Structural insight into nonribosomal peptide synthesis
Nonribosomal peptide synthetases (NRPSs) are large enzymes found in bacteria and fungi that catalyze peptide bond formation. Instead of making proteins, these megaenzymes produce a vast variety of small molecules with important and diverse biological activity. For example, NRPSs synthesize antifungals, antibacterials, antivirals, antitumours, siderophores, immunosuppressants, industrial agents and green chemicals. The impact of NRPS products on human health has been revolutionary: Penicillin, cyclosporin, cephalosporin and daptomycin have saved tens of millions of lives worldwide. NRPSs are true macromolecular machines, huge proteins with multiple active sites, moving parts and a coordinated synthetic process. They consist of a series of multidomain modules of ~1100 residues, with each module responsible for adding one specific amino acid to the growing peptide molecule, in assembly line fashion.
We have been studying linear gramicidin synthetase and bacillibactin synthetase to gain structural and functional insight into NRPSs. The initiation module of an antibiotic-producing NRPS, linear gramicidin synthetase includes the specialized tailoring formylation domain, and we capture states that represent every major step of the assembly-line synthesis in the initiation module. The structures show how the formylation domain is incorporated into the NRPS architecture and how it has evolved to act in concert with the other domains in the synthetic cycle. The transitions between the sequential conformations in the cycle are very large, with both the peptidyl carrier protein and the adenylation subdomain undergoing huge movements to transport substrate between distal active sites. The structures highlight the great versatility of NRPSs, as small domains repurpose and recycle their limited interfaces to interact with their various binding partners. In addition, X-ray crystallography of a cross-module construct of bacillibactin synthetase, and low resolution negative stain electron microscopy of dimodular bacillibactin synthetase indicate that intra-module mobility is even greater than that seen within a module and suggest that NRPSs lack a uniform, rigid supermodular architecture.
Martin Schmeing received his B.Sc. from McGill University (1998), before obtaining his M.Sc. and Ph.D. with Dr. Thomas Steitz at Yale University (2002, 2004). He then carried out postdoctoral research with Dr. V. Ramakrishnan at the Laboratory of Molecular Biology, Cambridge, UK (2006-2010). He was appointed Assistant Professor at the Department of Biochemistry, McGill University in 2010, and was promoted to Associate Professor in 2016. He holds a Canadian Research Chair in Macromolecular Machines and serves as the Associate Director of the Centre for Structural Biology. The main focus of his research is on elucidating the structures and functions of nonribosomal peptide synthetases (NRPSs). NRPSs are large microbial enzymes that synthesize their products through amide bond formation between building block monomers (most commonly amino acids). The chemical and biological properties of these compounds often make them useful to society as therapeutics (antibiotics, antivirals, anti-tumours, and immunosuppressants) and as natural green chemicals (emulsifiers, siderophores, and research tools). Two aspects of particular focus in Dr. Schmeing’s research are the catalytic event which links substrate building blocks, and the manner in which NRPS domains and modules work together in a complicated and productive catalytic cycle.