|Title||Structural Characterization of Novel Antimicrobial Therapeutic Targets and Crystallographic Examination of Enzymes Involved in Xenobiotic Metabolism|
Part I. Recently there has been an alarming rise in the number of infections due to pathogenic organisms that are insensitive to our current arsenal of pharmaceuticals, necessitating the identification of new antimicrobial targets. Here, we describe structural studies of two enzymes from the aspartate family of biosynthetic enzymes in order to assess their potential for drug targeting. Our first report details how an unlikely inhibitor with low millimolar binding characteristics, 5-hydroxy-4-oxo-norvaline, can effectively inactivate homoserine dehydrogenase HSD) through the formation of a tight-binding covalent adduct. This is followed by the structural characterization of the first phenolic-based non-amino acid inhibitor of HSD, which is shown to occupy the substrate binding site and make specific contacts with residues involved in catalysis. Together, these studies lay the foundation for further structure-based drug design of novel inhibitors of HSD. Following this, we report the first structure of the next enzyme in the aspartate pathway, homoserine transacetylase HTA). As the committal step in microbial methionine biosynthesis, this enzyme is essential to microbial survival. The high-resolution crystal structure identifies HTA as a new member of a larger structural family of enzymes known as the alpha/beta hydrolases. Using the structure as a guide, we propose a rationale for the previously reported ping-pong reaction mechanism for this enzyme. We conclude with a preliminary look into how the natural substrate, homoserine, binds in the active site and some subtle differences between HTAs from different sources. Part II. Microbes have long been admired for their ability to process virtually any chemical. In Part II of this work we will engage in the structural characterization of two enzymes involved in the microbial degradation of xenobiotic compounds. The first enzyme, cyclohexylamine oxidase CHAO), is the enabling step in the bacterial metabolism of cyclohexylamine. Preliminary crystallographic analysis of cofactor bound and ternary complexes of CHAO, will be used to highlight differences between this enzyme and its closest human homologue. This information will then be used to direct structure inspired mutagenesis studies in order to better understand the substrate specificity of this enzyme and how it might be altered. Finally, the last enzyme to be discussed in this work will be cyclohexanone monooxygenase CHMO). Also from the same biodegradation pathway as CHAO this enzyme is responsible for the stereo- and regio-specific conversion of small cyclic ketones into lactones. This reaction, known as the Baeyer-Villiger Oxidation, is of tremendous importance in the pharmaceutical industry as lactones often serve as the building blocks of other larger compounds. Surprisingly, despite many years of research there has never been a published report on the structure of this enzyme. We detail here, for the first time, the crystallographic structure of CHMO in multiple conformations. Together, the various structures of CHAO and CHMO presented will provide insight into how bacteria are able to process xenobiotics and how we might use this information for structure based protein design.
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