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

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.