This class of proteinase (E.C. 3.4.22) catalyzes peptide bond hydrolysis by a mechanism that involves the thiol group of the side-chain of a cysteine residue. These enzymes are also known as cysteine or sulfhydryl proteinases . The thiol group interacts with the carbonyl group of the Thiol groups can also open the ring of ethylenimine:
substrate’s peptide bond that is to be hydrolyzed and forms a thiol ester intermediate. A histidine side chain enhances the reactivity of the thiol group in much the same way that a serine hydroxyl group is activated in serine proteinases. Thiol proteinases tend to act at moderately acidic pH, about 5, and are the predominant hydrolases found in lysosomes and endosomes. A well-known thiol proteinase derived from the papaya plant is papain, the key ingredient in a commonly used meat tenderizer.
Any reagent that will react chemically with a sulfhydryl group will inactivate a thiol proteinase. Iodoacetate is such a nonspecific inactivator, whereas E64 [L-transepoxysuccinyl-leucyl-amido (4-guanidino) butane] is a more useful general inhibitor of thiol proteinases. Protein inhibitors present in blood plasma, and known as cystatins, block activity by covering the active site of the enzyme. Leupeptin, a peptide aldehyde, is another general thiol proteinase inhibitor that binds tightly to the catalytic center.
A thiol proteinase appears to be responsible for the activation of the precursor peptide that gives rise to the enkephalins, and E64 has been proposed as an inhibitor of prohormone processing (1). A thiol proteinase has been shown to be involved in the conversion of pre-renin to renin, which is the enzyme catalyzing the rate-determining step in the production of the hypertensive peptide, angiotensin II (2). Cruzipain, a thiol proteinase, has a critical function in the infectious disease trypanosomiasis, which affects millions of Central and South Americans and, hence, is a target for selective inhibition (3).
An important subclass of the cysteine proteinases includes the interleukin-lb converting enzyme, which converts an inactive precursor to a proinflammatory cytokine (4). These enzymes specifically cleave peptide bonds after aspartic acid residues and, hence, are called caspases. One member of this class, caspase-3, cleaves poly (ADP-ribose) polymerase and hence plays a key role in programmed cell death (see Apoptosis) (5), a process of fundamental importance to biology.
