Cystein protease

The cystein-proteases or proteases with Cystéine are peptidases which in common have a catalytic mechanism which implies an Amino-acid cystein present in the catalytic triad.

Catalytic reaction

the first stage of the catalytic reaction of the cystein-proteases is the deprotonation of the Thiol of the cystein of the active site of the enzyme by an adjacent amino-acid having a basic side chain, usually the Histidine.

the following stage is an attack Nucléophile of a carbon of the substrate of the enzyme by the sulfur of déprotonée cystein. A fragment of the substrate is then salted out, whereas the residue histidine of the enzyme is restorée in its déprotonée form. An intermediate thioester is formed flexible the cystein of the active site to the remainder of the substrate. The connection thioester is then hydrolized salting out the remainder of the substrate and restorant cystein in its initial form. The protein substrate is thus hydrolized in two fragments.

Examples of cystein-proteases

One can quote four examples of cystein-proteases:

the Papain
the Cathepsine S
the caspases
the Calpaïne S

Regulation of the cystein-proteases

The cystein-proteases, like all the proteases, are strongly controlled in order to avoid an anarchistic proteolysis which would be disastrous for the cell. Some as the Caspase S are synthesized in inactive form (the pro-caspases) and must be cleaved and dimerized to be active. The Calpaïne S must also be car-protéolysées to be active. They require especially the presence of the Calcium to be active.

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