The electron distribution in the “activated” state of cytochrome c oxidase

Abstract

Cytochrome c oxidase catalyzes reduction of O2 to H2O at a catalytic site that is composed of a copper ion and heme group. The reaction is linked to translocation of four protons across the membrane for each O2 reduced to water. The free energy associated with electron transfer to the catalytic site is unequal for the four electron-transfer events. Most notably, the free energy associated with reduction of the catalytic site in the oxidized cytochrome c oxidase (state O) is not sufficient for proton pumping across the energized membrane. Yet, this electron transfer is mechanistically linked to proton pumping. To resolve this apparent discrepancy, a high-energy oxidized state (denoted OH) was postulated and suggested to be populated only during catalytic turnover. The difference between states O and OH was suggested to be manifested in an elevated midpoint potential of CuB in the latter. This proposal predicts that one-electron reduction of cytochrome c oxidase after its oxidation would yield re-reduction of essentially only CuB. Here, we investigated this process and found ~5% and ~6% reduction of heme a3 and CuB, respectively, i.e. the apparent redox potentials for heme a3 and CuB are lower than that of heme a.

Activation of O2and NO in heme-copper oxidases – mechanistic insights from computational modelling

“…Several experimental investigations on different A-type of CcOs, designed to prepare the O H state, could not find an increase in the Cu B (II) midpoint potential as compared to the resting O state. [76][77][78] The discussion so far has mainly been concerned with the reduction potentials in the A family. Another puzzling experimental result is the very low Fe b3 (III) reduction potentials observed for the cbb 3 type of CcOs.…”

“…11 corresponds to the situation without any gradient across the membrane, and each reduction step is shown as a single step, including both electron and proton transfer. Since the low-spin heme a has a reduction potential close to that of the ultimate cytochrome c donor, 78 the electron transfer step from cytochrome c to low-spin heme a is not shown separately, but included implicitly in each reduction step, with electron transfer from cytochrome c to the BNC. All barriers associated with the reduction steps are mainly due to proton motion through the protein, and in each reduction step the transfer of both chemical and pumped protons are shown as one single barrier.…”

A Systematic DFT Approach for Studying Mechanisms of Redox Active Enzymes

“…By subtracting the parameterized energy from each O-H bond strength, the exergonicity relative to the cytochrome c donor is obtained, and by comparing to the midpoint potential of cytochrome c (0.25 V) the midpoint potential can be estimated for each cofactor. The first result to be discussed concerns the proton coupled reduction potential for Cu B , for which the experimental measurements have given low values, 0.2–0.4 V (Jancura et al, 2006; Brand et al, 2007; Vilhjámsdóttir et al, 2018). In contrast, the previous calculations indicate a much higher value (0.9–1.0 V) during catalytic turnover (Blomberg and Siegbahn, 2015a), and an explanation for the low experimental values has been suggested based on the computational results (Blomberg and Siegbahn, 2015b).…”

Copper and copper nanoparticles toxicity and their impact on basic functions in the body

“…Because copper can change oxidative status from I+ to II+ and back (the oxidative status III+ is uncommon in metabolism), it is an ideal cofactor for oxidoreductases. The enzymes from the group of oxidoreductases: Cu-Zn superoxide dismutase catalyzing dismutation of superoxide to oxide or hydrogen peroxide (14,15), nitrite reductase catalyzing reduction of nitrite to nitric oxide (16,17), Cu containing amine oxidase catalyzes oxidative deamination of primary amines to aldehydes with the contemporary release of hydrogen peroxide and ammonia (18)(19)(20)(21), mitochondrial cytochrome c oxidase (complex IV) as a part of oxidative phosphorylation (22,23), tyrosinase responsible for oxidation of aromatic structures like neurotransmitter dopamine and amino acid tyrosine to quinones (24) and bilirubin oxidase involved in the oxidative conversion of bilirubin to biliverdin (25,26) can be mentioned as the most meaningful one. Principle of Cu-Zn superoxide dismutase catalyzed reaction with indicated oxidative states of copper is given in Figure 1.…”