Table 2.

Molecular approaches towards enhanced enzymatic properties of cellulases

Microorganism	Gene	Characteristics of recombinant cellulase	References
Batocera horsfieldi	Bh-EGase I	SA: 1030.87 IU/mg, 40 °C and pH 4.0	Mei et al. (2016); Zhong et al. (2012)
Corbicula japonica	Glycoside hydrolase family (GHF) 9 β-1,4-glucanase gene, CjCel9A	GHF45 (CjCel45A, CjCel45B). Both genes encode ORF of 627 bp corresponding to 208 amino acids Key role as cellulose decomposer in estuarine environments	Sakamoto and Toyohara (2009)
Bacillus subtilis	EG gene	Introduced those benefits of commercial bioconversion of complex lignocellulosic materials to biofuels	Ghadiri et al. (2020)
Streptomyces sp., Thermobispora bispora, Cellulomonas fimi, Streptomyces halstedii, Clostridium thermocellum		Comparative sequence alignment, molecular modeling, and docking analyses were performed taking 5 microbial (3 bacteria and 2 fungi) cellulase enzymes Cellulose-binding site in endoglucanase E of Clostridium thermocellum Ser131, Met263, Gln298, and His310 which are reported to have differed with Ala119, Ala255, Ser298, and Cys304 in Streptococcus sp. Molecular modeling and docking studies revealed that different microbial cellulase enzymes have the potential towards the use of cellulose as a substrate for the high yield of bioethanol	Paul et al. (2020)
Talaromyces emersonii	Cellulase gene (cbh2)	cbh2 has an open reading frame of 1377 bp, which encodes a putative polypeptide of 459 amino acids Expression of the T. emersonii cbh2 gene induced by cellulose, xylan, xylose, gentiobiose, and repressed by glucose	Murray et al. (2003)
Fosmid library of Buffalo rumen metagenomic DNA	Acid-cellulase gene (Cel-1)	ORF encoding cellulase consisted of 1176-bp, corresponding to a protein of 391 amino acid and has catalytic domain belonging to glycosyl hydrolase family 5	Dadheech et al. (2018)
Trichoderma reesei Rut-C30	egl1 at the ace1 locus	Cellulase production by T. reesei QS305 performed in the 5-L fermentor, cellulases activity of 10.7 FPU/mL achieved at 108 h, 75.4% higher than that produced by T. reesei Rut-C30	Meng et al. (2018)
Aspergillus niger		PCR product was 1311 bp, xylanase activity of the reconstructed A. niger was 3.28 U/mL. The activity of the exoglucanase from the constructed strain was increased to 1.95 U/mL from 0.21 U/mL of the original A. niger. Reconstructed A. niger cellulase could hydrolyze wheat straw to release 19.7 g/L of glucose, compared to 12.01 g/L of that released by the cellulase from the original A. niger	Xue et al. (2019)
Acinetobacter sp.		These docking studies revealed that cellulase has the greater potential towards the cellotetraose as a substrate for the high yield of ethanol	Selvam et al. (2017)
Cellulase produced by bacteria in gayal rumen	Umcel-1	Putative gene Umcel-1 product belonged to the glycosyl hydrolase family 5 and showed the highest homology to the cellulase (GenBank accession no. YP_004310852.1) from Clostridium lentocellum DSM 5427, with 44% identity and 62% similarity. Open reading frame of 942 base pairs that encoded a product of 313 amino acids The activity of purified recombinant Umcel-1 was assessed, and the results revealed that it hydrolyzed carboxymethyl cellulose with optimal activity at pH 5.5 and 45 °C	LI et al. (2016a)
Aspergillus niger		Exoglucanase increased from the original 0.21 U/mL to 0.89 U/mL of the transformant. Endoglucanase increased from the original 4.51 U/mL to 15.12 U/mL of the transformant. FPA increased nearly 7.1-fold from 0.63 to 4.47 U/mL Cellulase from the transformant retained the halo-stable ability	Xue et al. (2017b)
Trichoderma reesei cellulase	(TrCBH2)	The final titer of more than 18 g/L of secreted protein was produced under controlled conditions in small-scale bioreactor cultivations after 60–70 h. High concentration of secreted enzyme in P. pastoris	Mellitzer et al. (2014)
Trichoderma reesei	Beta-glucosidase gene from Penicillium decumbens	The beta-glucosidase activity of the enzyme complexes from two selected transformants increased six- to eightfold and their filter paper activity (FPAs) enhanced by 30% on average	Ma et al. (2011)
Coprinus cinereus	Cellulase gene mfc	The highest activities of endo-β-1,4-glucanase (34.234 U/mL) and endo-β-1,4-xylanase (263.695 U/mL) were reached by the L. edodes gpd promoter	Cheng et al. (2009)
C. shiraiana	CscelA	CsCelA expressed in P. pastoris high pH stability and affinity. The active protein of 55.3 kDa recombinant cellulase (CsCelA) was 17.44 U/mL and 135 U/g for freeze-dried powder The Kmax and Vmax of CsCelA for sodium carboxymethylcellulose (CMC) were 4.6 mg/mL and 107.2 U/mg, respectively A stable activity from pH 4.0 to 9.0, and at temperatures ranging from 30 to 55 °C	Lü et al. (2015)
Teleogryllus emma	Novel endogenous β-1,4-endoglucanase (EG) gene belonging to the glycosyl hydrolase family 9 (GHF 9) TeEG-I	Signature motifs and catalytic domains, of GHF 9 members, sharing high levels of identity with the termite, Mastotermes darwiniensis (64% protein sequence identity), and the cockroach, Panesthia cribrata (62%), GHF 9 cellulases 47-kDa, Optimal pH and temperature of pH 5.0 and 40 °C Km and Vmax values for digestion of carboxymethyl cellulose 5.4 mg/mL and 3118.4 U/mg results indicate that TeEG-I distributed throughout the entire digestive tract of T. emma, suggesting a functional role of endogenous TeEG-I in a sequential cellulose digestion process T. emma digestion tract	Kim et al. (2008)
Saccharomyces cerevisiae	PSE1 and SOD1	cel6A of Neocallimastix patriciarum, β-glucosidase encoded cel3A of Saccharomycopsis fibuligera resulted in heterologous protein activity ranging between 10 and 373% high compared to the parental strains grown in complex media	Kroukamp et al. (2013)
Coprinopsis cinerea	Multi-functional cellulase gene mfc	Growth substrate, highest activities of endo-β-1,4-glucanase (0.48 U/mL), total cellulase (0.26 U/mL), and endo-β-1,4-xylanase (38.10 U/mL) in culture liquid Cabm44, uses a 275 bp gpd fragment from A. bisporus	Yang et al. (2011)
Cricket Teleogryllus emma	Novel endogenous β-1,4-endoglucanase (EG) gene belonging to the glycosyl hydrolase family 9 (GHF 9)	Optimal pH and temperature of pH 5.0 and 40 °C. Km and Vmax values for digestion of carboxymethyl cellulose 5.4 mg/mL and 3118.4 U/mg	Kim et al. (2008)