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Two Distinct Mechanisms for C-C Desaturation by Iron(II)- and 2-(Oxo)glutarate-Dependent Oxygenases: Importance of α-Heteroatom Assistance.

著者 Dunham NP , Chang WC , Mitchell AJ , Martinie RJ , Zhang B , Bergman JA , Rajakovich LJ , Wang B , Silakov A , Krebs C , Boal AK , Bollinger JM
J Am Chem Soc.2018 Apr 30 ; ():.
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Hydroxylation of aliphatic carbons by non-heme Fe(IV)-oxo (ferryl) complexes proceeds by hydrogen-atom (H•) transfer (HAT) to the ferryl and subsequent coupling between the carbon radical and Fe(III)-coordinated oxygen (termed rebound). Enzymes that use H•-abstracting ferryl complexes for other transformations must either suppress rebound or further process hydroxylated intermediates. For olefin-installing C-C desaturations, it has been proposed that a second HAT to the Fe(III)-OH complex from the carbon alpha to the radical preempts rebound. Deuterium (H) at the second site should slow this step, potentially making rebound competitive. Desaturations mediated by two related L-arginine-modifying iron(II)- and 2-(oxo)glutarate-dependent (Fe/2OG) oxygenases behave oppositely in this key test, implicating different mechanisms. NapI, the L-Arg 4,5-desaturase from the naphthyridinomycin biosynthetic pathway, abstracts H• first from C5 but hydroxylates this site (leading to guanidine release) to the same modest extent whether C4 harbors H or H. By contrast, an unexpected 3,4-desaturation of L-homoarginine (L-hArg) by VioC, the L-Arg 3-hydroxylase from the viomycin biosynthetic pathway, is markedly disfavored relative to C4 hydroxylation when C3 (the second hydrogen donor) harbors H. Anchimeric assistance by N6 permits removal of the C4-H as a proton in the NapI reaction, but, with no such assistance possible in the VioC desaturation, a second HAT step (from C3) is required. The close proximity (≤ 3.5 Å) of both L-hArg carbons to the (hydr)oxo group in an x-ray crystal structure of VioC harboring a vanadium-based ferryl mimic supports and rationalizes the sequential-HAT mechanism. The results suggest that, although the sequential-HAT mechanism is feasible, its geometric requirements may ensure competing hydroxylation, thus explaining why nearly all natural substrates for Fe/2OG desaturases have α-heteroatoms.
PMID: 29708749 [PubMed - as supplied by publisher]
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