Cysteine proteases are present in all living organisms. Besides their fundamental functions of catabolism and protein processing, cysteine proteases perform diverse functions. Cysteine proteases of parasites play key role in hemoglobin hydrolysis, blood cell invasion, egress, surface proteins processing. In mammals, a main group of cysteine proteases is known as lysosomal cathepsins. Bioinformatics analysis reveals that human genome encodes 11 cysteine cathepsins, i.e., the cathepsins B, C, F, H, K, L, O, S, V, X, and W, existing at the sequence level. Cathepsins and other cysteine proteases from parasites as well as viruses may become good targets for major diseases such as arthritis, osteoporosis, AIDS, immune-related diseases, atherosclerosis, cancer, and for a wide variety of parasitic diseases such as malaria, amebiasis, chagas disease, leishmaniasis, or African sleeping sickness. 
Cysteine proteases contain a Cys–His–Asn triad at the active site. A histidine residue, presents in the active site act as proton donor and enhance the nucleophilicity of the cysteine residue. In processing of cysteine protease, pH change has great importance. Auto-catalysis involves the cleavage of prodomain by catalytic site present inside the catalytic cleft of the enzyme under the influence of pH change. Trans-activation involve cleavage by the another molecule of the same enzyme or some other proteases that cleave within the residues lying at the junction of the prodomain and the mature domain such as pepsin, aspartic cathepsin D, and legumain/asparaginyl endopeptidase.

References

1.Verma S,et al. Front Pharmacol. 2016 Apr 25;7:107.