Biomimetic synthesis and optimization of cyclic peptide antibiotics

Molecules in nature are often brought to a bioactive conformation by ring formation (macrocyclization)(1). A recurrent theme in the enzymatic synthesis of macrocyclic compounds by non-ribosomal and polyketide synthetases is the tethering of activated linear intermediates through thioester linkages to carrier proteins, in a natural analogy to solid-phase synthesis(2). A terminal thioesterase domain of the synthetase catalyses release from the tether and cyclization(3,4). Here we show that an isolated thioesterase can catalyse the cyclization of linear peptides immobilized on a solid-phase support modified with a biomimetic linker, offering the possibility of merging natural-product biosynthesis with combinatorial solid-phase chemistry. Starting from the cyclic decapeptide antibiotic tyrocidine A, this chemo-enzymatic approach allows us to diversify the linear peptide both to probe the enzymology of the macrocyclizing enzyme, TycC thioesterase, and to create a library of cyclic peptide antibiotic products. We have used this method to reveal natural-product analogues of potential therapeutic utility; these compounds have an increased preference for bacterial over eukaryotic membranes and an improved spectrum of activity against some common bacterial pathogens.