The biosynthesis of methionine begins with aspartate being phosphorylated into L-aspartyl-4-phosphate through an ATP driven aspartate kinase. L-Aspartyl-4-phosphate is then catabolized through an NADPH-dependent aspartic beta-semialdehyde dehydrogenase resulting in the release of L-aspartate semialdehyde which is transformed into a homoserine through a homoserine dehydrogenase. Homeserine, in turn, is acetylated through a homoserine O-trans-acetylase resulting in the release of O-acetyl-L-homoserine. The latter compound interacts with hydrogen sulfide through an O-acetylhomoserine (thiol)-lyase resulting in the release of L-homocysteine. The latter compound then reacts with 5-methylterahydropteroyltri-L-glutamate through an N5-methyltetrahydropteroyltrigluatamate homocysteine methyltransferase resulting in the release of a tetrahydropteroyltri-l-glutamate and methionine. The degradation of methionine begins with methionine being used to synthesize S-adenosylmethionine through an S-adenosylmethionine synthetase. The S-adenosylmethionine reacts with a demethylated methyl donor resulting in the release of a methylated methyl donor, a hydrogen ion, and an S-adenosylhomocysteine. The latter compound then reacts with an S-adenosyl-L-homocysteine hydrolase resulting in the release of adenosine and homocysteine where the cycle can begin again. The salvage of methionine begins with S-methyl-5'-thioadenosine (a product of spermine biosynthesis) being phosphorylated through a 5-methylthioadenosine phosphorylase resulting in the release of adenine and S-methyl-5-thio-alpha-D-ribose 1-phosphate. This last compound is isomerized into 5-methylthioribulose 1-phosphate. The latter compound is then dehydrated through a methylthioribulose 1-phosphate dehydratase resulting in 5-(methylthio)-2,3-dioxopentyl 1-phosphate. This resulting compound is then dephosphorylated through a 2,3-dioxomethiopentane-1-phosphate enolase/phosphatase resulting in a 1,2-dihydroxy-5-(methylthio)pent-1-en-3-one. This latter compound can react spontaneously or through an acireductone dioxygenase resulting in the release of a 2-oxo-4-methylthiobutanoate. This latter compound is then turned into methionine through an aromatic amino acid aminotransferase II.

Methionine Metabolism and Salvage References