Coupling between Electron and Proton Transfer

Adelroth P, Sigurdson H, Halldn S, Brzezinski P. Kinetic coupling between electron and proton transfer in cytochrome c oxidase simultaneous measurements of conductance and absorbance changes. Proc Natl Acad Sci USA 1996 93 12292-7. 

Statistical-mechanical calculations have suggested that the hydrophobic cavity may contain 3-4 water molecules, at least transiently, and these may be crucial not only for proton transfer as such, but also in the mechanism of coupling between electron and proton transfer, as outlined below. Finally, it should be emphasised that the hydrophobic cavity is next to the binuclear oxygen reduction site, and that it is likely that the product of the redox reaction, water, is initially ejected into the hydrophobic cavity during the... 

The charge transfer properties of 1 were monitored optically by following the formation and decay of Zn porphyrin cation on picosecond time scales. The forward and reverse PCET rate constants of protonated 1 were = 5.0 x lO s and kt,ack = 1-0 x lO s h The deuterated analog of 1 gave a rate constant of kf j = 3.0 X 101 g-i and ki33 ]j= 6.2 x 10 s i, yielding KIEs of 1.7 and 1.6 for the forward and reverse PCET reactions, respectively. As theoretically elaborated [20, 95], it is this observed deuterium isotope effect that reveals coupling between electron and proton. 

Cyclic voltammetry studies reveal striking differences between complex 13 and the analogous complex [HFe(depp)(dmpm)(CH3CN)f (17) in which the NMe group of 13 has been replaced by a methylene group. At normal scan rates the Fe " couple is reversible for complex 17, but irreversible for 13. Scan rate dependence measurements and potential step experiments indicated that this difference in behavior arises from a rapid transfer of the proton of the Fe hydride to the N atom of the pendant base with a rate constant of 1.1 x 10 s at room temperature. This proton transfer results in an irreversible Fe " " couple at low scan rates. A similar process cannot occur for 17, and the Fe " " couple remains reversible, even at slow scan rates in the presence of an external base. These results indicate that pendant bases in the second coordination sphere can facilitate the coupling of electron and proton transfer reactions. 

When, however, the rates of electron and proton transfer are comparable and coupled to each other, intermediate slopes are obtained (between -0.5 and -1.0) [76-78]. Accordingly, the measured Marcus slopes of-0.72 and -0.71 support the assignment of a concerted PCET process for the oxidation of the substituted phenols by the Cu metal complexes. Additionally, KIEs of 1.21 to 1.56 are also consistent with an asynchronous PCET process in which there is some proton motion but a large ET component in the transition state. 

Three kinds of equilibrium potentials are distinguishable. A metal-ion potential exists if a metal and its ions are present in balanced phases, e.g., zinc and zinc ions at the anode of the Daniell element. A redox potential can be found if both phases exchange electrons and the electron exchange is in equilibrium for example, the normal hydrogen half-cell with an electron transfer between hydrogen and protons at the platinum electrode. In the case where a couple of different ions are present, of which only one can cross the phase boundary — a situation which may exist at a semiperme-able membrane — one obtains a so called membrane potential. Well-known examples are the sodium/potassium ion pumps in human cells. 

In addition to NAD and flavoproteins, three other types of electron-carrying molecules function in the respiratory chain a hydrophobic quinone (ubiquinone) and two different types of iron-containing proteins (cytochromes and iron-sulfur proteins). Ubiquinone (also called coenzyme Q, or simply Q) is a lipid-soluble ben-zoquinone with a long isoprenoid side chain (Fig. 19-2). The closely related compounds plastoquinone (of plant chloroplasts) and menaquinone (of bacteria) play roles analogous to that of ubiquinone, carrying electrons in membrane-associated electron-transfer chains. Ubiquinone can accept one electron to become the semi-quinone radical ( QH) or two electrons to form ubiquinol (QH2) (Fig. 19-2) and, like flavoprotein carriers, it can act at the junction between a two-electron donor and a one-electron acceptor. Because ubiquinone is both small and hydrophobic, it is freely diffusible within the lipid bilayer of the inner mitochondrial membrane and can shuttle reducing equivalents between other, less mobile electron carriers in the membrane. And because it carries both electrons and protons, it plays a central role in coupling electron flow to proton movement. 

These interfacial pH effects have been investigated by probing the voltammetry in buffered solutions. Figure 5.16 shows that for 1.0 < pFl < 10.6, E0 depends linearly on pH, with a slope of 63 3 mV. This value is indistinguishable from the slope of 59 mV pH-1 expected for a coupled proton/electron transfer and indicates that the H2Q species is produced when the monolayer is reduced. Between pH 10.6 and 12.0, the slope decreases to 25 4 mV pH-1, which compares favorably with the slope expected (29.5 mV pH-1) for a two-electron, one-proton transfer reaction. Therefore, over this pH range Q is reduced to HQ- and the p/12.0, E0 is independent of pH, thus indicating that the pKa of the HQ-/Q2- couple is 12.0 0.2. 

Krab, K. Wikstrom, M. (1987). Principles of coupling between electron transfer and proton translocation with special reference to proton-translocation mechanisms in cytochrome oxidase. Biochim. Biophys. Acta 895,25-39. 

Mitchell, P. Moyle, J. (1985). The role of ubiquinone and plastoquinone in chemiosmotic coupling between electron transfer and proton translocation in coenzyme Q. In (Lenaz, G., ed.) pp. 145-163, Wiley, Chichester. 

In biological electron transfer, the role of H+ depends on the strength of coupling between the electron and proton ... 

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