The 2-oxoglutarate-dependent hydroxylases and halogenases employ similar reaction mechanisms involving hydrogen-abstracting Fe(IV)-oxo intermediates. A crucial distinction between their active sites is the replacement of the carboxylate residue from the "His2(Asp/Glu)1 facial triad" of iron ligands found in the hydroxylases by alanine in the halogenases, which permits a halide ion to coordinate.1 The divergent outcomes have been rationalized to result from alternative "radical-rebound" steps (Scheme): hydroxyl (OH) radical in the hydroxylases (top) versus halogen (X) radical in the halogen-ases (bottom).1,2 The reportedly strict rebound selectivity in the latter could reflect, at least in part, an inherently greater reactivity of the coordinated halogen.3 Accordingly, one might anticipate hydroxylation by a halogenase variant with the carboxylate residue restored by mutagenesis, but the A118E variant of halogenase SyrB2 reportedly does not mediate hydroxylation.1 I will present an experimental analysis of the reactions of SyrB2 and its carboxylate-restored A118E variant with alternative substrates, which reveals the mechanism by which the enzyme ensures selective halogenation.
(1) Blasiak, L. C.; Vaillancourt, F. H.; Walsh, C. T.; Drennan, C. L. Nature 2006, 440, 368.
(2) Hanauske-Abel, H. M.; GŸnzler, V. J. Theor. Biol. 1982, 94, 421.
(3) Rohde, J.-U.; Stubna, A.; Bominaar, E. L.; MŸnck, E.; Nam, W.; Que, L., Jr. Inorg. Chem. 2006, 45, 6435.