L-Methionine: Methionine is one of the common sulfur-containing amino acids. The biosynthesis of methionine initially occurs by the condensation of homoserine and succinyl-CoA via the action of homoserine acyltransferase. Subsequently, cystathionine g-synthase displaces the succinate group with cysteine to give cystathionine. Cystathionine a-lyase then hydrolyzes cystathionine to produce homocysteine. Transfer of a methyl group from N5-methyltetrahydrofolate to the homocysteine forms methionine. Methionine is a common methyl-group donor to various substrates, such as creatine, epinephrine, ergosterol, and choline. Methionine is a relatively hydrophobic amino acid residue, and as such is frequently buried in protein three-dimensional structure, making modification of methionine residues difficult.