Identification of histidine 118 in the D1 polypeptide of photosystem II as the axial ligand to chlorophyll Z

Experiments were performed to identify two chlorophyll-ligating histidines, D1-H118 and D2-H117, as ligands to accessory chlorophylls in the reaction center of photosystem II (PSII). Results showed that D1-H118 was the axial ligand to the redox-active chlorophyll Z, while D2-H117 was identified as the axial ligand of the redox-inactive accessory chlorophyll. These results illustrate the two-fold structural symmetry of PSII similar to those shown in redox-active tyrosines.