Mechanistic studies of HPP epoxidase and DXP reductoisomerase: applications to biosynthesis and antibiotic development

Abstract

The focus of this dissertation is the study of two enzymes, DXR and HppE. DXR catalyzes the first committed step in the MEP pathway, which is the pathway most eubacteria, archeabacteria, algae, and the plastids of plants use for the biosynthesis of isoprenoid. Since mammals utilize the mevalonate pathway and isoprenoids are essential for survival, all enzymes in the MEP pathway are excellent antibiotic targets. One antibiotic that has promise in the fight against malaria is the natural product fosmidomycin, whose antibiotic activity is due to its ability to bind and inhibit DXR. With a deeper understanding of DXR's catalyzed reaction, it will be possible to design a more sophisticated and potent antibiotic. To probe the mechanism of DXR, two fluorinated substrate analogues, 3F-DXP and 4F-DXP, and a fluorinated product analogue, FCH₂-MEP were designed and analyzed as possible substrates or inhibitors. To further analyze the mechanism of DXR, a 2° [²H]-KIE study was conducted using the equilibrium perturbation method. The second enzyme this dissertation examines is HppE, which catalyzes the final step in the biosynthesis of the antibiotic, fosfomycin. Fosfomycin is a clinically useful antibiotic for the treatment of limb-threatening diabetic foot infections and urinary tract infections. Chemically speaking, HppE is unique for two reasons. First, HppE's epoxidation differs from Nature's standard method of epoxide formation by alkene oxidation, where the epoxide oxygen is derived from molecular oxygen. For HppE, the epoxide is formed through the dehydrogenation of a secondary alcohol; thus the epoxide oxygen is derived from the substrate. Second, HppE is a unique member of the mononuclear non-heme iron-dependent family of enzymes. HppE differs from all other mononuclear non-heme iron-dependent enzymes by requiring NADH and an external electron mediator for turnover but not requiring [alpha]-KG, pterin, ascorbate, or an internal iron-sulfur cluster. After a study was published on the activity of zinc-reconstituted HppE from Streptomyces wedmorensis, the proposed iron and NADH dependent mechanism of HppE was reevaluated and was reconfirmed. The HppE from Pseudomonas syringae (Ps-HppE) was also purified and was characterized biochemically and spectroscopically. The results of [²H] and [¹⁸O]-KIE studies on Ps-HppE are also reported.