The Flavin-Dependent N-Hydroxylating Ornithine Monooxygenase from Aspergillus fumigatus and the Heme-Dependent O2-Generating Chlorite Dismutase from Dechloromonas aromatica: Studies of Oxygen Consumption and Production by Two Microbial Enzymes

The appearance of oxygen in the earth's atmosphere approximately 2.3 billion years ago led to considerable changes in the environment. The species that inhabited earth, and wished to remain, had to adapt to an oxygen-rich atmosphere. At the biochemical level, this forced the production of enzymes and cofactors that were able to react with and bind oxygen, while limiting and elminating the toxic side products of its reduction. Modern species contain several oxygen-reactive biomolecules that are critical for survival; the following describes an in-depth examination of two enzyme systems with evolved highly sophisticated mechanisms for the use and production of molecular oxygen, respectively. The first part of the thesis describes mechanistic studies of a member of a novel subclass of N-hydroxylating flavin-dependent monooxygenases. The enzyme, L-ornithine monooxygenase (OMO), activates oxygen to a reactive flavin hydroperoxide intermediate to hydroxylate the primary amine of L-ornithine in a critical step for the biosynthesis of iron-chelating siderophores. The second half describes the CDE (Chlorite dismutase, DyP-type peroxidase, EfeB) protein-family. This microbial protein family contains the enzyme chlorite dismutase (Cld), a recently discovered enzyme from perchlorate (ClO4-) respiring bacteria. These organisms have achieved the ability to respire oxochlorates and make use of the man-made molecules for their survival. Cld is a heme-dependent enzyme that catalyzes the conversion of chlorite (ClO2-), the toxic end product of perchlorate respiration, to molecular oxygen (O2) and chloride (Cl-). Studies to understand Cld's detailed chemical mechanism and the protein residues essential for its function are described. The large CDE protein-family, to which Cld belongs, is described in detail with structural and sequence based analyses. Work shows that several annotated Cld genes are found in non-perchlorate respiring bacteria and likely have as yet unidentified non-dismutase functions. In the last chapter, biochemical and microbiological data are presented with an effort towards determining the biological function of a Cld homolog in the pathogenic species Staphyloccoccus aureus. The biological roles of both the L-ornithine monooxygenase and chlorite dismutase center around redox biochemistry, and each story will be described within the context of microbial redox cofactor acquisition, synthesis, storage, and utilization. The importance of each is described in regard to virulence in pathogenic species and the potential development of new antimicrobial targets.