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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.
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