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Kinetic models of redox-coupled proton pumping.

ABSTRACT: Cytochrome c oxidase, the terminal enzyme of the respiratory chain, pumps protons across the inner mitochondrial membrane against an opposing electrochemical gradient by reducing oxygen to water. To explore the fundamental mechanisms of such redox-coupled proton pumps, we develop kinetic models at the single-molecule level consistent with basic physical principles. We demonstrate that pumping against potentials >150 mV can be achieved purely through electrostatic couplings, given an asymmetric arrangement of charge centers; however, nonlinear gates are essential for highly efficient real enzymes. The fundamental requirements for proton pumping identified here highlight a possible evolutionary origin of cytochrome c oxidase pumping. The general design principles are relevant also for other molecular machines and suggest future applications in biology-inspired fuel cells.

SUBMITTER: Kim YC

PROVIDER: S-EPMC1892981 | biostudies-literature | 2007 Feb

REPOSITORIES: biostudies-literature

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Kinetic models of redox-coupled proton pumping.

Kim Young C YC Wikström Mårten M Hummer Gerhard G

Proceedings of the National Academy of Sciences of the United States of America 20070207 7

Cytochrome c oxidase, the terminal enzyme of the respiratory chain, pumps protons across the inner mitochondrial membrane against an opposing electrochemical gradient by reducing oxygen to water. To explore the fundamental mechanisms of such redox-coupled proton pumps, we develop kinetic models at the single-molecule level consistent with basic physical principles. We demonstrate that pumping against potentials >150 mV can be achieved purely through electrostatic couplings, given an asymmetric a ...[more]

PMID: 17287344

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Project description:To test the effect of varying the proton donor-acceptor distance in proton-coupled electron transfer (PCET) reactions, the oxidation of a bicyclic amino-indanol (2) is compared with that of a closely related phenol with an ortho CPh(2)NH(2) substituent (1). Spectroscopic, structural, thermochemical, and computational studies show that the two amino-phenols are very similar, except that the O···N distance (d(ON)) is >0.1 Å longer in 2 than in 1. The difference in d(ON) is 0.13 ± 0.03 Å from X-ray crystallography and 0.165 Å from DFT calculations. Oxidations of these phenols by outer-sphere oxidants yield distonic radical cations (•)OAr-NH(3)(+) by concerted proton-electron transfer (CPET). Simple tunneling and classical kinetic models both predict that the longer donor-acceptor distance in 2 should lead to slower reactions, by ca. 2 orders of magnitude, as well as larger H/D kinetic isotope effects (KIEs). However, kinetic studies show that the compound with the longer proton-transfer distance, 2, exhibits smaller KIEs and has rate constants that are quite close to those of 1. For example, the oxidation of 2 by the triarylamminium radical cation N(C(6)H(4)OMe)(3)(•+) (3a(+)) occurs at (1.4 ± 0.1) × 10(4) M(-1) s(-1), only a factor of 2 slower than the closely related reaction of 1 with N(C(6)H(4)OMe)(2)(C(6)H(4)Br)(•+) (3b(+)). This difference in rate constants is well accounted for by the slightly different free energies of reaction: ΔG° (2 + 3a(+)) = +0.078 V versus ΔG° (1 + 3b(+)) = +0.04 V. The two phenol-amines do display some subtle kinetic differences: for instance, compound 2 has a shallower dependence of CPET rate constants on driving force (Brønsted α, Δ ln(k)/Δ ln(K(eq))). These results show that the simple tunneling model is not a good predictor of the effect of proton donor-acceptor distance on concerted-electron transfer reactions involving strongly hydrogen-bonded systems. Computational analysis of the observed similarity of the two phenols emphasizes the importance of the highly anharmonic O···H···N potential energy surface and the influence of proton vibrational excited states.

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Kinetic models of redox-coupled proton pumping.

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Kinetic models of redox-coupled proton pumping.

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