PARENT SESSION
Posters P6A Type II reaction centres: Excited state dynamics and donor side. Abstracts (313-346)

Altered charge transfer yield in Photosystem II D1-E130-pheophytin mutants. Ling Xiong¹, Haether Johnston², Michael Seibert³, Michael Wasielewski⁴, Terry Gustafson², Richard Sayre^*,1, ¹ Department of Plant Cellular and Molecualr Biology, Columbus, Ohio, USA² Department of Chemistry, Columbus, Ohio, USA³ National Renewable Energy Lab, Golden, Colorado, USA⁴ Department of Chemistry, Evanston, Illinois, USA

ABSTRACT- The primary electron acceptor in photosystem II is pheophytin (Pheo). The photosystem II (PSII) reaction center complex contains two Pheos that occupy mirror image positions in two potential electron transfer chains that cross the membrane. Only the Pheo located near the QA plastoquinone-binding site participates in electron transfer in intact PSII complexes, however. To determine the protein environmental factors which differentiate the functions of these two Pheos we have mutagenized glutamate residue 130 of the D1 protein which forms a strong hydrogen bond to the ring V carbonyl of the active branch Pheo-active. Replacement of the D1-E130 residue with a leucine residue results in a 90% reduction in oxygen evolving activity associated with the loss of variable chlorophyll (Chl) a fluorescence in intact membranes. Spectroscopic analyses of PSII reaction center particles shows that D1-E130L mutation induces a 3 nm blue shift in both the ground state and transiently reduced state of the Pheoactive Qx absorption band. In contrast, D1-E130H mutants, having intermediate hydrogen bonding strength, have only a 40% reduction in oxygen evolving activity, near wild-type levels of variable Chl a fluorescence and no observable spectroscopic shifts in the ground state or transiently reduced Pheoactive Qx band. The D1-E130Q mutant is phenotypically similar to the D1-E130H mutant with the exception that the transiently reduced Pheo-active Qx band is blue shifted unlike the Pheo-active ground state. Surprisingly, the magnitude of the S2-mutiline Mn EPR signals, attributed to the water-splitting complex, as well as the photo-accumulated QA-Fe2+ EPR signals are similar for all D1-E130 mutants and wild type. These results suggest that the charge separated state, P680+/ Pheo-active-, is not stabilized at room temperature in D1-E130 mutants that have reduced hydrogen-bonding interactions with the ring V carbonyl oxygen of Pheo-active.

KEY WORDS: Pheophytin, reaction center, Photosystem II, Chlamydomonas