Fig. 8

ACIII as a redox-driven proton pump. Proposed coupling between electron transfer and proton translocation. Upon quinol oxidation (black arrows), electrons are transferred to FeS1 and subsequently through the electron wires (red arrows); electrons conducted through the heme wire reduce a soluble electron carrier such as cytochrome c or HiPIP, or the caa₃ oxygen reductase, as observed before^{8, 10, 11}; clusters FeS2–4 may be involved in further electron transfer (red dashed arrow). Concurrently, a protonation or conformation change of Asp169^C at the quinol-binding site may propagate through the conserved, neighboring residues Glu122^D, Ser245^C, and Tyr284^F (colored circles). The conformational changes of these residues, located in one plane at the end of the putative proton half-channels in ActC and ActF, will allow further conformational changes at the proton-conducting channels for proton transport across the membrane (blue arrows). ActH was not included for clarity