Mechanisms of Polyketide, Amino Acid and Polypeptide Biosynthesis

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The major objectives of the proposed research in chemical biology, peptide synthesis and natural products involve combination of methods used in organic chemistry (e.g. synthesis, spectroscopic analysis) and biochemistry (e.g. molecular biology, protein characterization, enzyme inhibition and structure of proteins). It is mostly divided into two areas: I. synthesis, structure-activity relationships and mechanism of bacterial antimicrobial peptides (polymers of amino acids) and II. mechanism of fungal polyketide synthases (PKS), large proteins from fungi that polymerize acetic acid to make bioactive molecules, some of which are important human drugs. In terms of background for the antimicrobial peptides, over 50% of hospital-acquired infections are antibiotic-resistant, and the corresponding bacterial pathogens now pose a serious health threat. Fortunately, certain peptides can destroy these pathogens very effectively. Our goals are to understand the structures and mechanisms of action of antimicrobial peptides from bacteria. Antimicrobial peptides produced by bacteria are often orders of magnitude more potent than conventional antibiotics, can kill resistant pathogens, and frequently bind a receptor molecule in the membrane of target bacteria. They can be ribosomally made (bacteriocins) or produced by nonribosomal peptide synthases (e.g. lipopeptides). A central theme is synthetic modification of such peptides to make simpler accessible analogs that can be used in humans to destroy resistant pathogens. The targets include lantibiotics (nisin), sulfur to a-C linked peptides (thuricins) and lasso peptides (microcin J25). A key objective is understanding interactions of such peptides with lipids and receptors as well as resulting membrane pore structures that lead to bacterial cell death. Targets include lipopeptides (tridecaptins) & cyclic bacteriocins (carnocyclin A). The bacterial species producing carnocyclin A (MicocinTM) has been approved for use in food for preservation of meat in USA and Canada, and has great potential as a therapeutic agent for infections resistant to conventional antibiotics. In terms of background, polyketides are secondary metabolites constructed by large (>200 kDa) enzymes (proteins) known as polyketide synthases (PKS) whose operation resembles fatty acid synthases. More than 20 important drugs are derived from known polyketides. Ultimately their function is to polymerize acetate, propionate or other small precursors into chains having specific length, functionality & stereochemistry. Our goals are to elucidate the mechanisms of highly reducing fungal PKS enzymes, which are remarkable molecular machines. The targets include enzymes that assemble lovastatin (a cholesterol-lowering drug and precusor to simvastatin (Zocor)), equisetin (inhibits HIV), dehydrocurvularin (an immunomodulator phytotoxin), cytochalasin E (an anti-angiogenesis anticancer agent) and hypothemycin (a kinase inhibitor & anticancer candidate).