Highly Selective but Multifunctional Oxygenases in Secondary Metabolism

Biosynthesis of bioactive natural products frequently features oxidation at multiple sites. Starting from a relatively reduced chemical scaffold that is assembled by controlled polymerization of small precursors, for example, acetate or amino acids, a diverse range of redox reactions can generate very complex and highly oxygenated structures. Their formation often involves C-H activation reactions catalyzed by oxygenase enzymes, either monooxygenases or dioxygenases. The former category includes the cytochrome P450s and flavin-dependent oxygenases, whereas examples of the latter are the non-heme iron α-ketoglutarate-dependent oxygenases. Oxygenases can catalyze a plethora of reactions ranging from hydroxylations and epoxidations to dehydrogenations, cyclizations, and rearrangements. The specific transformations are usually possible only with the use of these enzymatic catalysts. Aside from the ability of oxygenases to specifically oxidize unactivated carbon skeletons, some have recently been demonstrated to possess a fascinating ability to catalyze multiple reactions in a highly ordered fashion at different sites starting with a single substrate molecule. In the past, oxygenases associated with secondary metabolite pathways were considered to be highly regio-, stereo-, and substrate specific, with one oxidizing enzyme encoded in the gene cluster corresponding to one oxidation location in the natural product itself. However, it is becoming progressively clear that this "one oxygenase, one oxidation site" relationship is not necessarily a valid assumption. Multifunctional oxidases are known to occur in higher plants, fungi, and bacteria. Natural product gene clusters that contain multifunctional oxidase enzymes are responsible for production of lovastatin (a cholesterol-lowering agent and precursor to simvastatin), scopolamine (an anticholinergic drug), and cytochalasin E (an angiogenesis inhibitor), among many others. As opposed to simply being substrate promiscuous, these enzymes show very high substrate specificity and catalyze several oxidative reactions in a single pathway, with each oxidation being a prerequisite for the next. The basis for their specificity and highly ordered sequence is not yet well understood. In the lovastatin pathway, LovA is a cytochrome P450 that introduces a double bond and a hydroxyl group. H6H is an α-ketoglutarate-dependent oxygenase that hydroxylates (-)-atropine and then closes the newly introduced oxygen onto a neighboring methylene to generate the epoxide of scopolamine. CcsB is a flavin-dependent Baeyer-Villigerase that converts a ketone to a carbonate by double oxidation, a reaction not possible without enzymes. Recent crystallographic studies of other multifunctional oxygenases, such as AurH, a cytochrome P450 from Streptomyces thioluteus involved in aureothin biosynthesis, have indicated a steric switch mechanism. After the initial hydroxylation reaction catalyzed by AurH, the enzyme is thought to undergo a substrate-induced conformational change. In this Account, advances in our knowledge of these fascinating multifunctional enzymes and their potential will be explored.

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