Biosynthesis
As an essential amino acid, methionine is not synthesized de novo in humans, who must ingest methionine or methionine-containing proteins. In plants and microorganisms, methionine is synthesized via a pathway that uses both aspartic acid and cysteine. First, aspartic acid is converted via β-aspartyl-semialdehyde into homoserine, introducing the pair of contiguous methylene groups. Homoserine converts to O-succinyl homoserine, which then reacts with cysteine to produce cystathionine, which is cleaved to yield homocysteine. Subsequent methylation of the thiol group by folates affords methionine. Both cystathionine-γ-synthase and cystathionine-β-lyase require pyridoxyl-5'-phosphate as a cofactor, whereas homocysteine methyltransferase requires vitamin B12 as a cofactor.
Enzymes involved in methionine biosynthesis:
- aspartokinase
- β-aspartate semialdehyde dehydrogenase
- homoserine dehydrogenase
- homoserine O-transsuccinylase
- cystathionine-γ-synthase
- cystathionine-β-lyase
- methionine synthase (in mammals, this step is performed by homocysteine methyltransferase or betaine—homocysteine S-methyltransferase)
Read more about this topic: Methionine