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Cystathionine is an intermediate in the synthesis of cysteine from homocysteine. It is produced by the transsulfuration pathway and is converted into cysteine by cystathionine gamma-lyase (CTH). Biosynthetically, cystathionine is generated from homocysteine and serine by cystathionine beta synthase (upper reaction in the diagram below).
Cystathionine-β-synthase, also known as CBS, is an enzyme (EC 4.2.1.22) that in humans is encoded by the CBS gene. It catalyzes the first step of the transsulfuration pathway , from homocysteine to cystathionine : [ 5 ]
Reaction 5 is catalyzed by cystathionine beta-synthase while reaction 6 is catalyzed by cystathionine gamma-lyase. The required homocysteine is synthesized from methionine in reactions 1, 2, and 3. The transsulfuration pathway is a metabolic pathway involving the interconversion of cysteine and homocysteine through the intermediate cystathionine.
In enzymology, a cystathionine gamma-synthase (EC 2.5.1.48) is an enzyme that catalyzes the formation of cystathionine from cysteine and an activated derivative of homoserine, e.g.: O 4 -succinyl-L-homoserine + L-cysteine {\displaystyle \longrightarrow } L-cystathionine + succinate
Cysteine synthesis: Cystathionine beta synthase catalyzes the upper reaction and cystathionine gamma-lyase catalyzes the lower reaction. In animals, biosynthesis begins with the amino acid serine. The sulfur is derived from methionine, which is converted to homocysteine through the intermediate S-adenosylmethionine.
Thus, the substrate of this enzyme is L-cystathionine, whereas its 3 products are homocysteine, pyruvate, and ammonia. [2] [3] [4] Found in plants, bacteria, and yeast, cystathionine beta-lyase is an essential part of the methionine biosynthesis pathway as homocysteine can be directly converted into methionine by methionine synthase.
Cystathionine γ-synthase (metB) which joins an activated homoserine ester (acetyl or succinyl) with cysteine to form cystathionine; Cystathionine β-lyase (metC) which splits cystathionine into homocysteine and a deaminated alanine (pyruvate and ammonia) in the direct sulfurylation pathway for methionine biosynthesis:
L-Cysteine is also consumed in pantothenate/coenzyme A biosynthesis. ... cystathionine γ-lyase [4] pyridoxal phosphate 3-mercapto-pyruvate: → cysteine transaminase [7]