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

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 O-2 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 O-2 activation in MIOX and such cooperation is well regulated by the spin-dependent inner-sphere electron transfer. These findings of O-2 activation by MIOX may have far-reaching implications on other related nonheme diiron monooxygenases.

Automated summary

The study demonstrates a spin-regulated inner-sphere electron transfer mechanism involved in catalytic reactions of MIOX, leading to the formation of an unprecedented Fe2(III)Fe1(II)-peroxyhemiketal intermediate responsible for reductive O-O cleavage. The cooperation of both Fe sites in O-2 activation in MIOX is well regulated by the spin-dependent inner-sphere electron transfer, which may have significant implications on other related nonheme diiron monooxygenases.