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Research during the 54 years since the discovery of non-heme iron oxygenase enzymes has revealed a remarkable diversity in the strategies used for O2 activation. Many of these are represented in dioxygenase enzymes that introduce oxygen atoms from O2 into aromatic compounds (1). These include direct oxygen activation by external electron transfer to the redox-active mononuclear iron in Rieske dioxygenases, Fe(II)-mediated internal electron transfer from the substrate to O2 in extradiol dioxygenases, and Fe(III)-mediated spin inversion during internal electron transfer from substrate to O2 in intradiol dioxygenases. Recently, the exploration of these mechanisms has moved forward through the combined use of transient kinetics, spectroscopic characterizations, high level computations, and X-ray crystallographic techniques that allow the visualization of discrete intermediates. For each enzyme class, catalysis is found to depend on an elegant balance between the Lewis acidity, redox, and ligand exchange properties of the metal, the traditional structural and catalytic properties of the second sphere residues of the active site, and the less-appreciated contributions from structural dynamics that encompass the entire enzyme. Together, these mediate not only specific catalysis, but just as importantly, regulated catalysis that mitigates against the activation of O2 without attendant substrate oxygenation.
(supported by NIH GM24689)