Total Synthesis of Circular Bacteriocins by Butelase 1.

Circular bacteriocins, ranging from 35 to 70 amino acids, are the largest cyclic peptides produced by lactic acid bacteria to suppress growth of other bacteria. Their end-to-end cyclized backbone that enhances molecular stability is an advantage to survive in pasteurization and cooking processes in food preservation, but becomes a disadvantage and challenge in chemical synthesis. They also contain unusually long and highly hydrophobic segments which pose an additional synthetic challenge. Here we report the total synthesis of the three largest circular bacteriocins, AS-48, uberolysin, and garvicin ML, by an efficient chemoenzymatic strategy. A key feature of our synthetic scheme is the use of an Asn-specific butelase-mediated cyclization of their linear precursors, prepared by microwave stepwise synthesis. Antimicrobial assays showed that the AS-48 linear precursor is inactive at concentrations up to 100 μM, whereas the macrocyclic AS-48 is potently active against pathogenic and drug-resistant bacteria, with minimal inhibitory concentrations in a sub-micromolar range.