Figure 4 - full size

 

Figure 4.
Fig. 4. Proposed proton-pumping mechanism. The iron, porphyrin, and formyl side group of heme a are shown by Fe[a], Pr, and CHO, respectively. The COOH on Pr denotes one of the propionate groups of heme a. The brackets ([]1+ and[]0) indicate the net charge of the six-coordinated heme a. The diagrams shadowed and in the dotted squares show the structures in the stable and intermediate states, respectively. The thick arrows in a and d-f and the thin arrows in d and e indicate the electrostatic influence of the net positive charge of heme a and the proton transfers upon heme a oxidation, respectively. The dotted lines in the diagrams denote the hydrogen bond network connecting Arg-38 with Asp-51, including the peptide bond that blocks the reverse proton transfer from the intermembrane side.