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Publication Abstract from PubMed

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 ((2)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 (1)H or (2)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 (2)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 A) 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 alpha-heteroatoms.

Two Distinct Mechanisms for C-C Desaturation by Iron(II)- and 2-(Oxo)glutarate-Dependent Oxygenases: Importance of alpha-Heteroatom Assistance.,Dunham NP, Chang WC, Mitchell AJ, Martinie RJ, Zhang B, Bergman JA, Rajakovich LJ, Wang B, Silakov A, Krebs C, Boal AK, Bollinger JM Jr J Am Chem Soc. 2018 Apr 30. doi: 10.1021/jacs.8b01933. PMID:29708749^[4]

From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine.