The broad area of interest to this laboratory is the molecular basis of biological catalysis with focus on the structure and function of enzymes. Three of the several ongoing projects illustrate our approaches.
We are investigating enzymes in bacterial cell wall biosynthesis, either to analyze the mechanism of action of known antibiotics (vancomycin, fosfomycin, cycloserine) or to identify novel targets for drug design. For example, we have cloned, sequenced, expressed, overproduced, and purified to homogeneity eight enzymes in the peptidoglycan biosynthetic pathway and studied their mechanism of catalysis and inhibition. We have also studied the molecular mechanism of development of clinical resistance to vancomycin and identified the function of five genes necessary and sufficient for resistance, overproduced and purified the proteins and elucidated their catalytic function to decode the structural changes that produce a vancomycin-resistant peptidoglycan and thereby dangerously pathogenic bacteria.
We have initiated research in collaboration with Roberto KolterÕs group into the biosynthesis of thiazole- and oxazole-containing peptide antibiotics with specific focus on the E. coli 43 residue peptide microcin B17. We have purified the three enzyme complex that converts prepromicrocin to mature microcin that has undergone modification of 6 gly, 4 cys, and 4 ser residues to yield 4 thiazole and 4 oxazole residues with gain of antibiotic activity. We are studying structure/function issues to see if we can get enzymatic synthesis of microcin analogs that still inactivate DNA gyrase.
During the synthesis of fatty acids, of polyketide natural products (with structures that include erythromycin, amphotericin, rapamycin, and adriamycin), and non-ribosomal peptides (such as cyclosporin, vancomycin), a common post translational modification converts the apo-form of acyl carrier protein (ACP) domains to the holoforms by enzyme-catalyzed phosphopantetheinylation of a conserved serine residue. This introduces the sulfhydryl of the P-pantetheinyl moiety (from cosubstrate CoASH) covalently into the protein and the SH group serves to carry acyl groups in the several transfers in fatty acid, polyketide, and polypeptide antibiotic assembly. We are studying the specificity and mechanism of the enzymes that carry out this phosphopantetheinyl group transfer from CoASH to serine side chains of ACP domains in both polyketide and nonribosomal peptide synthases.
Selected Publications:
A. Gehring, I. Mori, R. Perry, C. Walsh (1998). The Nonribosomal Peptide Synthetase HMWP2 Forms a Thiazoline Ring During Biogenesis of Yersiniabactin, an Iron Chelating Virulence Factor of Y. pestis, Biochemistry 37:11637-11650.
D.E.Cane, C.Walsh, C. Khosla (1998). Harnessing the Biosynthetic Code: Combinations, Permutations, and Mutations, Science 282:63-68.
N. Kelleher, P. Belshaw, C. Walsh (1998). Regioselectivity and Chemoselectivity
Analysis of Oxazole and Thiazole Ring Formation by the Peptide-Heterocyclizing
Microcin B17 Synthetase Using High Resolution MS/MS, J. Am Chem Soc, 120:9716-9717.