δ‐L‐(α‐Aminoadipoyl)‐L‐cysteinyl‐D‐valine synthetase: the order of peptide bond formation and timing of the epimerisation reaction

Abstract

δ‐L(α‐Aminoadipoyl)‐L‐cysteinyl‐D‐valine (ACV) synthetase catalyses the formation of the common precursor tripeptide of both the penicillin and cephalosporin antibiotics from the l‐enantiomers of its constituent amino acids. Replacement of cysteine with l‐O‐methylserine in preparative‐scale incubations led to the isolation of both t.‐O‐methylserinyl‐L‐valine and l‐O‐methylserinyl‐D‐valine dipeptides. The dipeptides were characterized with the aid of authentic synthetic standards by both ¹H NMR and electrospray ionization MS. A revised mechanism for ACV biosynthesis involving formation of the cysteinyl‐valine peptide bond before the epimerisation of valine and subsequent condensation with the δ‐carboxyl of L‐α‐aminoadipate is therefore proposed.