Electric fields control water-gated proton transfer in cytochrome c oxidase

Significance

Cytochrome c oxidase (C cO) powers aerobic life by reducing oxygen to water. This redox reaction creates a proton motive force across a biological membrane that drives the synthesis of adenosine triphosphate (ATP). C cO transfers the protons both across the membrane and to its active site responsible for oxygen reduction, but the gating principles of this reaction remain unsolved. Here we show that internal redox changes in C cO create orientated electric fields that sort the protons along the chemical and pumping pathways, while preventing back leakage reactions. These redox-triggered electric fields show distinct similarities to other energy-converting enzymes and may be a general principle of enzyme catalysis.

Abstract

Aerobic life is powered by membrane-bound enzymes that catalyze the transfer of electrons to oxygen and protons across a biological membrane. Cytochrome c oxidase (C cO) functions as a terminal electron acceptor in mitochondrial and bacterial respiratory chains, driving cellular respiration and transducing the free energy from O ₂ reduction into proton pumping. Here we show that C cO creates orientated electric fields around a nonpolar cavity next to the active site, establishing a molecular switch that directs the protons along distinct pathways. By combining large-scale quantum chemical density functional theory (DFT) calculations with hybrid quantum mechanics/molecular mechanics (QM/MM) simulations and atomistic molecular dynamics (MD) explorations, we find that reduction of the electron donor, heme a, leads to dissociation of an arginine (Arg438)–heme a ₃ D-propionate ion-pair. This ion-pair dissociation creates a strong electric field of up to 1 V Å ⁻¹ along a water-mediated proton array leading to a transient proton loading site (PLS) near the active site. Protonation of the PLS triggers the reduction of the active site, which in turn aligns the electric field vectors along a second, “chemical,” proton pathway. We find a linear energy relationship of the proton transfer barrier with the electric field strength that explains the effectivity of the gating process. Our mechanism shows distinct similarities to principles also found in other energy-converting enzymes, suggesting that orientated electric fields generally control enzyme catalysis.