The peripheral accessory chlorophylls (Chls) of the photosystem II (PSII) reaction center (RC) are coordinated by a pair of symmetry-related histidine residues (D1-H118 and D2-H117). These Chls participate in energy transfer from the proximal antennae complexes (CP43 and CP47) to the RC core chromophores. In addition, one or both of the peripheral Chls are redox-active and participate in a low-quantum-yield electron transfer cycle around PSII. We demonstrate that conservative mutations of the D2-H117 residue result in decreased Chl fluorescence quenching efficiency attributed to reduced accumulation of the peripheral accessory Chl cation, Chlz+. In contrast, identical symmetry-related mutations at residue D1-H118 had no effect on Chl fluorescence yield or quenching kinetics. Mutagenesis of the D2-H117 residue also altered the line width of the Chlz+ EPR signal, but the line shape of the D1-H118Q mutant remained unchanged. The D1-H118 and D2-H117 mutations also altered energy transfer properties in PSII RCs. Unlike wild type or the D1-H118Q mutant, D2-H117N RCs exhibited a reduced CD doublet in the red region of Chi absorbance band, indicative of reduced energetic coupling between P680 and the peripheral accessory Chl. In addition, transient absorption measurements of D2-H117N RCs, excited on the blue side of the Chl absorbance band, exhibited a (≈400 fs) pheophytin Qx band bleach lifetime component not seen in wild-type or D1-H118Q RCs. The origin of this component may be related to delayed fast-energy equilibration of the excited state between the core pigments of this mutant.
|Number of pages||6|
|Journal||Proceedings of the National Academy of Sciences of the United States of America|
|Publication status||Published - Mar 19 2002|
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