Cysteine proteases, also known as thiol proteases, are hydrolase enzymes that degrade proteins. These proteases share a common catalytic mechanism that involves a nucleophilic cysteine thiol in a catalytic triad or dyad. Discovered by Gopal Chunder Roy in 1873, the first cysteine protease to be isolated and characterized was papain, obtained from Carica papaya. Cysteine proteases are commonly encountered in fruits including the papaya, pineapple, fig and kiwifruit. The proportion of protease tends to be higher when the fruit is unripe. In fact, the latex of dozens of different plant families are known to contain cysteine proteases. Cysteine proteases are used as an ingredient in meat tenderizers. Classification The MEROPS protease classification system counts 14 superfamilies plus several currently unassigned families (as of 2013) each containing many families. Each superfamily uses the catalytic triad or dyad in a different protein fold and so represent convergent evolution of the catalytic mechanism. For superfamilies, P indicates a superfamily containing a mixture of nucleophile class families, and C indicates purely cysteine proteases. superfamily. Within each superfamily, families are designated by their catalytic nucleophile (C denoting cysteine proteases). {| class="wikitable" |+Families of cysteine proteases ! Superfamily !! Families !! Examples |- | CA || C1, C2, C6, C10, C12, C16, C19, C28, C31, C32, C33, C39, C47, C51, C54, C58, C64, C65, C66, C67, C70, C71, C76, C78, C83, C85, C86, C87, C93, C96, C98, C101 |Papain (Carica papaya), bromelain (Ananas comosus), cathepsin K (liverwort) and calpain (Homo sapiens) |- | CD||C11, C13, C14, C25, C50, C80, C84||Caspase-1 (Rattus norvegicus) and separase (Saccharomyces cerevisiae) |- | CE||C5, C48, C55, C57, C63, C79||Adenain (human adenovirus type 2) |- | CF||C15||Pyroglutamyl-peptidase I (Bacillus amyloliquefaciens) |- | CL||C60, C82||Sortase A (Staphylococcus aureus) |- | CM||C18||Hepatitis C virus peptidase 2 (hepatitis C virus) |- | CN||C9||Sindbis virus-type nsP2 peptidase (sindbis virus) |- | CO||C40||Dipeptidyl-peptidase VI (Lysinibacillus sphaericus) |- | CP||C97||DeSI-1 peptidase (Mus musculus) |- | PA||C3, C4, C24, C30, C37, C62, C74, C99||TEV protease (tobacco etch virus) |- | PB||C44, C45, C59, C69, C89, C95||Amidophosphoribosyltransferase precursor (Homo sapiens) |- | PC||C26, C56||Gamma-glutamyl hydrolase (Rattus norvegicus) |- | PD||C46||Hedgehog protein (Drosophila melanogaster) |- | PE||P1||DmpA aminopeptidase (Brucella anthropi) |- | unassigned||C7, C8, C21, C23, C27, C36, C42, C53, C75|| |} Catalytic mechanism The first step in the reaction mechanism by which cysteine proteases catalyze the hydrolysis of peptide bonds is deprotonation of a thiol in the enzyme's active site