Chlorophyll Z (Chl(Z)) is a redox-active chlorophyll (Chl) which is photooxidized by low-temperature (< 100 K) illumination of photosystem II (PSII) to form a cation radical, Chlz+. This cofactor has been proposed to be an 'accessory' Chl in the PSII reaction center and is expected to be buried in the transmembrane region of the PSII complex, but the location of Chlz is unknown. A series of single-replacement site-directed mutants of PSII were made in which each of two potentially Chl-ligating histidines, D1-H118 or D2-H117, was substituted with amino acids which varied in their ability to coordinate Chl. Assays of the wild-type and mutant strains showed parallel phenotypes for the D1-118 and D2-117 mutants: noncoordinating or poorly coordinating residues at either position decreased photosynthetic competence and impaired assembly of PSII complexes. Only the mutants substituted with glutamine (D1-H118Q and D2-H117Q) had phenotypes comparable to the wild-type strain. The Chlz+ cation was characterized by low-temperature electron paramagnetic resonance (EPR), near-infrared (IR) absorbance, and resonance Raman (RR) spectroscopies in wild-type, H118Q, and H117Q PSII core complexes. The quantum yield of Chlz+ formation is the same (~2.5% per saturating flash at 77 K) for wild-type, H118Q, and H117Q, indicating that its efficiency of photooxidation is unchanged by the mutations. Similarly, the EPR and near-IR absorbance spectra of Chlz+ are insensitive to the mutations made at D1-118 and D2-117. In contrast, the RR signature of Chlz+ in H118Q PSII, obtained by selective near-IR excitation into the Chlz+ cation absorbance band, is significantly altered relative to wild-type PSII while the RR spectrum of Chlz+ in the H117Q mutant remains identical to wild- type. Shifts in the RR spectrum of Chlz+ in H118Q reflect a change in the structure of the Chl ring, most likely due to a perturbation of the core size and/or extent of doming caused by a change in the axial ligand to Mg(II). Thus, we conclude that the axial ligand to Chlz is H118 of the D1 polypeptide. Furthermore, we propose that H117 of the D2 polypeptide is the ligand to a homologous redox-inactive accessory Chl which we term Chl(D). The Chl Z and D terminology reflects the 2-fold structural symmetry of PSII which is apparent in the redox-active tyrosines, Y(Z) and Y(D), and the active/inactive branch homology of the D1/D2 polypeptides with the L/M polypeptides of the bacterial reaction center.
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