Structural and mechanistic studies of enzymes involved in the biosynthesis of peptidic natural products.

摘要

Peptidic natural products are produced by diverse organisms ranging from bacteria to humans. These secondary metabolites can be assembled by the ribosome or by nonribosomal peptide synthetase (NRPS) enzymatic assembly lines. The architectural complexity and biological activity of such compounds make them interesting targets for study. Frequently, nonribosomal peptides contain nonproteinogenic amino acid building blocks, and the biosynthetic routes to both ribosomal and nonribosomal peptides often utilize tailoring enzymes. These specialized enzymes catalyze mechanistically challenging reactions and provide peptidic natural products with structural motifs not normally found in proteins. Structural studies of these tailoring enzymes will further our understanding of biosynthetic pathways, and engineered tailoring enzymes could find use as promiscuous catalysts for the chemoenzymatic synthesis of natural product analogs.;The L-tyrosine 2,3-aminomutase SgTAM catalyzes the formation of beta-tyrosine from L-tyrosine, and is used in the biosynthetic pathway to the enediyne antitumor antibiotic C-1027. This enzyme contains the rare electrophilic prosthetic group 4-methylideneimidazole-5-one (MIO) and is homologous to the histidine ammonia lyase family of enzymes. While lyases form alpha,beta-unsaturated carboxylates as products, SgTAM catalyzes additional chemical steps that result in an overall 2,3-amino shift. The precise mechanistic role of MIO in the ammonia lyase and aminomutase families of enzymes was actively debated for over 50 years. Here, we report the first x-ray crystal structure of an MIO-dependent aminomutase and detail the synthesis and characterization of mechanistic probes for this enzyme. Furthermore, we report several structures of SgTAM bound to substrate analogs. These co-crystal structures reveal how SgTAM achieves substrate recognition and suggest a specific role for MIO in catalysis. The results of our studies allow for the rational engineering of MIO-based aminomutases and ammonia lyases with altered physical properties and substrate specificities.;Additionally, we are currently studying several enzymes involved in the biosynthesis of the tricyclic depsipeptide microviridin J. This ribosomal peptide natural product contains two lactones and one lactam, which are introduced by two enzymes belonging to the ATP-grasp ligase superfamily of proteins. Here, we detail the overexpression of these enzymes, MdnJ-B and MdnJ-C, in E. coli and report the optimization of conditions which lead to the crystallization of both enzymes. The structural characterization of MdnJ-B and MdnJ-C will lead to a greater understanding of macrocycle formation in ribosomal peptide biosynthesis, and engineered variants of these enzymes may find use as macrocyclization catalysts.