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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] L-serine + L-homocysteine L-cystathionine + H 2 O
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.
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 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.
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 γ-lyase (no common gene name) which joins an activated serine ester (acetyl or succinyl) with homocysteine to form cystathionine; Not Cystathionine β-synthase which is a PLP enzyme type II; cysteine biosynthesis from serine: O-acetyl serine sulfhydrylase (cysK or cysM) which adds a thiol group to an activated serine ester
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.
cystathionine: → cystathionine γ-lyase [4] pyridoxal phosphate 3-mercapto-pyruvate: → cysteine transaminase [7] pyridoxal phosphate References