Fig. 4.
Trans-membrane heme ligation in the cytochrome b6f complex. The central Fe atom of heme bp and bn is axially ligated, respectively, by residues His86/His187 and His100/His202 which bridge the B and D trans-membrane helices of the cytochrome b subunit, Heme cn is covalently attached to the protein via Cys35. Heme cn is unique as it lacks an amino acid as axial ligand; central Fe-atom penta-coordinated. The only ligand of heme cn supplied by H2O or OH− ligand on heme bn side. Central Fe of heme cn separated by 4.0 Å from propionate oxygen of heme bn, resulting in electronic coupling, characterized by a high spin g =12 EPR signal (Zatsman et al. 2006; Baymann et al. 2007)) and an oxidase reaction with nitric oxide (Twigg et al. 2009).