7lo1
FAD-dependent monooxygenase AfoD from A. nidulans
Structural highlights
FunctionAFOD_EMENI FAD-dependent monooxygenase; part of the gene cluster that mediates the biosynthesis of asperfuranone, a probable antitumor agent (PubMed:19199437). The polyketide synthase afoG is responsible for producing the 3,5-dimethyloctadienone moiety from acetyl-CoA, three malonyl-CoA, and two S-adenosyl methionines (SAM) (PubMed:19199437). The 3,5-dimethyloctadienone moiety is then loaded onto the SAT domain of afoE and extended with four malonyl-CoA and one SAM, which leads to the formation of 2,4-dihydroxy-6-(5,7-dimethyl-2-oxo-trans-3-trans-5-nonadienyl)-3-methylbenzaldehyde (compound 2) after reductive release and aldol condensation (PubMed:19199437). AfoD is the next enzyme in the biosynthesis sequence and hydroxylates the side chain at the benzylic position of compound 2 (PubMed:19199437). After benzylic hydroxylation, a furan ring is formed after five-member ring hemiacetal formation and water elimination (PubMed:19199437). AfoF and afoC are proposed to oxidize the R-diketone proton and to reduce the unconjugated carbonyl group, respectively, to generate asperfuranone (PubMed:19199437). Since no intermediates could be isolated from afoF and afoC deletants, the sequence of these two enzymes is not fully understood (PubMed:19199437). Moreover, since afoC deletant still produces a small amount of asperfuranone, other endogenous oxidoreductases might catalyze the same reaction with much less efficiency (PubMed:19199437).[1] Publication Abstract from PubMedControlling the selectivity of a reaction is critical for target-oriented synthesis. Accessing complementary selectivity profiles enables divergent synthetic strategies, but is challenging to achieve in biocatalytic reactions given enzymes' innate preferences of a single selectivity. Thus, it is critical to understand the structural features that control selectivity in biocatalytic reactions to achieve tunable selectivity. Here, we investigate the structural features that control the stereoselectivity in an oxidative dearomatization reaction that is key to making azaphilone natural products. Crystal structures of enantiocomplementary biocatalysts guided the development of multiple hypotheses centered on the structural features that control the stereochemical outcome of the reaction; however, in many cases, direct substitutions of active site residues in natural proteins led to inactive enzymes. Ancestral sequence reconstruction (ASR) and resurrection were employed as an alternative strategy to probe the impact of each residue on the stereochemical outcome of the dearomatization reaction. These studies suggest that two mechanisms are active in controlling the stereochemical outcome of the oxidative dearomatization reaction: one involving multiple active site residues in AzaH and the other dominated by a single Phe to Tyr switch in TropB and AfoD. Moreover, this study suggests that the flavin-dependent monooxygenases (FDMOs) adopt simple and flexible strategies to control stereoselectivity, which has led to stereocomplementary azaphilone natural products produced by fungi. This paradigm of combining ASR and resurrection with mutational and computational studies showcases sets of tools for understanding enzyme mechanisms and provides a solid foundation for future protein engineering efforts. Deciphering the evolution of flavin-dependent monooxygenase stereoselectivity using ancestral sequence reconstruction.,Chiang CH, Wymore T, Rodriguez Benitez A, Hussain A, Smith JL, Brooks CL 3rd, Narayan ARH Proc Natl Acad Sci U S A. 2023 Apr 11;120(15):e2218248120. doi: , 10.1073/pnas.2218248120. Epub 2023 Apr 4. PMID:37014851[2] From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine. References
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