Fig. 1.

Schematic of CcO function. (A) Flux diagram of a Lundegårdh–Mitchell loop in which an electric potential across a protonically sealed membrane is created by taking up protons and electrons from opposite sides. (B) Flux diagram of a true proton pump. J_el is the electron flux, J_up is the proton uptake flux from the N side, J_pump is the net proton pumping flux to the P side, and J_prod is the product flux. Flux conservation requires that J_prod = J_el = J_up − J_pump. (C) CcO proton pump. Electron transfer from cytochrome c via Cu_A and heme a to the binuclear center (heme a₃ and Cu_B) is indicated in red. Light blue arrows indicate proton translocation, including uptake of both chemical and pumped protons from the negatively charged N side and release of pumped protons on the positively charged P side. Blue arrows indicate uptake of dioxygen and release of water. (D) Kinetic scheme of the three-site model. Circles and squares show proton and electron sites, respectively. Arrows indicate proton (light blue) and electron (red) transfer reactions with intrinsic rate coefficients κ_μν. The blue arrow denotes the irreversible product formation.