title: Analysis of the kinetics of the membrane potential generated by cytochrome c oxidase upon single electron injection.
abstract: In a recent work from this group (Popovic, D. M.; Stuchebrukhov A. A. FEBS Lett. 2004, 566, 126), a model of proton pumping by cytochrome c oxidase (CcO) was proposed. The key element of the model is His291 (bovine notation), a histidine ligand to enzyme's CuB redox center, which plays the role of the pump element. The model assumes that upon electron transfer between heme a and the binuclear catalytic center of the enzyme, two sequential proton transfers occur: First, a proton from Glu242 is transferred to an unprotonated His291, then a second proton, after reprotonation of Glu242 from the negative side of the membrane, is transferred to a hydroxyl group in the binuclear center, a water molecule is formed, and the first proton, due to proton-proton repulsion, is expelled from His291 to the positive side of the membrane, resulting in a pumping event. In the process the free energy of water formation (i.e., reduction of oxygen) is transformed into a proton gradient across the membrane. The model possesses specific kinetic features. It assumes, for example, that upon electron transfer the first proton is transferred to the proton-loading site of the pump, His291, and not to the catalytic center of the enzyme. Here, we analyze the kinetic properties of the proposed model, and calculate the time dependence of the membrane potential generated by CcO upon a single electron injection into the enzyme. These data are directly compared with recent experimental measurements of the membrane potential generated by CcO. Specifically, F to O, and O to E transitions will be discussed. Several enzymes from different organisms (bovine, two bacterial enzymes, and several mutants) are compared and discussed in detail. The kinetic description, however, is phenomenological, and does not include explicitly the nature of the groups involved in proton translocation, except in terms of their position depth within the membrane; thus, the kinetic equations developed here are in fact describe a generic model, similar, e.g., to that proposed earlier by Peter Rich (P.R. Rich, Towards an understanding of the chemistry of oxygen reduction and proton translocation in the iron-copper respiratory oxidases. Aust. J. Plant Physiol. 22 (1995) 479-486), and which is based on the idea of displacement of the pumped protons by the chemical ones.
authors.affiliation: Chemistry Division, Argonne National Laboratory, Argonne, IL 60439, USA.
authors.forename: D M
authors.initials: DM
authors.lastname: Medvedev
authors.forename: E S
authors.initials: ES
authors.lastname: Medvedev
authors.forename: A I
authors.initials: AI
authors.lastname: Kotelnikov
authors.forename: A A
authors.initials: AA
authors.lastname: Stuchebrukhov
mesh_terms: D000818:Animals; D002417:Cattle; D004579:Electron Transport; D003576:Electron Transport Complex IV / Q000235:genetics / Q000378:metabolism*; D007700:Kinetics; D008564:Membrane Potentials; D008954:Models, Biological; D009154:Mutation / Q000235:genetics; D010231:Paracoccus denitrificans / Q000201:enzymology / Q000378:metabolism; D011522:Protons; D012242:Rhodobacter sphaeroides / Q000201:enzymology / Q000378:metabolism
chemical_list: D011522:Protons; D003576:Electron Transport Complex IV