Schematic of CcO. A single electron enters the enzyme at site and travels subsequently to sites , , and, finally, to site . Protons, taken at the -side of the membrane, move to site , which can donate protons both to the pump site and to the catalytic site . The prepumped proton is transferred from site to the -side of the membrane via site .
Kinetics of electron and proton transfers in CcO for , , , and . The time axis (in ms) is shown in a logarithmic scale starting at . The process begins at , when a single electron populates site , and a single proton is located on site . (a) Time dependence of the electron populations (blue dotted line), (green dashed line), (red dashed-dotted line), and (black continuous line). (b) The proton populations, (blue dotted line), (red dashed-dotted line), (green dashed line), and (black continuous line), vs time. (c) The number of pumped protons, , as a function of time. The first phase of the process, where the electron moves from site to site , corresponds to the maximum of the population at the moment . In the second phase, both the electron population of the -site, , and the proton population of the -site, , peak at . The third phase of the process is marked by the significant population of the proton site and the electron site at . In this phase site is depopulated, and the prepumped proton is partially transferred to the -side of the membrane, . In the final phase, at , the electron site and the catalytic proton site are occupied, , , and about one proton is translocated to the -side of the membrane.
The total energy of the system, (black continuous line), and the average energy of the electron, , as functions of time (in ms, logarithmic scale). The electron energy is varied in the range from at , to the value at . The whole electron-proton system dissipates less energy, , than its electron component, which loses about 550 meV during the pumping process, indicating the energy transfer to the proton subsystem.
The efficiency of the pump, , where , as a function of the transmembrane voltage at the temperatures (blue dashed line); (green continuous line), and (red dashed-dotted line). For the physiological range of transmembrane voltages, , the pump operates with a maximum efficiency at room temperatures . At high temperatures and high enough transmembrane voltages, the efficiency takes negative values, suggesting that at these conditions the protons flow back, from the positive to the negative side of the membrane.
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