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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).
The reverse transsulfuration pathway depicting the conversion of homocysteine to cysteine in reactions 5 and 6. 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.
Transsulfuration, catalyzed by CBS, converts homocysteine to cystathionine, which cystathione gamma lyase converts to cysteine. [12] CBS occupies a pivotal position in mammalian sulfur metabolism at the homocysteine junction where the decision to conserve methionine or to convert it to cysteine via the transsulfuration pathway, is made.
Cystathionine β-synthase catalyses the condensation of homocysteine and serine to give cystathionine. This reaction uses pyridoxine (vitamin B 6 ) as a cofactor. Cystathionine γ-lyase then converts this double amino acid to cysteine, ammonia, and α-ketobutyrate.
Other members include cystathionine γ synthase, cystathionine β lyase, and methionine γ lyase. [8] It is also a member of the broader aspartate aminotransferase family. [1] [8] Like many other PLP-dependent enzymes, cystathionine γ-lyase is a tetramer with D2 symmetry. [8] Pyridoxal phosphate is bound in the active site by Lys 212. [2]
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.
Homocystinuria (HCU) [2] is an inherited disorder of the metabolism of the amino acid methionine due to a deficiency of cystathionine beta synthase or methionine synthase. [3] It is an inherited autosomal recessive trait, which means a child needs to inherit a copy of the defective gene from both parents to be affected.
cystathionine: → cystathionine γ-lyase [4] pyridoxal phosphate 3-mercapto-pyruvate: → cysteine transaminase [7] pyridoxal phosphate References