ACV synthetase (ACVS), encoded by pcbAB, is a large non-ribosomal peptide synthetase (NRPS) that assembles the tripeptide ACV from α-aminoadipic acid, L-cysteine, and L-valine. Importantly, the valine residue is epimerised to D-valine during assembly. ACVS then passes ACV to IPNS (isopenicillin N synthase, encoded by pcbC), which cyclises ACV to isopenicillin N — the first penicillin ring structure.