title: How cytochrome c oxidase can pump four protons per oxygen molecule at high electrochemical gradient.
abstract: Experiments have shown that the A-family cytochrome c oxidases pump four protons per oxygen molecule, also at a high electrochemical gradient. This has been considered a puzzle, since two of the reduction potentials involved, Cu(II) and Fe(III), were estimated from experiments to be too low to afford proton pumping at a high gradient. The present quantum mechanical study (using hybrid density functional theory) suggests a solution to this puzzle. First, the calculations show that the charge compensated Cu(II) potential for CuB is actually much higher than estimated from experiment, of the same order as the reduction potentials for the tyrosyl radical and the ferryl group, which are also involved in the catalytic cycle. The reason for the discrepancy between theory and experiment is the very large uncertainty in the experimental observations used to estimate the equilibrium potentials, mainly caused by the lack of methods for direct determination of reduced CuB. Second, the calculations show that a high energy metastable state, labeled EH, is involved during catalytic turnover. The EH state mixes the low reduction potential of Fe(III) in heme a3 with another, higher potential, here suggested to be that of the tyrosyl radical, resulting in enough exergonicity to allow proton pumping at a high gradient. In contrast, the corresponding metastable oxidized state, OH, is not significantly higher in energy than the resting state, O. Finally, to secure the involvement of the high energy EH state it is suggested that only one proton is taken up via the K-channel during catalytic turnover.
authors.affiliation: Department of Organic Chemistry, Arrhenius Laboratory, Stockholm University, SE-106 91 Stockholm, Sweden. Electronic address: mb@organ.su.se.
authors.forename: Margareta R A
authors.initials: MRA
authors.lastname: Blomberg
authors.affiliation: Department of Organic Chemistry, Arrhenius Laboratory, Stockholm University, SE-106 91 Stockholm, Sweden.
authors.forename: Per E M
authors.initials: PEM
authors.lastname: Siegbahn
mesh_terms: D002384:Catalysis; D020134:Catalytic Domain; D003198:Computer Simulation; D003300:Copper / Q000378:metabolism; D003576:Electron Transport Complex IV / Q000737:chemistry / Q000378:metabolism*; D004735:Energy Transfer; D007501:Iron / Q000378:metabolism; D008954:Models, Biological; D008958:Models, Molecular; D010084:Oxidation-Reduction; D010100:Oxygen / Q000737:chemistry / Q000378:metabolism*; D011487:Protein Conformation; D017494:Proton Pumps / Q000737:chemistry / Q000378:metabolism*; D011522:Protons
chemical_list: D017494:Proton Pumps; D011522:Protons; D003300:Copper; D007501:Iron; D003576:Electron Transport Complex IV; D010100:Oxygen
keywords: Cytochrome c oxidase; Density functional theory; Energy profile; Proton pumping; Reduction potential