Spin-Regulated Inner-Sphere Electron Transfer Enables Efficient O—O Bond Activation in Nonheme Diiron Monooxygenase MIOX

Abstract

The combined molecular dynamics (MD) simulations and quantum mechanical/molecular mechanics (QM/MM) calculations have been performed to address the longstanding issue of the “dioxygen activation” by the nonheme diiron monooxygenase myo-inositol oxygenase (MIOX). MIOX utilizes a mixed-valence Fe2(III)Fe1(II) cluster for catalysis. It is well recognized that the Fe2(III) site is responsible for the substrate myo-inositol (MI) binding, while the Fe1(II) site is responsible for O2 binding and activation. However, it is enigmatic how the O–O bond of oxygen is reductively cleaved in the absence of additional reductants. In this study, we demonstrate a spin-regulated inner-sphere electron-transfer mechanism that is involved in the catalytic reactions of MIOX. Because of the Pauli principle and exchange-enhanced reactivity, the spin-regulated inner-sphere electron transfer enables the formation of an unprecedented Fe2(III)Fe1(II)-peroxyhemiketal intermediate that is responsible for the reductive O–O cleavage. In contrast to Fe1(III)-mediated O–O cleavage in the Fe2(II)Fe1(III)-peroxyhemiketal intermediate proposed previously, our calculations demonstrate that the proton transfer-triggered Fe1–O cleavage in Fe2(III)Fe1(II)-peroxyhemiketal intermediate is the most favorable pathway, leading to MI-OOH intermediate and the Fe1(II) species. The following Fe1(II)-mediated O–O homolysis in MI-OOH generates the substrate radical and Fe(III)–OH species, during which the Fe1(IV)═O intermediate would be bypassed. Thus, our calculations show that both Fe sites are cooperately involved in O2 activation in MIOX and such cooperation is well regulated by the spin-dependent inner-sphere electron transfer. These findings of O2 activation by MIOX may have far-reaching implications on other related nonheme diiron monooxygenases.

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