  Not determined, NA Not available, as these proteins have been claimed to exhibit the activity, but no activity parameters have been mentioned, Kiwi TLP Kiwi fruit thaumatin-like protein, MCha-Pr Momordica charantia pathogenesis-related protein, Fraction PTA2c  Pinto bean antifungal peptide, WSG Withania somnifera glycoprotein, IWF4 Intercellular washing fluid, EAFP  Eucommia antifungal peptide, GtAFP Gentiana triflora antifungal protein, MBP-1  Maize basic peptide, CLAP  Chickpea cyclophilin-like antifungal protein, VaD1 Vigna angularis variegate 1, TDSC Trichosanthes dioica seed chitinase

Anti-viral Activity of plant peptides and proteins

Anti-HIV Activity

Acquired Immunodeficiency Syndrome (AIDS) is the fourth leading cause of death triggered by the Human immunodeficiency virus (HIV) (Irvin and Uckun 1992). Two variants of HIV are HIV-1 and HIV-2, each being etiologically and genetically different. Medically, these types vary with the disease's pace of progression, with HIV-1 being faster than HIV-2 (Irvin and Uckun 1992). The mode of action of HIV-1 involves host and viral membrane interaction through binding of the envelope glycoproteins (g120 and gp41) to CD4, CCR5 and CXCR4 receptors of the host cell. Subsequently, the virus enters the cell along with the integration of the viral genome into the host genome (Wang 2012). Preventing protein maturation and viral RNA replication to DNA are some of the treatment options available to enhance the infected's survivability. Nevertheless, no proper vaccine is available yet due to: (i) Advent of viral strains that are highly resistant to current anti-HIV drugs, (ii) Incapability to annihilate latent viruses, (iii) Toxicity, (iv) Lack of proper route of administration (Irvin and Uckun 1992). Hence, as mentioned earlier, the scientific community is probing novel drug molecules to curb the obstacle. Within this framework, therapeutic plant peptides are seen as prospective contestants. As an alternative, plant peptides can be used as an excellent medication due to their highly specific nature, increased bioactivity, non accumulated in our organs and less to negligible toxicity (Barbieri et al. 1982; Barbosa Pelegrini et al. 2011). Many antiviral plant proteins belong to the family of cyclotides endowed with a highly stable peptide framework. Cyclotides are cyclic structures that are 28–37 amino acid residues long. They consist of a cyclic cysteine knot motif (CCK) made up of highly conserved cysteine residues linked together by three disulfide bonds. Surface-exposed hydrophobic patches formed by the CCK motif and its cyclicity are some of the reasons for its anti-HIV activity (Gerlach and Mondal 2012). Some other plant proteins including RIPs (Ribosome Inactivating Proteins) such as TCS (Trichosanthin) and PAP (Pokeweed antiviral Protein-N-glycosidase that exhibits antiviral activity against several viruses) have strong anti-HIV potential with some present in clinical trials. TCS has been shown to lower HIV-1 p24 antigen levels in AIDS patients (Leader et al. 2008). MAP30 (Momordica anti-human immunodeficiency virus protein) is a highly potent anti-HIV agent and a type-I RIP, with an IC50 of only 0.33 nM (Lee-Huang et al. 1990). Due to the strong IC50 value of PAP, the conjugation of PAP and immunoconjugates have been used as inhibitors for HIV infection (Irvin and Uckun 1992). An example of this is TXU-PAP, wherein PAP has been conjugated with TXU and attacks the CD7 antigen of HIV-infected cells, thereby inhibiting the infection (Lee-Huang et al. 1990).

Being prone to microbial infections, the combined activity of both anti-HIV and anti-microbial peptides could create new opportunities for HIV therapy. Aforementioned proteins have properly recorded structures, but not much research has been done to understand their mode of action. The most widely accepted hypothesis is attacking the viral envelope (Bokesch et al. 2004). The cyclotides work by viral membrane disruption leading to the formation of the pore (Gerlach and Mondal 2012). These cyclotides (Kalata 1) get bound to the phospholipid-rich viral coat with the help of its hydrophobic patches, resulting in an oligomeric form that penetrates the viral coat. This leads to the formation of discrete pores, thereby causing the coat to collapse (Wang 2012). As viral coat has glycoproteins in it, plant peptides like ricin and con A, possessing carbohydrate-binding sites in them, have been considered as potential candidates for inhibiting HIV at initial stages (Mazalovska and Kouokam 2018). RIPs like PAP, MAP30, TCS stop HIV-1 replication through depurination of long terminal repeats (LTRs) present in the DNA (Kaur et al. 2012). Another RIP saporin impedes the activity of HIV1 integrase for processing the 3 end of the viral DNA disintegrating genome and its mRNA (Yadav and Batra 2015). If we can decipher the role of such proteins at different phases of the viral infection, anti-HIV activity can be exploited. Steps are to be taken to extract and characterize much more powerful anti-HIV agents that are less toxic. The available anti-HIV peptides are reported in Table 3.

Table 3.

List of anti-HIV peptides/proteins from plants

S. No Plant and its part Protein Nature M. Wt. (kDa) Peptide Sequence Mode of action *IC50 References
1 Phaseolus lunatus (Seeds) Lunatusin Peptide 7 KTCENLADTFRGPCFATSNC HIV-1 reverse transcriptase inhibition 120 µM Wong and Ng (2005a)
2 Phaseolus vulgaris (Seeds) Vulgin Polypeptide 5 VDVGTVLTATFIEQFFKHRNDQAPEGKGFYTYNAFISAAR HIV-1 reverse transcriptase inhibition 58 µM Ye and Ng (2003)
Fraction PTA2c Peptide 258 µM Ye and Ng (2001)
3 Lens culinaris (Seeds)  LTI Protein 16 GDKKQAYTDTYLSTRSQPP HIV-1 reverse transcriptase inhibition 30 mM Cheung and Ng (2007)
4 Vigna sesquipedalis (Ground Beans) Sesquin Peptide 7 KTCENLADTY HIV-1 reverse transcriptase inhibition ND Wong and Ng (2005b)
N.A Heterodimeric lectin 60 73 µM Wong and Ng (2003b)
5 Acacia confusa (Seeds) Acaconin Protein 32 HIV-1 reverse transcriptase inhibition 10 ± 2.3 µM Lam and Ng (2010)
6 Gelonium multiflorum (Seeds) GAP 31 Protein 31 HIV-1 reverse transcriptase inhibition 0.32 nM Lee-Huang et al. (1991b)
Inhibition of syncytium formation 0.28 nM
Viral core protein p24 inhibition 0.23 nM
7 Dianthus caryophyllus (Leaves) DAPs 30 Protein 30 ATAYLNLAPSASQYSXF HIV-1 reverse transcriptase inhibition 0.88 nM Lee-Huang et al. (1991b)
Inhibition of syncytium formation 0.76 nM
Viral core protein p24 inhibition 0.85 nM
DAPs 32 Protein 32 AVKTILNLVSPSANRYATF HIV-1 reverse transcriptase inhibition 0.76 nM
Inhibition of syncytium formation 0.76 nM
Viral core protein p24 inhibition 0.71 nM
8 Momordica charantia (Seeds) MAP 30 Protein 30 DVNFDLSTATAKTYTFIEDFRATLPF HIV-1 reverse transcriptase inhibition 0.33 nM Lee-Huang et al. (1990)
Inhibition of viral core protein p24 expression 0.22 nM
Inhibition on syncytium formation 0.83 nM
9 Trichosanthes kirilowii (Root tubers) TAP 29 Protein 29 Inhibition of syncytium formation 0.34 nM Lee-Huang, et al. (1991a)
Inhibition of viral core protein p24 expression 0.37 nM
Inhibition of viral-associated reverse transcriptase activity 0.46 nM
10 Dorstenia contrajerva (Leaves) Contrajervin Peptide 5 ERDDHRCGPDYGNPSCSGDRCCSIYNWCGGGSSYCSGGSCRYQCWY HIV-1 inhibition by binding to gp120 and gp41  > 4.9 µM Bokesch et al. (2004)
11 Treculia obovoidea (Bark) Treculavirin Dimeric peptide 10 PGCEERPDHQCGPDYGNPGCGAGRCCSIHGWCGSSADYCSGTSCQYQCSC HIV-1 inhibition by binding to gp120 and gp41  > 2.5 µM Bokesch et al. (2004)
12 Dolichos lablab (Seeds) Dolichin Protein 28 GAVGSVINASLFEQLLKHRNDQDPEGKG HIV-1 reverse transcriptase inhibition  < 180 µM Ye et al. (2000)
13 Oldenlandia affinis Kalata B1 (Whole Plant) Macrocyclic Peptides 2.89 GLPVCGETCVGGTCNTPG HIV inhibition by cell envelope disruption 3.5 µM Daly et al. (2004)
Kalata B8 (Aerial Parts) Macrocyclic Peptides 3.28 GSVLNCGETCLLGTCYTTG 11 µM Daly et al. (2006)
14 Chassalia parvifolia Circulin A (Crude Extract) Macrocyclic Peptides 3.17 GIPCGESCVW IPCISAALGC SCKNKVCYRN HIV replication inhibition 0.05 µM Gustafson et al. (1994)
Circulins B (Crude Extract) Macrocyclic Peptides 3.3 GVIPCGESCV FIPCISTLLG CSCKNKVCYR N 0.05 µM
Circulins C (Stems) Macrocyclic Peptides 3.1 NA NA Gustafson et al. (2000)
Circulins D (Stems) Macrocyclic peptides 3.39 NA
Circulins E (Stems) Macrocyclic peptides 3.39 NA
Circulins F (Stems) Macrocyclic peptides 3.05 NA
15 Peganum harmala (Seeds) PHP Homodimeric protein 18 HIV-1-RT inhibition 1.26 µM Ma et al. (2013)
16 Palicourea condensata (Bark) Palicourein Polypeptide 3.9 RNGDPTFCGETCRVIPVCTYSAALGCTCDDRSDGLCK HIV-1 replication inhibition 1.5 µM Bokesch et al. (2001)
17 Trichosanthes kirilowii (Root tubers) TCS or (GLQ 223) Protein 26 HIV-1 replication inhibition 0.46 nM Shu et al. (2009)
18 Leonia cymosa (Bark) Cycloviolin A Macrocyclic peptides 3.2 SCVFIPCISAAIGCSCKNKVCY NA 0.56 μM Hallock et al. (2000)
Cycloviolin B 2.8 SCYVLPCFTVGCTCTTSSQ
Cycloviolin C 3.1 SCVFIPCLTTVAGCSCKNK
Cycloviolin D 3.1 SCVFIPCISAAIGCSCKNKCY
19 Viola odorata Cycloviolacin O2 (Whole Plant) Macrocyclic peptides 3.1 HIV inhibition by cell membrane disruption NA Gerlach et al. (2010)
Cycloviolacin O13 (Aerial Parts) 3.12 6.4 µM Ireland et al. (2008)
Cycloviolacin O14 (Aerial Parts) 3.17 4.8 μM
Cycloviolacin O24 (Aerial Parts) 3.04 6.17 μM
20 Viola yedoensis (Whole Plant) Cycloviolin Y1 Macrocyclic peptides 3 NA 4.47 μM Wang et al. (2008)
Cycloviolin Y4 1.72 μM
Cycloviolin Y5 1.76 μM
21 Viola tricolor (Whole Plant) Varv E Macrocyclic peptides 2.99 NA 3.98 μM Wang et al. (2008)
22 Viola hederacea (Leaves) Vhl-1 Macrocyclic peptides 3.33 NA 0.87 μM Chen et al. (2005a)
23 Vicia faba cv. Giza 843 (Seeds) VFTI-G1 Protein 15 HIV-1-RT inhibition 0.76 µM Dia and Krishnan (2016)
24 Gymnocladus chinensis Baill (Beans) Gymnin Peptide 6.5 HIV-1-RT inhibition 200 µM Wong and Ng (2003b)
25 Adzuckia angularia (Seeds) Fraction AB2 Peptide 5 HIV-1-RT inhibition 280 µM Ye and Ng (2001)
26 Bauhinia variegate (Seeds) Fraction BG2 Homodimeric lectin 64 HIV-1-RT inhibition 1.02 µM Chan and Ng (2015)
27 Momordica balsamina (Seeds) Balsamin Protein 28 HIV-1 replication inhibition 10.2 nM Kaur et al. (2012)
28 Phaseolus vulgaris (Seeds) Vulgarinin Peptide 7 KTCENLADTYKGPCFTSGGD HIV-1-RT inhibition 130 µM Wong and Ng (2005c)
29 Phytolacca americana (Leaves) PAP Protein 29 -30 Inhibited p24 production in HIV 0.5 nM Irvin and Uckun (1992)
PAP-I 29 HIV-1-RT inhibition 14 ± 2.1 nM Rajamohan et al. (1999)
PAP-II 30 26 ± 2.5 nM
PAP-III 30 17 ± 2.0 nM
30 Momordica charantia (Seeds) MRK29 Protein 28.6 Asp Val Asn Phe Arg Leu Ser Gly Ala Asp HIV-1-RT inhibition 18 µg/ml Jiratchariyakul et al. (2001)
31 Brassica juncea var. integrifolia (Seeds) Juncin Protein 18.9 HIV-1-RT inhibition 4.5 µM Ye and Ng (2009)
32 Panax ginseng (Roots) Panaxagin Homodimeric protein 53 HIV-1-RT inhibition NA Ng and Wang (2001)
33 Allium tuberosum (Shoot) Fraction MS3 Protein 36 EQHGSQAGGALHPGXLHYSKYGGYGGTTPDYYGDGQQ HIV-1-RT inhibition NA Lam et al. (2000)

*IC50  Concentration causing 50% inhibition, ND  Not determined, NA Not available, as these proteins have been claimed to exhibit activity, but no activity parameters have been mentioned. LTI  Lentil trypsin-chymotrypsin inhibitor, TAP 29  Trichosanthes anti-HIV protein, MAP 30  Momordica anti-HIV protein, Vhl-1  Viola hederaceae leaf cyclotide-1, MRK29  Thai bitter gourd protein, HIV-1-RT  Human immunodeficiency virus-1 reverse transcriptase

Anti-SARS-CoV-2 activity

SARS-CoV-2, also called COVID-19 (Coronavirus Disease 2019), has more than 130 million reported cases worldwide and has taken the lives of more than 2.8 million people since its onset in late 2019 (Zhou et al. 2020) and successive pandemic declarations by the WHO on 11 March 2020 (WHO 2021). Since the virus outbreak, a monumental effort has been made by researchers and drug companies worldwide to discover a vaccine. Multiple candidates were chosen from varied sources, most of them being in clinical trials. But so far, no definite cure has been developed. Only a few vaccines have been engineered as a contingency plan against the virus. Plant peptides have also been tested for vaccine production to broaden the range of candidates. Lectin extracted from red marine alga Griffithsia sp. (GRFT) have been shown to inhibit the cytopathic effect of SARS-CoV, enhancing the mortality of cells (O’Keefe et al. 2010). In the case of MERS-CoV (Middle East respiratory syndrome-CoV: Strain of SARS-CoV in the Middle East), GRFT acts by preventing its entry into the host cell through spike protein inhibition. Thus, GRFT serves as an effective inhibitor of MERS-CoV infection (Millet et al. 2016). In-silico methods using plant proteins have also been utilized to identify the potential lead compounds for COVID-19 vaccine design. Avenin from oats, α/β-gliadin from wheat, and ribulose bisphosphate carboxylase small chain from multiple sources have been utilized to generate effective binders to SARS-CoV-2 spike receptor-binding protein (RBD). When combined with certain oligopeptides (VQVVN, PISCR), these plant peptides / proteins might be employed as lead compounds in developing potent entry inhibitors (Luo et al. 2020). A wide variety of therapeutic plant peptides exist, out of which only a few have been explored (Mammari et al. 2021). Future research should focus on other plant-derived peptides, their mode of action, and their side effects in order to engineer a proper peptide vaccine for COVID-19.

Non-infectious Diseases

The diseases which are mainly caused due to environmental or genetic factors and not by pathogens are termed non-infectious diseases. Examples of non-infectious diseases include diabetes mellitus, most cancers, and cardiovascular diseases. These could be cured using therapeutic peptides obtained from various plant sources. Peptides are essential molecules that can attach to multiple cell surface receptors. The plant peptides used as drugs are increasing day by day. This review is majorly discuss the plant peptides with anti-diabetic, anti-cancer, and anti-hypertensive properties. When treated with proteolytic enzymes of plant proteins form protein hydrolysates and yield peptides. These therapeutic peptides could be used to treat various non-infectious diseases. Nineteen percent of the medicinal plant peptides are used to cure metabolic disorders, twelve percent are used to cure cancer, and almost three percent to cure cardiac related problems (Patil et al. 2020). The peptides obtained from various plant sources such as common bean, rice, pinto bean, hemp seeds, and mulberry have anti-diabetic properties. Peptides obtained from soybean, wheat, barley, and walnut have anti-cancer properties. Anti-hypertensive activity is observed in peptides purified from rice and walnut. This review focuses on the various peptides, their origins, sequences, and how they prevent non-infectious diseases (Table 4).

Table 4.

List of plant peptides/proteins used for non-infectious diseases

S. No Plant and its part M. Wt Sequence Inhibitor target Property References
1 Cannabis sativa L. (Seeds)

287.2 Da

568.4 Da

LR

PLMLP

Alpha-glucosidase inhibition Anti-diabetic Ren et al. (2016)
2 Morus alba L. (Leaves) 0.3–5 KDa WGVENAATYFWQTV Alpha-glucosidase inhibition Anti-diabetic Jha et al. (2018)
3 Phaseolus vulgaris L. (Fruit) Alpha-glucosidase inhibition Anti-diabetic Mojica and de Mejia (2016)
4 Phaseolus vulgaris L. (Fruit)  > 3 kDa Alpha-amylase inhibition Anti-diabetic Ngoh and Gan (2016)
5 Oryza sativa L. (Seeds) DPP-IV enzyme inhibitor Anti-diabetic Hatanaka et al. (2015)
6 Phaseolus vulgaris L. (Fruit) GLUT2 and SLUT1 inhibitor Anti-diabetic Patil et al. (2020)
7 Walnut (Fruit) 1033.42 Da WPERPPEIP ACE inhibitor Anti-hypertensive Liu et al. (2013)
8 Oryza sativa (Husk) ACE inhibitor Anti-hypertensive Shobako and Ohinata (2020)
9 Terminalia chebula Retz (Fruit) 1033 Da DENSKF ACE inhibitor Anti-hypertensive Sornwatana et al. (2015)
10 Oryza sativa (Husk) Anti-proliferative Kannan et al. (2010)
11

Glycine max

Triticum aestivum

Hordeum vulgare

Amaranthus- hypochondriacs (Fruit)

Anti-mitotic, anti-cancer Hernandez-Ledesma et al. (2009)
12 Juglans regia L (Fruit) 621.2795 Da CTLEW Causes apoptosis and autophagy Ma et al. (2015)

Anti-diabetic activity of plant peptides/proteins

Diabetes mellitus is widespread, and it is one of the most prevalent non-infectious diseases and its treatment is challenging. A study conducted in India, reports 80 million diabetic cases, and projected to be 140 million cases by 2037 (Deepthi et al. 2018). The increasing number of cases shows diabetic prevalence in India and the need for developing new strategy in controlling the disease. Several peptides in plants are reported to possess anti-diabetic property by controlling/inhibiting the enzymes and transporters associated with glucose metabolism (α-glucosidase inhibitors, α-amylase inhibitors, DPP-1V inhibitors, GLUT and SLUT) (Patil et al. 2020).

α-Glucosidase Peptide Inhibitors

The outcome of Ren et al. (2016) study reported that Cannabis sativa L. (hemp seeds) peptide (Leucine-Arginine and Proline-Leucine-Methionine-Leucine-Proline) has α-glucosidase inhibitory activity. The hydrophobic nature of the amino acids proline and leucine has shown to have α-glucosidase inhibitory activity, which can be incorporated in therapeutic peptide for further development of effective anti-diabetics. Similarity, 14 amino acids (Tryptophan-glycine-valine-glutamate-asparagine-alanine-alanine-threonine-tyrosine-phenylalanine-tryptophan-glutamine-threonine-valine) long peptide from Morus alba L. (Mulberry) and a peptide (Threonine-threonine-glycine-glycine-lysine-glycine-glycine-lysine) from Phaseolus vulgaris L. (black bean) were shown to have α-glucosidase inhibitory activity (Jha et al. 2018; Mojica and de Mejia 2016).

α-Amylase Peptide Inhibitors

The peptide CSP-1 (cumin seed peptide) obtained from Cuminum cyminum L., has shown 25 % of α-amylase inhibition property (Patil et al. 2020), whereas the peptide from Phaseolus vulgaris cv. Pinto (pinto beans) showed 62.10 % of inhibition. Seven peptides from pinto beans are reported to have α-amylase inhibition property and each of which are in 6–16 amino acids in length. One among the seven peptides which had higher inhibition activity is composed of proline-proline-histidine-methionine-leucine-proline (Ngoh and Gan 2016).

Dipeptidyl Peptidase-IV (DPP-IV) Peptide Inhibitors

DPP-IV facilitates the degradation of Glucagon-like peptide-1 (GLP-1), hence DPP-IV inhibitors are the prime molecules in controlling diabetics. The proteases Umamizyme G and Bioprase SP containing Leucine-Proline and Isoleucine-Proline amino acids from Oryza sativa were having inhibitory activity against DPP-IV. Among which, Isoleucine-Proline was the most potent DPP-IV enzyme inhibitor with the IC50 value of 2.5 mg/ml (Hatanaka et al. 2015).

GLUT and SLUT Plant-Based Peptide Inhibitors

GLUT and SLUT are to be inhibited during hyperglycemic condition where the blood glucose levels are highly elevated. Patil et al. 2020 reported that the peptides in black beans (Phaseolus vulgaris L.) have the ability to block the glucose transporters (GLUT-2 and SLUT-1) in order to control the elevated blood glucose level.

Anti-hypertensive activity of plant peptides/proteins

Hypertension, an elevated pressure in the blood vessels and it is one of the major causes of cardiovascular diseases. Renin-Angiotensinogen System (RAS) is mainly involved in the management of blood pressure. The inhibitors of these enzymes (renin and Angiotensin-I-Converting Enzyme (ACE) of RAS) inhibits the elevated vasodilators to control the blood pressure level. Daskaya-Dikmen et al. 2017 reported several plant-based peptides showing inhibitory activity against ACE towards the development of novel anti-hypertensive therapeutics.

Peptide Inhibitors of ACE

The peptide P-2a2 (Tryptophan-proline-glutamate-arginine-proline-proline-glutamine-isoleucine-proline) from walnut has the molecular weight of 1034 Da and it has shown higher level of inhibition profile with an IC50 value of 23.67 μg/ml against ACE, which prevents the breakdown of vasodilator, bradykinin (Liu et al. 2013). The peptide (Leucine–Arginine–Alanine) obtained from Oryza sativa and chebulin (Aspartate–Glutamate–Asparagine-Serine–Lysine–Phenylalanine) from Terminalia chebula Retz has shown anti-hypertension activity by inhibiting ACE. The walnut and the fruit of Terminalia chebula Retz have been used as a food supplement in the control the hypertension (Shobako and Ohinata 2020; Sornwatana et al. 2015).

Anti-oxidant activity of plant peptide/proteins

The reactive oxygen species (ROS) during metabolism are controlled by host antioxidant enzymes, however, excessive amount of ROS cause severe oxidative stress leads to cell damage which facilitate other diseases including cardiovascular, cancer and diabetes. Zou et al. 2016 reported that the antioxidant peptides possess higher level of hydrophobic amino acids than hydrophilic amino acids and contains 33.7 % of Glycine, Proline and Leucine, 18.7 % of Alanine, Tyrosine and Valine, 4.9 % of Methionine and Glutamine, 2 % of Cysteine and 40.7 % of other amino acids in its composition. Comparative study conducted by Nath et al. 2019 showed that papain-treated soybean milk peptide has higher antioxidant property than native soybean peptide. Similarly, Zhang et al. 2018 study shows the antioxidant peptides, valine-leucine-tyrosine-isoleucine-tryptophan (MW 673.1 Da) and serine-valine-proline-tyrosine-glutamate (MW 566.9 Da) were having potential antioxidant activity. Six peptides obtained from Pinto beans by Ngoh and Gan (2016) shown highest antioxidant activity.

Ribosome Inactivating proteins and peptides from plants

Ribosome-Inactivating Proteins (RIPs) are a category of proteins whose principal function is to impair ribosomes in an irreparable manner modifying rapidly through enzymatic pathways (Stirpe 2004). Considering their discovery in the last few decades, RIPs investigation and inculcation in therapeutics have garnered tremendous scientific attention. RIPs are present in bacteria and plants, yet many plant RIPs have been well-characterized and have been traced to their functions compared to bacterial RIPs (Walsh et al. 2013). By hydrolyzing a specific N-C glycosidic bond of the eukaryotic 28S rRNA (belonging to the large 60S ribosomal subunit), the integral N-glycosidase activity of RIPs liberates the adenine residue from the 3' end of its conserved GAGA tetraloop (sarcin/ricin loop), thereby impeding protein synthesis and irreversibly inactivating the ribosome (Walsh et al. 2013). RIPs have also been shown to exhibit RNase, DNase, polynucleotide adenosine glycosidase, superoxide dismutase activity (Park et al. 2006). RIPs have been classified into three subclasses, two of them being most prominently exploited for research purposes (Girish et al. 2006). The highly ubiquitous RIP-I is the most widely used RIP with a 26–35 kDa molecular weight. RIP-I launches itself into the cell by attaching to the LDL (Low-Density Lipoprotein) receptors (Walsh et al. 2013).

The example of Saporin (Type I RIP extracted from Saponaria officinalis) can be used to understand the mechanism of protein synthesis inhibition by RIP-I. Internalization of saporin takes place through endocytosis by binding to the member of the LDL receptor family, α2-macroglobulin/LPR1(low-density lipoprotein receptor-related protein1) existent in the host cell membrane (Vago et al. 2005). Saporin sets foot on cytoplasm through golgi independent pathway, thereby steering clear of low pH conditions of intracellular compartments. Once inside the cytoplasm, saporin inhibits protein synthesis by excising the adenine residue from the 3’ end of the particular site of the ribosome (Walsh et al. 2013). Another example of RIP-I, TCS (Trichosanthin-extracted from Trichosanthes kirilowii), associated with negatively charged phospholipid containing monolayer through electrostatic, hydrophobic interactions under acidic conditions (low pH), altering the charge of some residues, which is accompanied by salt-bridge breakage and charge to charge repulsion. This is followed by partial denaturation of TCS into a molten globular state, thus entering the host cell (Puri et al. 2012).

The process of protein synthesis inhibition is similar to that of any Type-I RIP. The RIP-II, is group of proteins is highly toxic. It is a heterodimeric carbohydrate-binding protein composed of 2 chains, A and B, held together by a disulfide bond. It has a molecular weight of 56–69 kDa, with each chain having a molecular weight of about 30 kDa (Girish et al. 2006). The A-chain exhibits vital N-glycosidase activity. The B-chain enables RIP-II to attach to the particular carbohydrate-containing cell receptors, as it has a strong affinity for carbohydrate moieties. This, in turn, leads to the migration of chain A across the cell membrane (Stirpe 2004). The entry process into cells for RIP-II is highly different from RIP-I because the latter lacks B-chain, which plays a vital role in its internalization process. Ricin (extracted from Ricinus communis) as almost all the Type-II RIPs are analogous to ricin, which has a well-identified for their mode of action (Puri et al. 2012). Binding to a particular receptor on the host cell membrane through the B-chain, ricin enters the cell either by clathrin-dependent or clathrin-independent endocytosis resulting in the origin of ricin containing endosomal vacuole (Puri et al. 2012). Eventually, ricin enters the trans-golgi network in COP-I vesicles. It is delivered to the early endosomes, either recycled by returning it to the cell surface or undergoes proteolytic degradation by the lysosome, finally reaching the E.R. lumen (Fujimura et al. 2004; Gustafson et al. 2000). The disulfide bond joining the two chains is degraded within the E.R. lumen, letting the remaining ricin transported by Endoplasmic Reticulum Associated Degradation (ERAD-Pathway for degradation of misfolded proteins) to the cytoplasm (Fujimura et al. 2004; Gustafson et al. 2000). Almost most of the toxin is degraded by 26s proteasome, leaving behind only a small portion that influences protein synthesis (Puri et al. 2012). Additionally, another class of RIP is not universal-Type-III RIPs. They show similar enzymatic activity to RIP-I as they have an identical N-terminal domain bound to the carboxyl domain with an unestablished function. Moreover, they are always synthesized in an inactive form (Girish et al. 2006). In the present scenario, in-depth research on RIPs has been encouraged due to their miscellaneous biological involvement in viral, HIV, and microbial infections (Pizzo and di Maro 2016).

RIPs have been coupled to specific antibodies to generate immunoconjugates in cancer and HIV therapy by targeting a specific cell due to their ability to hydrolyze N-glycosidase bond (Pizzo and di Maro 2016). Anti CD4-PAP is an immunoconjugate created by combining PAP with an antibody that targets HIV-infected CD4 T-cells and prevents HIV infection (Irvin and Uckun 1992). Another example is B43-PAP (anti-CD19 pokeweed antiviral protein), an immunotoxin made by combining B43 [an antibody-targeting CD19 antigen found on B-lineage acute lymphoblastic leukemia (ALL) cells] and PAP (Irvin and Uckun 1992). Alpha-momorcharin (0.12 nM), beta-momorcharin (0.11 nM), MAP30, balsamin, isomers of luffin (a—1.64 ng/ml and b—0.84 ng/ml), ricin (814 pM), abrin (500 pM), and other plant RIPs with extremely low IC50 values have been isolated. Cell-Free Protein Synthesis (CFPS-growing in vitro) has been demonstrated to be inhibited by these RIPs (Puri et al. 2012). Despite having many RIPs, only a minority have been fully identified. Therefore, the main challenge arises in exploring and identifying some potent plant RIPs with high therapeutic efficiency and less toxicity. The available ribosome-inactivating peptides are listed in Table 5.

Table 5.

List of ribosome-inactivating proteins from plants

S. No Plant and its part Protein Nature M. Wt. (kDa) Class of RIP Mode of action *IC50 References
1 Momordica balsamina (Seeds) Balsamin Protein 28 RIP-I 28S rRNA depurination with the liberation of RNA fragment of about 400 nucleotides 90.6 ng/ml Kaur et al. (2012)
2 Cucurbita foetidissima (Root) Foetidissimin Protein 63 RIP-II 28S rRNA depurination with the liberation of RNA fragment of about 550 nucleotides 25.9 nM Zhang and Halaweish (2003)
Foetidissimin II 61 28S rRNA depurination with the liberation of RNA fragment of about 450 nucleotides 0.251 µM Zhang and Halaweish (2007)
3 Cucurbita texana Texanin (Fruit) Protein 29.7 RIP-I 28S rRNA depurination NA Zhang and Halaweish (2007)
ME2 (Roots) Protein 27.5 NA Vivanco et al. (1999)
4 Abrus precatorius (Seeds) AGG Heterodimeric lectin 134 RIP-II 28S rRNA depurination 0.469 µg/ml Bhutia et al. (2016)
Abrin Homotetrameric protein 260 500 pM Ferreras et al. (2011)
5 Viscum album L. (Green Parts) Viscum Heterodimeric protein 60 RIP-II 28S rRNA depurination NA Olsnes et al. (1982)
6 Amaranthus viridis L. (Leaves) Amaranthin Protein 30 RIP-I 28S rRNA depurination 25 pM Kwon et al. (1997)
7 Beta vulgaris L. (Leaves) Beetin-27 Protein 27.59 RIP I 28S rRNA depurination 1.15 ng/ml Iglesias et al. (2005)
8 Citrullus colocynthis (L.) Schrad (Seeds) Colocin 1 Protein 26.3 RIP-I 28S rRNA depurination 0.04 nM Bolognesi et al. (1990)
Colocin 2 0.13 nM
9 Marah oreganus (Seeds) MOR-I Protein 27.98 RIP-I 28S rRNA depurination 0.063 nM Remi Shih et al. (1998)
MOR-II 27.63 0.071 nM
10 Momordica charantia L. (Seeds) MCL Heterotetrameric lectin 115 RIP-II 28S rRNA depurination 5 µg/ml Puri et al. (2012)
α-momorcharin Protein 28 RIP-I 0.12 nM
β-momorcharin 29 RIP-I 0.11 nM
MAP30 30 RIP-I 3.3 nM
γ-momorcharin 11.5 sRIP-I 55 nM
δ-momorcharin 30 RIP-I 0.15 nM
11 Trichosanthes kirilowii Maxim TCS(GLQ223) (Seeds) Protein 26 RIP-I 28S rRNA depurination 0.36 ng/ml (3.7 nM) Lee-Huang et al. (1991a); Schrot et al. (2015)
TAP 29 (Root tubers) 29 RIP-I 3.7 nM Lee-Huang et al. (1991a)
Trichosanthrip (Seeds) 10.96 sRIP-I 1.6 ng/ml Shu et al. (2009)
α-kirilowin (Seeds) 28.8 RIP-I NA 1.2–1.8 ng/ml Wong et al. (1996)
β-kirilowin (Seeds) 27.5 RIP-I 1.8 ng/ml
12 Basella rubra L. (Seeds) Basella RIP 2a protein fraction Protein 30.6 RIP-I NA 1.70 ng/ml Bolognesi et al. (1997)
Basella RIP 2b protein fraction 31.2 1.70 ng/ml
Basella RIP 3 31.2 1.66 ng/ml
13 Saponaria ocymoides L. (Seeds) Ocymoidin Protein 30.2 RIP-I 28S rRNA depurination 46 pM; 4.8 ng/ml Bolognesi et al. (1995), di Massimo et al. (1997)
14 Secale cereale (Seeds) RPSI (Seeds) Protein 30.1 RIP-I NA 0.42 µg/ml Minami et al. (1998)
15 Phytolacca americana L PAP (Leaves) Protein 29–30 RIP-I 28S rRNA depurination 0.29 nM Irvin and Uckun (1992), Poyet and Hoeveler (1997)
PAP-I (Leaves) 29 3 ± 0.2 pM Rajamohan et al. (1999)
PAP-II (Leaves) 30 4 ± 0.2 pM
PAP-III (Leaves) 30 3 ± 0.2 pM
PAP-S (Seeds) 30 36–83 nM; 1.09 -2.5 ng/ml Barbieri et al. (1982)
PAP-R (Roots) 25.0 0.05 nM Stirpe et al. (1986)
16 Trichosanthes lepiniate (Root tuber) Trichomaglin Protein 24.6 RIP-I 28S rRNA depurination 10.1 nM Chen et al. (1999)
17 Iris hollandica var. Professor Blaauw (Bulbs) IrisRIP Protein 28 RIP-I 28S rRNA depurination 0.1–0.16 nM Desmyter et al. (2003)
IrisRIP.A1 29 0.16 nM van Damme et al. (1997)
IrisRIP.A2 29 0.12 nM
IrisRIP.A3 29 0.10 nM
18 Viscum album L. (Leaves) ML-I Heterodimeric lectin 115 RIP-II NA 2.6 µg/mL Stirpe et al. (1980)
19 Momordica grosvenorii (Seeds) Momorgrosvin Glycoprotein 27.7 RIP-I NA 0.3 nM Tsang and Ng (2001)
20 Pisum sativum var. arvense Poir (Seeds) α pisavins Protein 20.5 RIP-I NA 0.5 nM Lam et al. (1998)
β pisavins 18.7
21 Vaccaria pyramidata (Seeds) Pyramidatine Protein 28.0 RIP-I 28S rRNA depurination 3.6 ng/ml di Massimo et al. (1997)
22 Cinnamomum porrectum (Seeds) Porrectin Glycoproteins 64.5 RIP-II 28S rRNA depurination 0.11 µM Li et al. (1996)
23 Cicer arietinum (Seeds) CLAP Protein 18 NA 20 µM Ye and Ng (2002a)
24 Phaseolus mungo (Seeds) Mungin Protein 18 NA 24 µM Ye and Ng (2000)
25 Adzuckia angularia (Seeds) Fraction AB2 Peptide 5 NA 11 µM Ye and Ng (2001)
26 Phaseolus vulgaris (Seeds) Fraction PTA2c Peptide 5 NA 9 µM Ye and Ng (2001)
27 Dianthus caryophyllus (Leaves) DAPs 30 Protein 30 RIP-I 28S rRNAdepurination 3.4 nM Lee-Huang et al. (1991b)
DAPs 32 32 2.3 nM
28 Gelonium multiflorum (Seeds) GAP 31 Protein 31 RIP-I 28S rRNAdepurination 4.1 nM Lee-Huang et al. (1991b)
29 Asparagus officinali (Seeds) Asparin 1 Protein 30.5 RIP-I NA 0.27 nM Bolognesi et al. (1990)
Asparin 2 29.8 0.15 nM
30 Luffa cylindriaRoem (Seeds) Luffin Protein 26 RIP-I NA 0.42 ng/ml Kishida et al. (1983)
Luffin a 28 1.64 ng/ml Ng et al. (1992b); Schrot et al. (2015)
Luffin b 29 0.84 ng/ml
31 Lychnis chalcedonica (Seeds) Lychnin Protein 26.6 RIP-I NA 0.17 nM Bolognesi et al. (1990)
32 Manihot palmata (Seeds) Mapalmin Protein 32.3 RIP-I NA 0.05 nM Bolognesi et al. (1990)
33 Bryonia dioica Bryodin-L (Leaves) Protein 28.8 RIP-I NA 0.09 nM Bolognesi et al. (1990)
Bryodin (Roots) 30 0.12 nM Stirpe et al. (1986)
34 Ricinus communis.L (Seeds) Ricin D = Ricin Glycoprotein 62.8 RIP-II 28S rRNAdepurination 5.5 ng/ml; 814 pM Battelli et al. (1997), Endo and Tsurugi (1987), Schrot et al. (2015), Wei and Koh (1978)
Ricin E 64 NA Schrot et al. (2015)
RCA 118–130 NA
35 Ricinus commnis.L USA (Seeds) Ricin 1 Glycoprotein 66 RIP-II 28S rRNAdepurination NA
Ricin 2
Ricin 3
36 Ricinus communis, India (Seeds) Ricin I Glycoprotein 64 RIP-II 28S rRNAdepurination NA
Ricin II
Ricin III
37 Trichosanthes cucumeroides (Ser.) Maxim (Root tubers) β-TCS Protein 28 RIP-I 28S rRNAdepurination 2.8 ng/ml; 0.1 nM Ng et al. (1992a); No et al. (1991); Yeung and Li (1987)
38 Saponaria officinalis L Saporin-L1 (Leaves) Protein 31.6 RIP-I 28S rRNAdepurination 0.25 nM Ferreras et al. (1993)
Saporin-L2 (Leaves) 31.6 0.54 nM
Saporin-R1 (Roots) 30.2 0.86 nM
Saporin-R2 (Roots) 30.9 0.47 nM
Saporin-R3 (Roots) 30.9 0.48 nM
Saporin-S5 (Seeds) 30.9 0.05 nM
Saporin-S6 (Seeds) 31.6 0.06 nM
39 Phaseolus vulgaris (Seeds) Vulgarinin Peptide 7 NA 13 µM Wong and Ng (2005c)
40 Adenia digitata (Roots) Modeccin Protein 57–63 RIP-II 28S rRNAdepurination 4 µg/ml Olsnes et al. (1978); Schrot et al. (2015)
Modeccin 6B 57 0.31 µg/ml Barbieri et al. (1980)
41 Panax ginseng (Roots) Panaxagin Homodimeric protein 53 NA 0.28 nM Ng and Wang (2001)
42 Allium tuberosum (Shoot) Fraction MS3 Protein 36 NA 850 nM Lam et al. (2000)

*IC50 Concentration causing 50% inhibition, ND  Not determined, NA  Not available, CAP30 Chenopodium album antiviral RIP, RPSI  Rye protein synthesis inhibitor, PAP  Pokeweed antiviral protein, IrisRIP = IRIP Type-1 ribosome-inactivating protein from iris bulbs, CLAP  Chickpea cyclophilin-like antifungal protein, Fraction AB2 Red bean antifungal peptide, Fraction PTA2c  Pinto bean antifungal peptide, DAPs 30  Dianthus anti-HIV proteins, GAP 31 Gelonium anti-HIV protein, RCA  Ricinus communis agglutinin, TAP 29 Trichosanthes anti-HIV protein, β-TCS  β-trichosanthin

Anti-carcinogenic activity of plant peptides/proteins

One of the causes of death in recent times is the various types of cancer. Cancer caused due to genetic effects is 5-10%, but almost 90-95 % of the cancers are caused due to the environment and lifestyle changes. Bioactive plant peptides can be used to cure cancer. Plant peptides prevent the proliferation of cancerous cells and cause their death-apoptosis (Hernandez-Ledesma and Hsieh 2017). A study conducted by Kannan et al. (2010) on Oryza sativa—heat stabilized defatted rice bran showed that when treated with alcalase (protease), peptide hydrolysates were produced, which are less than 5 kDa. This peptide hydrolysate was subjected to ion-exchange chromatography followed by an MTS assay. The peptide at 1000 μg/ml could show the highest inhibition for the colon and liver cancer cells for up to 84 %. This study further analysed for the amino acid composition from the peptide, and it was found that the peptide contains arginine, proline, and glutamic acid. The peptide chain was found to be glutamate-glutamine-arginine-proline-arginine, a short pentapeptide sequence. The peptide showed anti-proliferative effects on cancer cells. A peptide that prevents cancer is found in Glycine max (soybean), Triticum aestivum (wheat), Hordeum vulgare (barley) is called lunasin. Lunasin is an effective anticancer agent consisting of 43 amino acids. It has a presence of 8 aspartate residues in the C terminal; they are responsible for opposing mitosis, they play a role in the attachment of lunasin to chromatin. The amino acids arginine-glycine-aspartate are called cell adhesion motif they internalize lunasin into the cell's nucleus. The amino acids 23–31 target the lunasin to H3–H4 histones in DNA.

In vivo mouse models were used to check the effects of lunasin on cancer cells. Lunasin was also found in Amaranthus hypochondriacs. Lunasin obtained from soybean could be taken orally as it is resistant to enzymes present in our body like pepsin and pancreatin. This property of lunasin makes it an ideal plant peptide that could cure the cancer. The amount of lunasin found was 4.4–70.5 mg lunasin/g of protein in Glycine max, the highest among the other plants like wheat and barley (Hernandez-Ledesma et al. 2009). A study was conducted by Ma et al. (2015) on Juglans regia L (walnut). The walnut protein was treated with different proteases, followed by purification steps to obtain the pure peptide. The peptide was further subjected to its anti-cancer activity on cells. The walnut protein hydrolyzed with papain exhibited inhibitory actions on the MCF-7 cell line (human breast cancer cell line). The peptide was found to be cysteine-threonine-leucine-glutamate-tryptophan. This peptide CTLEW induces the process of apoptosis and autophagy. The reported anti-carcinogenic proteins are listed in Table 6.

Table 6.

List of anti-carcinogenic peptides/proteins from plants

S. No Plant and its part Protein Nature Sequence Mode of action M. Wt. (kDa) *IC50 References
1 Acacia confuse (Seeds) Acaconin Protein 32 128 ± 9 µM Lam and Ng (2010)
2 Clausena lansium (Lour) (Seeds) CLTI Homodimeric protein DPLLDFPGNEVEASRAYYVVSVIRGAG Prevents the growth of human hepatoma cells and leukemia cells 54 100 µM Ng et al. (2003)
NA
3 Momordica charantia (Seeds) BG-4 Peptide RDSDCLAQCICVDGHCG Apoptosis of human colon cancer cells 4 134.4 µg/ml Dia and Krishnan (2016)
217.0 µg/ml
MAP 30 RIP-I Apoptosis in liver cancer cells 30 28.6 µM Fang et al. (2012b)
MCL Lectin (RIP-II) - Antitumor activity toward human nasopharyngeal carcinoma cells 115 6.9 µM Fang et al. (2012a)
7.4 µM
α-MMC RIP-I 28 NA Fan et al. (2015)
4 Castanopsis chinensis (Seeds) CCL Homotetrameric lectin NFEETILGSK Prevents growth of HepG2 cells 120 NA Wong et al. (2008)
5 Phaseolus lunatus (Seeds) Lunatusin Peptide KTCENLADTFRGPCFATSNC Inhibits growth of MCF-7, breast cancer cell line 7 5.71 µM Wong and Ng (2005a)
6 Vigna sesquipedalis (Seeds) Sesquin Peptide KTCENLADTY Anti tumour activity 7 NA Wong and Ng (2005b)
7 Phaseolus coccineus cv. ‘Major’ (Seeds) Coccinin Peptide KQTENLADTY Prevents proliferation in leukemia cell lines 7 30 µM Ngai and Ng (2004)
40 µM
8 Arachis hypogaea (Seeds) Hypotin Protein CDVGSVISASLFEALQKHRN Anti-proliferative activity 30.4 296 µg/ml Wang et al. (2007)
9 Cicer arietinum (Seeds) C-25 Lectin TKTGYINAAF Anti-proliferative activity 25 37.5 µg/ml Kumar et al. (2014)
10 Corydalis cava (Tubers) Fraction 18 Protein Prevents the growth of human carcinoma cells 30 NA Nawrot et al. (2010)
11 Arisaema tortuosum Schott (Tubers) ATL Homotetrameric lectin 54 NA Dhuna et al. (2005)
12 Phaseolus vulgaris cv. Blue tiger king (Seeds) BTKL Dimeric lectin 60 35.2 ± 2.7 µM Fang et al. (2011b)
347.9 ± 24.5 µM
494.6 ± 70.4 µM
13 Canavalia ensiformis (Seeds) Con A Homotetraeric lectin Anti-hepatoma effect 104 5 µg/ml Lei and Chang (2009), Liu et al. (2009)
10 µg/ml
10 µg/ml
20 µg/ml
NA
14 Withania somnifera (Fruit) Asparginase Homodimeric protein Anti-tumour activity 72 ± 0.5 1.45 ± 0.05 IU/ml Oza et al. (2010)
15 Glycine max (Seeds) BBI Peptide Colorectal chemopreventive agents 8 32 to 73 µM Clemente and del Carmen Arques (2014); Kennedy (1998)
NA
NA
IBB1 Protein DDESSKPCCDQCACTKSNPPQCRCSDMRLNSCHSACKSCICALSYPAQCFCVDITDFCYEPCKPSEDDKEN Colorectal chemopreventive agents 10–12 39.9 ± 2.3 µM Clemente et al. (2010)
IBBD2 Protein SDQSSSYDDDEYSKPCCDLCMCTRSMPPQCSCEDIRLNSCHSDCKSCMCTRSQPGQCRCLDTNDFCYKPCKSRDD colorectal chemopreventive agents 10–12 48.3 ± 3.5 µM
16 Abrus precatorius (Seeds) Abrin Homotetrameric protein 260 3.70 pM Lin et al. (1971); Olsnes and Pihl (1973)
AGG Heterodimeric glycoprotein 134 NA Bhutia et al. (2016); Mukhopadhyay et al. (2014)
17 Trichosanthes kirilowii (Root Tuber) TCS Protein 26–27 31.6 µM Fang et al. (2012c)
20.5 µM
130 µM
28.6 µM
NA Tsao et al. (1986)
18 Gynura procumbens (Lour.) Merr. (Leaves) SN-F11/12 Mixture of proteins 25 3.8 µg/ml Hew et al. (2013)
19 Allium sativum (Bulbs) Alliumin Protein 13 8.33 µM Xia and Ng (2005)
20 Cucurbita foetidissima (Roots) Foetidissimin II Proteins 61 70 nM Zhang and Halaweish (2007)
70 nM
21 Viola arvensis (Whole plant) Varv A Macrocyclic peptides 2.87 3.56 µM Lindholm et al. (2002)
1.34 µM
4.88 µM
11.03 µM
3.24 µM
3.19 µM
6.35 µM
Varv F Macrocyclic peptides 2.95 7.13 µM
7.49 µM
7.07 µM
5.90 µM
6.31 µM
NA
22 Viola odorata (Whole plant) Cycloviolacin O2 Macrocyclic peptides 3.14 0.11 µM Lindholm et al. (2002)
0.12 µM
0.26 µM
0.12 µM
0.12 µM
0.10 µM
1.32 µM
23 Viola biflora (Aerial parts) Vibi D Macrocyclic peptides 2.9  > 30 µM Herrmann et al. (2008)
Vibi E 3.08 3.2 µM
Vibi G 3.2 0.96 µM
Vibi H 3.27 1.6 µM
24 Viola philippica (Whole plant) Viphi A Macrocyclic peptides 3.17 4.91 ± 0.04 µM He et al. (2011)
15.5 ± 0.06 µM
1.75 ± 0.05 µM
Viphi B 2.98 NA
Viphi C 3.05 NA
Viphi D 3.08 2.51 ± 0.03 µM
5.24 ± 0.40 µM
NA
Viphi E 3.15 2.51 ± 0.03 µM
5.24 ± 0.40 µM
NA
Viphi F 3.14 1.03 ± 0.03 µM
6.35 ± 0.31 µM
2.91 ± 0.06 µM
Viphi G 3.17 1.03 ± 0.03 µM
6.35 ± 0.31 µM
2.91 ± 0.06 µM
Viphi H 3.09 NA
Viba 15 2.86 1.32 ± 0.15 µM
10.2 ± 0.43 µM
3.10 ± 0.06 µM
Viba17 2.84 1.32 ± 0.15 µM
10.2 ± 0.43 µM
3.10 ± 0.06 µM
Varv A 2.87 1.32 ± 0.15 µM
10.2 ± 0.43 µM
3.10 ± 0.06 µM
Kalata B1 2.89 1.32 ± 0.15 µM
10.2 ± 0.43 µM
3.10 ± 0.06 µM
25 Viola labridorica (Whole Plant) Vila A Macrocyclic peptides 3.16 7.08 µg/ml Tang et al. (2010a)
5.13 µg/ml
 > 10 µg/ml
5.08 µg/ml
5.80 µg/ml
 > 10 µg/ml
Vila B Macrocyclic peptides 3.16 34.65 µg/ml
8.25 µg/ml
 > 10 µg/ml
6.34 µg/ml
6.25 µg/ml
 > 10 µg/ml
Vila D Macrocyclic peptides 2.94 49.59 µg/ml
 > 10 µg/ml
 > 10 µg/ml
 > 10 µg/ml
 > 10 µg/ml
 > 10 µg/ml
Varv D Macrocyclic peptides 2.87 46.62 µg/ml
 > 10 µg/ml
 > 10 µg/ml
 > 10 µg/ml
 > 10 µg/ml
 > 10 µg/ml
26 Psychotria leptothyrsa (Whole Plant) Psyle A Macrocyclic peptides 2.91 26 µM Gerlach et al. (2010)
NA
Psyle B Macrocyclic peptides .01 NA
Psyle C Linear cyclotide 2.84 3.5 µM
NA
Psyle D Macrocyclic peptides 3.25 NA
Psyle E Macrocyclic peptides 3.25 0.76 µM
NA
Psyle F Macrocyclic peptides 3.21 NA
27 Viola abyssinica (Whole Plant) Vaby A Macrocyclic peptides 2.86 7.6 µM Yeshak et al. (2011)
Vaby D Macrocyclic peptides 3.06 2.8 µM
28 Viola tricolor (Whole Plant) Varv A Macrocyclic peptides 2.87 3 µM Tang et al. (2010b)
6 µM
37.18 µg/ml
 > 10 µg/ml
 > 10 µg/ml
 > 10 µg/ml
 > 10 µg/ml
Varv D Macrocyclic peptides 2.87 NA
46.62 µg/ml
 > 10 µg/ml
 > 10 µg/ml
 > 10 µg/ml
 > 10 µg/ml
Varv E Macrocyclic peptides 2.99 4 µM
4 µM
38.84 µg/ml
 > 10 µg/ml
 > 10 µg/ml
 > 10 µg/ml
 > 10 µg/ml
Varv F Macrocyclic peptides 2.95 6 µM
7 µM
44.49 µg/ml
 > 10 µg/ml
 > 10 µg/ml
 > 10 µg/ml
 > 10 µg/ml
Varv H Macrocyclic peptides 3.05 NA
44.70 µg/ml
 > 10 µg/ml
 > 10 µg/ml
 > 10 µg/ml
 > 10 µg/ml
Varv He Macrocyclic peptides 3.08 NA
55.43 µg/ml
 > 10 µg/ml
 > 10 µg/ml
 > 10 µg/ml
 > 10 µg/ml
Varv Hm Macrocyclic peptides 3.06 NA
74.39 µg/ml
 > 10 µg/ml
 > 10 µg/ml
 > 10 µg/ml
 > 10 µg/ml
Vitri A Macrocyclic peptides 3.15 3.90 µg/ml
4.94 µg/ml
3.07 µg/ml
3.69 µg/ml
NA
6.03 µg/ml
NA
Vitri B Macrocyclic peptides 2.87  > 10 µg/ml
 > 10 µg/ml
 > 10 µg/ml
 > 10 µg/ml
NA
45.21 µg/ml
NA
Vitri C Macrocyclic peptides 2.96  > 10 µg/ml
 > 10 µg/ml
 > 10 µg/ml
 > 10 µg/ml
NA
46.96 µg/ml
NA
Vitri D Macrocyclic peptides 3.04  > 10 µg/ml
 > 10 µg/ml
 > 10 µg/ml
 > 10 µg/ml
NA
51.65 µg/ml
NA
Vitri E Macrocyclic peptides 2.92  > 10 µg/ml
 > 10 µg/ml
 > 10 µg/ml
 > 10 µg/ml
NA
54.39 µg/ml
NA
Vitri F Macrocyclic peptides 3.21 3.58 µg/ml
5.36 µg/ml
3.44 µg/ml
2.74 µg/ml
6.31 µg/ml
29 Vicia faba cv. Giza 843 (Seeds) VFTI-G1 Protein 15 30 µM Fang et al. (2011a)
30 Asparagus officinalis (Seeds) Asparin 1 Protein 29.7  > 3.33 µM Bolognesi et al. (1990)
0.61 µM
0.18 µM
 > 3.33 µM
NA
Asparin 2 Protein 28.1  > 3.33 µM
0.21 µM
0.18 µM
 > 3.33 µM
NA
31 Citrullus colocynthis (Seeds) Colocin 1 Glycoprotein 20.4  > 3.33 µM
0.54 µM
0.01 µM
0.32 µM
0.23 µM
Colocin 2 Glycoprotein 19.5 1.41 µM
0.25 µM
0.004 µM
0.14 µM
0.10 µM
32 Lychnis chalcedonica (Seeds) Lychnin Glycoprotein 20.0  > 3.33 µM
2.11 µM
0.03 µM
1.53 µM
0.33 µM
33 Manihot palmata (Seeds) Mapalmin Glycoprotein 26.9  > 3.33 µM
1.68 µM
0.03 µM
1.64 µM
0.08 µM
34 Bryonia dioica Bryodin-L (Leaves) Glycoprotein 27.3  > 3.33 µM
0.77 µM
0.05 µM
 > 3.33 µM
NA
Bryodin (Roots) Glycoprotein 30 0.86 µΜ Stirpe et al. (1986)
0.90 µΜ
0.15 µΜ
2.24 µΜ
1.01 µΜ
35 Bauhinia variegate var. variegate (Seeds) BvvL Homodimeric lectin 64 12.8 µΜ Chan and Ng (2015)
Bauhinia variegate (Seeds) BG2 Homodimeric lectin 1.4 µM Lin and Ng (2008)
0.18 µM
36 Dioclea lasiocarpa (Seeds) DlasiL Homotetrameric lectin 52 ± 2 nM Gondim et al. (2017)
224 ± 10 nM
275 ± 4 nM
167 ± 1 nM
37 Lens culinaris (Seeds) Bowman-Birk Isoinhibitor Peptide 7.5 32 ± 2 µM Caccialupi et al. (2010)
38 Pisum Sativum (Seeds) TI1B Peptide 7.9 31 µΜ Clemente et al. (2012)
39 Canavalia brasiliensis (Seeds) ConBr Lectin 30 108 ± 14 nM Grangeiro et al. (1997)
95 ± 14 nM
1146 ± 24 nM
529 ± 8 nM
40 Canavalia maritima (Seeds) ConM Tetrameric lectin 102 67 ± 2 nM Delatorre et al. (2006)
62 ± 4 nM
1382 ± 17 nM
176 ± 2 nM
41 Dioclea sclerocarpa (Seeds) DsclerL Lectin Anti-cancer 50.8 64 ± 4 nM Gondim et al. (2017)
102 ± 8 nM
1250 ± 9 nM
264 ± 1 nM
42 Aspidistra elatior Blume (Rhizomes) AEL Heterotetramer lectin 56 NA Xu et al. (2007)
43 Soybean (Cotyledon) Lunasin Peptide MTKFTILLIS LLFCIAHTCS 5.5 181 µM Hernandez-Ledesma et al. (2013)
14 µM
62 µM
44 Saponaria officinalis L Saporin-L1 (Leaves) Protein MKSWIMLVVT WLIILQTTVT 31.6  > 3300 nM Ferreras et al. (1993)
120 nM
13 nM
Saporin-L2 (Leaves) Protein 31.6  > 3300 nM
160 nM
25 nM
Saporin-R1 (Roots) Protein 30.2 340 nM
490 nM
76 nM
Saporin-R2 (Roots) Protein 30.9 170 nM
230 nM
33 nM
Saporin-R3 (Roots) Protein 30.9 3200 nM
84 nM
34 nM
Saporin-S5 (Seeds) Protein 30.9 420 nM
7 nM
2 nM
Saporin-S6 (Seeds) Protein 31.6 310 nM
18 nM
6 nM
45 Ricinus communis (Seeds) Ricin Protein 64 34.1 ng/ml Trung et al. (2016)
46 Basella rubra L. (Seeds) Basella RIP 2 Mixture of two proteins 30.6–31.2 63.7 ± 15.6 nM Bolognesi et al. (1997)
166 ± 24 nM
16.6 ± 3.7 nM
169 ± 87 nM
353 ± 5.7 nM
Basella RIP 3 Protein 31.2 43.8 ± 9.2 nM
315 ± 25 nM
9.3 ± 0 nM
110 ± 75 nM
700 ± 369 nM
47 Vaccaria pyramidata (Seeds) Pyramidatine Protein 28.0 6.3 nM Bolognesi et al. (1995)
179 nM
142 nM
5.7 nM
4.3 nM
48 Saponaria ocymoides L. (Seeds) Ocymoidin Protein 30.2 11.7 nM
493 nM
 > 3330 nM
9.3 nM
8.7 nM
49 Viscum album L. var. coloratum (Arial parts) VCA Heterodimeric lectin Anti-tumour 60 125 ng/ml Han et al. (2015)
125 ng/ml
50 Viscum album L. (N.A.) ML-I Heterodimeric lectin 115 NA Franz et al. (1981)
7 ng/ml
ML-II Heterodimeric lectin 60 NA
ML-III Heterodimeric lectin 50 NA
51 Dianthus superbusvar longicalycinus (Whole Plant) Longicalycinin A Cyclic peptide Cyclo(Gly1–Phe2–Tyr3–Pro4–Phe5– Cytotoxic to HepG2 cancer cell line 0.611 13.52 µg/ml Hsieh et al. (2005)
52 Phaseolus vulgaris (Seeds) Vulgarinin Peptide K T CENLADTYKGP CFTS G GD Inhibition of proliferation in leukemia cell lines 7 NA Wong and Ng (2005c)
53 Brassica juncea var. Integrifolia (Seeds) Juncin Protein 18.9 5.6 µM Kwon et al. (1997)
6.4 µM
54 Peganum harmala(Seeds) PHP Homodimeric protein ITCPQVTQSLAPCVPYLISG Anti-proliferative activity against cancer cells 18 0.7 µM Ma et al. (2013)
2.74 µM
3.13 µM
1.47 µM
55 Allium tuberosum (Shoot) Fraction MS3 Protein 36 NA Lam et al. (2000)
56 Zingiber officinalis (Rhizome) G-24 Protein Inhibition of human oral cancer cell line 24 NA Gill et al. (2012)

*IC50  Concentration causing 50% inhibition, ND  Not determined, NA  Not available, CLTI  Clausena lansium trypsin inhibitor, VFTI-G1  Bowman birk type trypsin inhibitor, BG-4  Bitter gourd-4, MAP 30  Momordica anti-human immunodeficiency virus protein, MCL  Momordica charantia lectin, α-MMC  α-Momorcharin, CCL  Castanopsis chinensis lectin, ATL Arisaema tortuosum lectin, BTKL Blue Tiger King Lectin, Con A  Concanavalin A, BBI  Bowmans birk inhibitor, IBB1 and IBB2  Bowmans birk isoinhibitors, TCS  Trichosanthin or Tin Hua Fen or GLQ223, BvvL Bauhinia variegate var variegata lectin, DlasiL Dioclea lasiocarpa lectin, TI1B Bowman birk isoinhibitor, ConBr Canavalia brasiliensis Lectin, ConM Canavalia maritime lectin, DsclerL Dioclea sclerocarpa lectin, VCA  Viscum album L. var coloratum agglutinin, ML-I,II,III Mistletoe lectin-I,II,III, PHP  Peganum harmala protein, AEL Aspidistra elatior Blume lectin, AGG  Abrus agglutinin

Plant Peptides for Drug Design

Rational drug design is the process of designing drug molecules that bind to a target. Cyclotides are a new type of microproteins with a unique topology that includes a head-to-tail cyclized backbone structure that is further stabilised by three disulfide bonds that form a cystine knot. They are disulphide rich peptides and their basic function is plant defence. When compared to linear peptides of equal size, they have a unique molecular architecture that renders them extremely resistant to physical, chemical, and biological destruction. Apart from the conserved regions composing the cystine knot, the cyclotides are orally accessible and able to traverse cellular membranes to alter intracellular protein–protein interactions (PPIs) in vitro and in vivo. They are ideal scaffolds for numerous biotechnological applications, including drug development, because to their unique characteristics (Camarero and Campbell 2019). It does not involve trial and error like traditional drug design. The cyclotide sequences are updated on Cybase regularly. The example, plant cyclotide used is Kalakata B1, the peptide sequence is converted to cyclotide scaffold because of the cysteine knot. Grafting of sequences from myelin oligodendrocyte glycoprotein (MOG) into kalakata B1 has been used to design drugs for multiple sclerosis (Craik and Du 2017). By applying molecular grafting of bioactive epitopes or even molecular evolution methods, it is possible to create cyclotides with unique biological properties. Cyclotides which can target a wide range of protein targets have been developed and evaluated using these methods, largely in vitro but also in animal models. Despite the early success of using the cyclotide scaffold to target specific proteins and modify their biological activity, no cyclotides have yet been tested in humans. Potential immunogenicity and oral bioavailability are two obstacles that bioactive cyclotides must overcome before entering the clinic. More research into the biopharmaceutical properties of these fascinating new micro-proteins is expected to be released soon (Camarero and Campbell 2019).

Conclusion

Finally, this review encapsulates the therapeutic plant peptides and their prospective applications. They can serve as future treatments that are both unique and effective. Although many plant peptides have been explored for therapeutic applications, only a handful have progressed to the next stages. Usually, drug development constitutes in vitro examinations, in vivo corroboration and clinical trial review. Regrettably, almost all the research involving protein therapies reaches a dead-end in vitro, with only a handful of them being marketed as medicine. Various strategies have been applied to overcome such disadvantages (low bioavailability, high toxicity). One such strategy is bioconjugation and it has improved target selectivity, lower toxicity, and enhanced retention time with a regulated release in the target tissue. As these intricate component systems become more ubiquitous, research into bioconjugate treatments should become more focused due to their peculiarity in contrast to single-molecule drug organization. New formulation strategies have to be developed to design new drug candidates and bring out the peptide's full potential. To summarise, substantial research into medicinal plant proteome could identify novel plant-based peptide drugs. Many therapies involving proteins could be discovered due to research in this approach. Plant-derived peptide therapeutics is still the primary source of bioactive compounds worldwide.

Acknowledgements

We thank SRM Institute of Science and Technology and Department of Biotechnology, School of Bioengineering for their constant support.

Author Contributions

SM, SBB and VV collected the literature data and prepared Tables. SR and DP prepared the figure. SM, SBB, VV, DP, SR, PR and PV wrote the manuscript. PR, PR and PV revised the manuscript. All authors reviewed the final version of the manuscript for submission.

Funding

This work was supported by a grant received from SRM Institute of Science and Technology under Faculty Selective Excellence Program - 2021. 

Declarations

Conflict of interest

The authors have no conflicts of interest to declare that are relevant to the content of this article.

Footnotes

Publisher's Note

Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

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