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Biosynthesis of Cysteine in Mammals

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Cysteine is not an essential amino acid, but it arises in mammals from methionine, which is essential, and serine, which is not. In the absence of dietary methionine, animals cannot make cysteine. Mammals require homocysteine (from the degradation of Methionine) as source of sulfur and produce cysteine. Homocysteine reacts with serine in a reaction catalyzed by CBS (Cystathionine Beta-Synthase) to yield Cystathionine. CTH (Cystathionine Gamma-lyase), a pyridoxal phosphate enzyme, catalyzes the cleavage of Cystathionine to yield free cysteine, with Alpha-ketobutyrate and NH3 as the products. Cysteine is an allosteric inhibitor of Cystathionine Gamma-lyase (Ref.1). While the sulfur in Cysteine comes from homocysteine, the rest of the molecule comes from the initial serine residue.

Cysteine is an important amino acid for all [...]


1.Reaction mechanism and regulation of cystathionine beta-synthase.
Banerjee R, Evande R, Kabil O, Ojha S, Taoka S.
Biochim Biophys Acta. 2003 Apr 11; 1647(1-2):30-5. Review.
2.Cystathionine {beta}-synthase: structure, function, regulation and location of homocystinuria-causing mutations.
Miles EW, Kraus JP.
J Biol Chem. 2004 Apr 15
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