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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. Two transsulfurylation pathways are known: the forward and the reverse. [1]
Methionine (symbol Met or M) [3] (/ m ɪ ˈ θ aɪ ə n iː n /) [4] is an essential amino acid in humans.. As the precursor of other non-essential amino acids such as cysteine and taurine, versatile compounds such as SAM-e, and the important antioxidant glutathione, methionine plays a critical role in the metabolism and health of many species, including humans.
This diagram shows the biosynthesis (anabolism) of amino acids aspartate, asparagine, threonine, methionine, lysine from the precursor oxaloacetate. The associated enzymes are subject to regulation via feedback inhibition and/or repression at the genetic level. As is typical in highly branched metabolic pathways, additional regulation at each ...
The biosynthetic pathways of synthesis of γ-glutamylpeptides and alliins are still ambiguous. γ-Glutamylpeptides can be formed from cysteine (via γ-glutamylcysteine or glutathione) and can be metabolized into the corresponding alliins via oxidation and subsequent hydrolyzation by γ-glutamyl transpeptidases.
L-cysteine production pathways; Reactants → Enzyme Cofactors Notes O-acetyl-L-serine/hydrogen sulfide → cysteine synthase [9] pyridoxal phosphate not present in humans L-cystine/2 glutathione → glutathione-cystine transhydrogenase [10] cystathionine: → cystathionine γ-lyase [4] pyridoxal phosphate 3-mercapto-pyruvate: → cysteine ...
Homocysteine is a non-protein amino acid, synthesized from methionine and either recycled back into methionine or converted into cysteine with the aid of the B-group vitamins [citation needed]. About 50% of homocysteine [citation needed] is converted back to methionine by remethylation via the methionine synthase major pathway.
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:
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. Moreover, the transsulfuration pathway is the only pathway capable of removing sulfur-containing amino acids under conditions of excess. [9]