Serine carboxy peptidases

Enzyme which can hydrolyse the sericin is classified as proteolytic enzymes [63-65]. The proteolytic enzymes cleave the peptide/amide linkages and convert them into amino acid. Mainly there are three types of proteolytic enzymes such as zinc protease (e.g. carboxy peptidase A), serine protease (Chymotrypsin, Trypsin, Thrombin) and thiol protease (acts as cystine residue in the protein). The function of proteolytic enzymes in their degree of degumming depends on the pH of the bath and the optimum activity is found to be different at different pH for different enzymes. 

Yet another example of the catalytic triad has been found in carboxy-peptidase II from wheat. The structure of this enzyme is not significantly similar to either chymotrypsin or subtilisin (Figure 9.15). This protein is a member of an intriguing family of homologous proteins that includes esterases such as acetylcholine esterase and certain lipases. These enzymes all make use of histidine-activated nucleophiles, but the nucleophiles may be cysteine rather than serine. Finally, other proteases have been discovered that contain an active-site serine or threonine residue that is activated not by a histidine-aspartate pair but by a primary amino group from the side chain of lysine or by the N-terminal amino group of the polypeptide chain. 

Chymotrypsin is only one member of the family of serine proteases (enzymes that utilize an active-site serine (Ser, S) to cleave the peptide chain). Trypsin, also an endopeptidase obtained from bovine pancreas, is another member of the same family of serine (Ser, S) proteases. Interestingly, however, trypsin cleaves peptides on the carbon side of lysine (Lys, K) and arginine (Arg, R) groups. The catalytic triad discussed above for chymotrypsin also appears to apply here, while still other peptidases have other conserved units that allow them to catalyze the cleavage of peptides at specific sites. While it is not appropriate to provide an exhaustive list of peptidases that have been found to effect protein cleavage, it is important to be aware that in addition to serine (Ser, S) proteases, there are cysteine (Cys, C) proteases and aspartate (Asp, D) proteases and, in addition to endopeptidases, there are aminopeptidases that act on N-termini (N-terminal exopeptidases) and carboxy-peptidases (carboxyl-terminus exopeptidases) that act at the corresponding carboxylic acid termini. 

---