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Cystathionine gamma-lyase (CSE) (or cystathionase) is an enzyme which breaks down cystathionine into cysteine, α-ketobutyrate, and ammonia. Pyridoxal phosphate is a prosthetic group of this enzyme. Cystathionine gamma-lyase also catalyses the following elimination reactions: * L-homoserine to form H2O, NH3 and 2-oxobutanoate * L-cystine, producing thiocysteine, pyruvate and NH3 * L-cysteine producing pyruvate, NH3 and H2S In some bacteria and mammals, including humans, this enzyme takes part in generating hydrogen sulfide.〔 Hydrogen sulfide is one of a few gases that was recently discovered to have a role in cell signaling in the body. == Enzyme Mechanism == Cystathionase uses pyridoxal phosphate to facilitate the cleavage of the sulfur-gamma carbon bond of cystathionine, resulting in the release of cysteine.〔 Afterwards the external ketimine is hydrolyzed, causing the release of α-ketobutyrate. The lysine residue reforms the internal aldimine by kicking off the ammonia leaving group. The amino group on cystathionine is deprotonated and undergoes a nucleophilic attack of the internal aldimine. An additional deprotonation by a general base results in the formation of the external aldimine and removal of the lysine residue. The basic lysine residue is then able to deprotonate the alpha carbon, pushing electron density into the nitrogen of the pyridine ring.〔 Pyridoxal phosphate is necessary to stabilize this carbanionic intermediate; otherwise the proton's pKa would be too high.〔 The beta carbon is then deprotonated, creating an alpha-beta unsaturation and pushing a lone pair onto the aldimine nitrogen. To reform the aldimine, this lone pair pushes back down, cleaving the sulfur-gamma carbon bond, resulting in the release of cysteine.〔 A pyridoxamine derivative of vinyl glyoxylate remains after the gamma elimination. The lone pair from the pyridine nitrogen pushes electron density to the gamma carbon, which is protonated by lysine. Lysine then attacks the external aldimine, pushing electron density to the beta carbon, which is protonated by a general acid. The imine is then hydrolyzed to release α-ketobutyrate. Deprotonation of the lysine residue causes ammonia to leave, thus completing the catalytic cycle.〔 Cystathionine gamma lyase also shows gamma-synthase activity depending on the concentrations of reactants present. The mechanisms are the same until they diverge after formation of the vinyl glyoxylate derivative. In the gamma synthase mechanism, the gamma carbon is attacked by a sulfur nucleophile, resulting in the formation of a new sulfur-gamma carbon bond.〔〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Cystathionine gamma-lyase」の詳細全文を読む スポンサード リンク
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