Proteolytic enzymes are crucial for a variety of biological processes in organisms ranging from lower to higher organisms. Proteases cleave proteins into smaller fragments by catalyzing peptide bonds hydrolysis. Proteases are classified according to their catalytic site, and distributed into four major classes: cysteine proteases, serine proteases, aspartic proteases, and metalloproteases. Cysteine proteases, papain family enzymes, are involved in multiple functions such as extracellular matrix turnover, antigen presentation, processing events, digestion, immune invasion, hemoglobin hydrolysis, parasite invasion, parasite egress, and processing surface proteins. 
Inhibitors of proteases and related enzymes have versatile applications in medicine and other areas. They are used in the clinic for the treatment of cancer, hypertension, thrombosis, diabetes as well as viral and bacterial infections. Protease inhibitors typically mimic the peptide substrates of the target enzymes and feature specialized moieties that specifically interact with catalytic residues in the active centre. Cysteine proteases are traditionally targeted with activated electrophiles such as epoxides or vinyl sulfones. 
 S. Verma, R. Dixit, and K. C. Pandey, “Cysteine proteases: Modes of activation and future prospects as pharmacological targets,” Front. Pharmacol., vol. 7, no. APR, pp. 1–12, 2016, doi: 10.3389/fphar.2016.00107.
 L. Kaysser, “Built to bind: Biosynthetic strategies for the formation of small-molecule protease inhibitors,” Nat. Prod. Rep., vol. 36, no. 12, pp. 1654–1686, 2019, doi: 10.1039/c8np00095f.
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