Table 4.

Main sources and activity of proteases with potential in plant protein-based food applications (note that some of the commercial enzymes are mixtures). Details, including stereospecificity of the enzymes, are out of the scope of this review and are therefore omitted.

Enzyme	Major Sources	Action Site	Product	References
Trypsin	Porcine or bovine intestine	Highly specific. Cleaves C-terminal to arginine (R) and lysine (K) residues. Less effective if acidic residue (glutamate (E) or aspartate (D)) is near the cleavage site. May cleave before proline (P).	Small peptides	[76,77,78]
Pepsin	Porcine gastric mucosa	Broad specificity, with overall preference to cleave after bulky aromatic residues (maybe favoring phenylalanine (F)), leucine (L), and possibly methionine (M). Cleavage after histidine (H), lysine (K), arginine (R), proline (P) usually not as favored, unless adjacent to residues such as leucine (L) or phenylalanine (F).	Small peptides	[79,80,81]
Carboxy-peptidase (CP)	CP-A from bovine pancreas; CP-B from bovine or porcine pancreas; CP-Y (yeast CP) from baker’s yeast.	CPs are exopeptidases that cleave the carboxy end of proteins and peptides, usually one residue at a time. Depending on their substrate preference they can be classified as CPs-A (prefer aromatic and large aliphatic sidechains, hydrolyze slowly glycine (G) and acidic residues, rarely proline (P) and basic residues); CPs-B (with narrower specificity than CPs-A and preference towards the basic residues arginine (R), lysine (K) and some action on neutral amino acids); and CPs-C (can release proline (P) and other amino acids). CP-Y has broad specificity, similar to CP-A but cleaves rapidly glycine (G) and leucine (L), and slowly phenylalanine (F).	Typically single amino acids	[82,83,84,85]
Amino-peptidase (AP)	Microbes and porcine kidney.	APs are exopeptidases that cleave the amino end of proteins and peptides. They can be classified to aminoacylpeptidases, dipeptidyl- and tripeptidyl- peptidases (i.e., releasing single amino acids, dipeptides, tripeptides, respectively), with a tetra-peptidase recently reported. If acting only on di- or tri- peptides, they are di- and tri-peptidases, respectively. Based on substrate specificity they are classified into 2 categories: broad and narrow.	Amino acids, di-peptides, tri-peptides, rarely tetra-peptides	[86,87]
Alcalase	Microbes	Has broad specificity. Reported to cleave bonds on the carboxyl side of glutamic acid (E), methionine (M), leucine (L), tyrosine (Y), lysine (K), and glutamine (Q), also at phenylalanine (F), tryptophan (W), alanine (A), serine (S).	Small peptides	[88,89,90,91]
Plasmin or fibrinolysin	From bovine plasma or microbes	Has similar specificity to trypsin, but less efficient. Cleaves after arginine (R) and lysine (K) residues.	Small peptides	[92,93,94]
Flavor-zyme®	Microbial (Aspergillus oryzae)	Broad specificity, mostly endo activity	Small peptides and amino acids	[95,96]
Protamex	Microbial (Bacillussp.)	Broad specificity.	Small peptides	[95,97]
Neutrase®	Microbial (Bacillus amyloliquefaciens)	Broad specificity	Small peptides	[95]
Corolase 7089	Fungal neutral protease	Broad specificity	Small peptides	[97]
Pronase	Microbial (Streptomyces griseus)	Broad specificity	Amino acids and peptides	[98]
Prolidase	Microbial	Cleaves before proline (P) or hydroxylproline in dipeptides.	Amino acids	[99]
Ficin	Fig (Ficus carica)	Generally prefers to cleave after aromatic residues e.g., tyrosine (Y), phenylalanine (F); exact specificity depends on form.	Small peptides	[100,101,102]
Papain	Papaya (Carica papayaL.)	Has broad specificity, with reported preference to cleave bonds at arginine (R), lysine (K), and phenylalanine (F).	Small peptides	[52,88,103,104]
Bromelain	Fruit or stem of pineapple (Ananas comosusL.)	Broad specificity.	Small peptides	[88,100,105]