Photosystem II, water oxidation and electron transfer asymmetry
Photosystem II oxidizes water. Complex IV of the respiratory chain reduces oxygen to water. Sousa and colleagues (2016) compared the two, noting that in the bovine respirasome only one of two possible electron transfer branches is active, as in photosystem II. They said oxygen reduction in complex IV is in some ways the reverse of water oxidation in photosystem II. They suggested that it might be equally important in both cases to control the flow of four electrons.45 Both enzymes also take part in proton handling, which is covered below.67
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Complex IV as oxygen reducer
Complex IV is cytochrome c oxidase. Wallace (2013) listed three of its 13 polypeptides as encoded by mitochondrial DNA.1 Protasoni and Zeviani (2021) gave the same count, ten nuclear and three mtDNA subunits, and added a 14th subunit, NDUFA4, found in monomeric human complex IV.2 Borisov and Forte (2021) described the electron path. Electrons from cytochrome c are primarily accepted by CuA, then pass to heme a and on to the binuclear centre of heme a3 and CuB, where oxygen is reduced to water.3
One branch of two
Sousa, Mills, Vonck and Kühlbrandt (2016) reported that in the bovine respirasome only one of two possible electron transfer branches is active. They likened this to photosystem II, which has two near-symmetrical electron transfer chains.4 Citing Nelson and Yocum (2006), they stated that one of those chains has been turned off in evolution to avoid wasteful transfer to two identical quinol acceptors. They suggested that oxygen reduction in complex IV is in some ways the reverse of water oxidation. They proposed that it might matter in both cases to control the flow of four electrons by closing one of two paths.5 The wording is tentative.
Protons and the gradient
Proton handling is less settled. Morelli and colleagues (2019) noted that each oxygen reduced to water needs 4 H+. They reported a hypothesis that another 4 H+, eight in total, enter through the proton entrance half channel.6 They doubted that protons bound to water and protons moving as charge by the Grotthuss mechanism could be equivalent. On the photosynthetic side, Didaran and colleagues (2026) wrote that water oxidation at photosystem II and plastoquinol oxidation at cytochrome b6f deposit protons into the thylakoid lumen. This produces a gradient with chemical and electrical parts.7 They added that counter-ion fluxes typically dissipate the electrical part quickly, so continued deposition appears as lumen acidification.8
three (COI-III) of the 13 polypeptides of ETC complex IV (cytochrome c oxidase, COX)
Wallace DC, 2013 · Bioenergetics in human evolution and disease: implications for the origins of… · open at passage 20Our observation that in the respirasome only one of two possible electron transfer branches is active is reminiscent of the two near-symmetrical electron transfer chains in photosystem II
Sousa JS, Mills DJ, Vonck J, Kühlbrandt W, 2016 · Functional asymmetry and electron flow in the bovine respirasome · open at passage 25In some ways, oxygen reduction in complex IV is the reverse of the water oxidation reaction in photosystem II, and it might be equally important in both cases to control the flow of four electrons
Sousa JS, Mills DJ, Vonck J, Kühlbrandt W, 2016 · Functional asymmetry and electron flow in the bovine respirasome · open at passage 25for each molecule of oxygen reduced to water they need 4 H+, and it is hypothesized that another 4 H+ (for a total of 8) enter through this channel.
Morelli AM, Ravera S, Calzia D, Panfoli I, 2019 · An update of the chemiosmotic theory as suggested by possible proton currents… · open at passage 31in mammals it is composed of 13 subunits, 10 nuclear-encoded and 3 encoded by the mtDNA (MTCO1, MTCO2 and MTCO3), which form the functional core of the complex
Protasoni M, Zeviani M, 2021 · Mitochondrial Structure and Bioenergetics in Normal and Disease Conditions · open at passage 79Electrons from cytochrome c primarily accepted by CuA are transferred to heme a and then to the catalytic binuclear center composed of heme a3 and CuB in which the reduction of oxygen to water occurs
Borisov VB, Forte E, 2021 · Impact of Hydrogen Sulfide on Mitochondrial and Bacterial Bioenergetics · open at passage 18Water oxidation at photosystem II (PSII) and proton-coupled plastoquinol oxidation at the cytochrome b6f complex deposit protons into the thylakoid lumen, generating a pmf with two thermodynamic components
Didaran F, MacPherson S, Cherestes A, Zohrabi S, Lefsrud M, 2026 · Proton motive force partitioning links energy and redox balance to… · open at passage 1PSII water oxidation releases protons to the lumen and cytochrome b6f transfers additional protons to the lumen during plastoquinol oxidation via the Q-cycle.
Didaran F, MacPherson S, Cherestes A, Zohrabi S, Lefsrud M, 2026 · Proton motive force partitioning links energy and redox balance to… · open at passage 23