Structural Biochemistry/Enzyme/Cysteine Proteases

Cysteine Protease
This is part of a large family of peptide-cleaving enzymes or proteases. Cysteine Proteases is one of proteases enzyme that cleave protein by cleave the peptide bond. The strategy used by the cysteine proteases is most similar to that use to be chymotrypsin family that is to generate a nucleophile that attack the peptide carbonyl group. Also similar to chymotrypsin, it polarizes the peptide carbonyl group to get it activated for attack, and upon attack by the nucleophile, a stabilizing tetrahedral intermediate is generated. But different from chymotrypsin enzyme, in this enzyme, a cysteine residue, activated by histidine residue play a nucleophilic attack the peptide bond.

Most common Cysteine Proteases
Papain is a protein-cleaving enzyme derived from papaya fruit (Carica papaya) and certain other plants. Papain is used as a meat tenderizer and in medicine as a digestive aid. Cathepsins are members of the lysosomal cysteine protease (active site) family and the cathepsin family name has been synonymous with lysosomal proteolytic enzymes. In actuality, the cathepsin family also contains members of the serine protease (cathepsin A,G) and aspartic protease (cathepsin D,E) families as well. These enzymes exist in their processed form as disulfide-linked heavy and light chain subunits with molecular weights ranging from 20-35 kDa. Cathepsins have a vital role in mammalian cellular turnover, e.g. bone resorption. They degrade polypeptides and are distinguished by their substrate specificity