Evidence for C-H cleavage by an iron-superoxide complex in the glycol cleavage reaction catalyzed by myo-inositol oxygenase

摘要

myo-Inositol oxygenase (MIOX) activates O-2 at a mixed-valent nonheme diiron(II/III) cluster to effect oxidation of its cyclohexan-(1,2,3,4,5,6-hexa)-ol substrate [myo-inositol (MI)] by four electrons to D-glucuronate. Abstraction of hydrogen from C-1 by a formally (superoxo)diiron(III/III) intermediate was previously proposed. Use of deuterium-labeled substrate, 1,2,3,4,5,6-[H-2](6)-lMI (D-6-MI), has now permitted initial characterization of the C-H-cleaving intermediate. The MIOX-1,2,3,4,5,6-[H-2](6)-MI complex reacts rapidly and reversibly with O-2 to form an intermediate, G, with a g = (2.05, 1.98, 1.90) EPR signal. The rhombic g-tensor and observed hyper-fine coupling to Fe-57 are rationalized in terms of a (superoxo)diiron(III/III) structure with coordination of the superoxide to a single iron. G decays to H, the intermediate previously detected in the reaction with unlabeled substrate. This step is associated with a kinetic isotope effect of >= 5, showing that the superoxide-level complex does indeed cleave a C-H(D) bond of MI.