FIG 1.

Metabolic pathway of Met, Thr, Ile, and Val biosynthesis in E. coli. Homoserine (Hse) is a branch-point metabolite for the synthesis of Met and Thr. 2-Ketobutyrate, precursor for 2-aminobutyrate or Ile, can be produced from Thr by IlvA. Ile is synthesized from 2-ketobutyrate and pyruvate by the action of IlvBN/IlvIH (AHAS), IlvC, IlvD, and IlvE. Val is produced from pyruvate by same enzymes in the Ile biosynthesis. Abbreviations: CoA, coenzyme A; MetA, homoserine O-succinyltransferase; ThrB, homoserine kinase; ThrC, threonine synthase; IlvA, threonine deaminase; AvtA, valine-pyruvate aminotransferase; GshA, glutamate-cysteine ligase; GshB, glutathione synthase; IlvBN, acetohydroxy acid synthase I; IlvIH, acetohydroxy acid synthase III; IlvC, ketol-acid reductoisomerase; IlvD, dihydroxy-acid dehydratase; IlvE, branched-chain-amino-acid aminotransferase.