Flavin-dependent Enzymes in Natural Product Biosynthesis

Natural products are biologically active metabolites produced by fungi, bacteria, and plants that have an extended application in pharmaceutical and chemical industries. Because of their chemical versatility, flavoenzymes are commonly involved in natural product biosynthetic pathways. This has given rise to the identification of flavoenzymes that are promising candidates for biomedical and biotechnical applications. This dissertation discusses the characterization of three flavoenzymes involved in natural product biosynthesis. The class B flavin-dependent monooxygenases S-monoooxygenase from Allium sativum (AsFMO) and N-hydroxylating monooxygenase from Streptomyces sp. XY332 (FzmM) were studied. Both enzymes perform heteroatom oxidation as part of allicin or fosfazinomycin biosynthesis respectively. AsFMO was predicted to oxidize S-allyl-L-cysteine (SAC) to alliin in allicin biosynthesis. Surprisingly, AsFMO exhibited negligible activity with SAC, and instead was highly active with allyl mercaptan and NADPH. This contradicted the initial proposal and suggested that AsFMO is involved in an alternative path producing allicin directly from allyl mercaptan. FzmM was identified to perform multiple N-oxidations which lead to the formation of a nitro group. FzmM performed a highly coupled and specific reaction with L-aspartate and NADPH to produce nitrosuccinate. Both AsFMO and FzmM followed a kinetic mechanism representative of class B flavin-dependent monooxygenases with a rapid pro-R stereospecific reduction and the formation of a C(4a)-hydroperoxyflavin intermediate during oxidation. In addition, the AsFMO structure was obtained and consisted of two domains for FAD and NADPH binding signature of class B monooxygenases. The biochemical and structural study of the Acinetobacter baumannii siderophore interacting protein (BauF) was also accomplished. This enzyme is essential in acinetobactin mediated iron assimilation and is important for virulence. The characterization of the binding and reduction of acinetobactin-ferric iron complex revealed that BauF is specific for this substrate and does not utilize NAD(P)H as an electron donor. The unique activity and structure of BauF can aid future drug design.