The timing of proton migration in membrane-reconstituted cytochrome c oxidase

Abstract

In mitochondria and aerobic bacteria energy conservation involves electron transfer through a number of membrane-bound protein complexes to O2. The reduction of O2, accompanied by the uptake of substrate protons to form H2O, is catalyzed by cytochrome c oxidase (CcO). This reaction is coupled to proton translocation (pumping) across the membrane such that each electron transfer to the catalytic site is linked to the uptake of two protons from one side and the release of one proton to the other side of the membrane. To address the mechanism of vectorial proton translocation, in this study we have investigated the solvent deuterium isotope effect of proton-transfer rates in CcO oriented in small unilamellar vesicles. Although in H2O the uptake and release reactions occur with the same rates, in D2O the substrate and pumped protons are taken up first (D Х 200 s, ''peroxy'' to ''ferryl'' transition) followed by a significantly slower proton release to the other side of the membrane (D Х 1 ms). Thus, the results define the order and timing of the proton transfers during a pumping cycle. Furthermore, the results indicate that during CcO turnover internal electron transfer to the catalytic site is controlled by the release of the pumped proton, which suggests a mechanism by which CcO orchestrates a tight coupling between electron transfer and proton translocation.heme-copper oxidases ͉ electron transfer ͉ membrane protein ͉ respiratory chain I n membrane-bound proton pumps the endergonic proton translocation across the membrane is driven by an exergonic reaction, e.g., electron transfer from a low-potential donor to a high-potential acceptor. The protons are translocated through proton-transfer pathways, typically composed of networks of hydrogen-bonded protonatable and polar amino acid side chains and bound water molecules, that span the transmembrane part of the protein. To prevent short-circuiting of the electrochemical gradient it is crucial that there is never simultaneous access (i.e., a direct contact) to the two membrane sides. One way to control the proton access is to alter the position of e.g., an amino acid side chain such that the group exchanges protons rapidly with either one side or the other side of the membrane (but never both sides simultaneously), defining an input and an output state (for review, see refs. 1-6). Proton pumping would be accomplished, for example, if the reaction that drives proton translocation is energetically coupled to changes in the pK a value of the protonatable group such that it has a high pK a in the input state and a low pK a in the output state. In the discussion below, we will refer to such a group as a ''pumping element.'' Cytochrome c oxidase (CcO), which belongs to the hemecopper oxidase superfamily, is an example of a redox-driven proton pump (for review, see refs 4, 5, and 7-12). The CcOs are found in the cytoplasmic membrane in bacteria or the inner membrane of mitochondria, where they catalyze the reduction of molecular oxygen to water. The electrons are donated by...

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“…When the flow-flash reaction is performed in H 2 O buffer, the rates of proton uptake and release during the P R →F transition are essentially the same, and they appear coincident in time (Salomonsson et al 2005). This is observed in phospholipid vesicles.…”

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