Fig. 1.

Direct-sulfurylation and trans-sulfurylation methionine biosynthesis pathways in bacteria. The first step in methionine biosynthesis involves the activation of homoserine through an acylation step catalyzed by two enzymes encoded by metAs and metXa genes. In the trans-sulfurylation pathway (blue box), the enzyme homoserine succinyl transferase (HST, MetAs) converts homoserine and succinyl-CoA into O-succinyl-L-homoserine (OSH). Cysteine and O-succinyl-L-homoserine (OSH) are converted into cystathionine by cystathionine-γ-synthase (CgS, MetB). Cystathionine is converted into homocysteine by cystathionine-β-lyase (CbL, MetC). In the direct-sulfurylation pathway, the enzyme homoserine acetyl transferase (HAT, MetXa) converts homoserine and acetyl-CoA into O-acetyl-L-homoserine (OAH) which in a single step is further converted into homocysteine by O-acetylhomoserine sulfhydrylase (OAHS, MetY). In both pathways, the enzyme methionine synthase (MetE/H, not shown in the figure) converts homocysteine to methionine.