Alternative enzymatic pathways to penicillin antibiotics.

The discovery of penicillin, more than a century ago, has been one of the most significant advances in medicine. Despite the growing threat of antimicrobial resistance, which has rendered many other antibiotics ineffective, penicillin derivatives remain among the most widely prescribed antibiotics. Penicillin is biosynthesised by a large nonribosomal peptide synthetase (NRPS) enzyme, which assembles a tripeptide precursor ACV. This intermediate is subsequently cyclised by isopenicillin N synthase (IPNS) to form penicillin. ACV is similar in structure to glutathione, a ubiquitous, tripeptide antioxidant essential for aerobic life forms. Unlike ACV, glutathione is assembled using simpler ligase enzymes rather than complex NRPS machinery. In this paper, we describe an alternative pathway to penicillins that uses stand-alone ligase and epimerase enzymes to generate peptide precursors, which can be transformed to penicillin derivatives using an engineered IPNS enzyme. Unlike the native NRPS assembly line, the ligase pathway provides direct access to therapeutically relevant penicillin G, penicillin V and ampicillin, which are currently produced by semi-synthesis.