Cysteine protease
3710395
195627425
2008-03-03T20:18:02Z
192.117.113.122
+he
{{Unreferenced|date=June 2007}}
[[Image:L-cysteine-skeletal.png |thumb|100px|[[Cysteine]]]]
[[Image:Cysteinprotease Reaktionsmechanismus.svg|thumb|300px|Reaction mechanism of the cysteine protease mediated cleavage of a peptide bond.]]
[[Protease]]s are [[enzyme]]s that degrade [[protein|polypeptides]]. '''Cysteine proteases''' have a common catalytic mechanism that involves a [[nucleophile|nucleophilic]] [[cysteine]] [[thiol]] in a [[catalytic triad]]. The first step is deprotonation of a thiol in the enzyme's active site by an adjacent [[amino acid]] with a basic [[side chain]], usually a [[histidine]] residue. The next step is nucleophilic attack by the deprotonated cysteine's [[anion|anionic]] [[sulfur]] on the substrate [[carbonyl]] [[carbon]]. In this step, a fragment of the substrate is released with an [[amine]] terminus, the histidine residue in the protease is restored to its deprotonated form, and a [[thioester]] intermediate linking the new carboxy-terminus of the substrate to the cysteine thiol is formed. The thioester bond is subsequently hydrolyzed to generate a [[carboxylic acid]] moiety on the remaining substrate fragment, while regenerating the free enzyme.
==Examples of Cysteine Proteases==
* [[Papain]]
* [[Cathepsins]]
* [[Caspases]]
* [[Calpains]]
==Protease Regulation==
Proteases are usually synthesized as large precursor proteins called [[zymogens]], such as the [[serine protease]] precursors [[trypsinogen]] and [[chymotrypsinogen]], and the [[aspartic protease]] precursor [[pepsinogen]]. The protease is activated by removal of an inhibitory segment or protein. Activation occurs once the protease is delivered to a specific intracellular compartment (e.g. [[lysosome]]) or extracellular environment (e.g. [[stomach]]). This system prevents the [[cell (biology)|cell]] that produces the protease from being damaged by it.
Protease [[Enzyme inhibitor|inhibitor]]s are usually proteins with [[structural domain|domain]]s that enter or block a protease [[active site]] to prevent [[Substrate (biochemistry)|substrate]] access. In [[competitive inhibitor|competitive inhibition]], the inhibitor binds to the active site, thus preventing enzyme-substrate interaction. In [[competitive inhibitor|non-competitive inhibition]], the inhibitor binds to an [[allosteric regulation|allosteric site]], which alters the active site and makes it inaccessible to the substrate.
==Examples of Protease Inhibitors==
* [[Serpin]]s
* [[Stefin]]s
* [[Inhibitor of apoptosis|IAP]]s
* [[TIMP]]s
==External links==
* {{MeshName|Cysteine+endopeptidases}}
{{Proteases}}
{{Cysteine proteases}}
[[Category:EC 3.4.22|*]]
[[de:Cysteinprotease]]
[[fr:cystéine protéase]]
[[he:ציסטאין פרוטאז]]