Figure 1

Proposed valinomycin biosynthetic pathway in the host E. coli. Pyr pyruvate, l -Lac lactate, α-Kiv α-ketoisovalerate, d -Hiv d-hydroxyisovalerate, l -Val l-valine, d -Val d-valine, ilvBN acetohydroxy acid synthase I, ilvC acetohydroxy acid isomeroreductase, ilvD dihydroxy acid dehydratase, ilvE branched chain amino acid aminotransferase, A adenylation domain, T thiolation domain, C condensation domain, KR ketoreductase domain, E epimerase domain, TE thioesterase domain. A domains select and activate the substrates. T domains are responsible for the translocation of the bound aminoacyl or peptidyl intermediate between adjacent catalytic positions. C domains catalyze the formation of peptide bond and elongate the peptide chain. The KR domain in module 1 reduces α-Kiv to d-Hiv. The E domain in module 2 transfers l-Val to d-Val. The KR domain in module 3 reduces Pyr to l-Lac. The four modules of valinomycin synthetase are iteratively reused to assemble three tetradepsipeptide monomers, which are eventually oligomerized and macrolactonized to form the cyclododecadepsipeptide valinomycin.