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

Substitution of chlorophyll into the inactive branch pheophytin-binding site impairs charge separation in Photosystem II. Ling Xiong¹, Seibert Michael², Michael WASIELEWSKI³, Craig Hemann⁴, C. Russ Hille⁴, Richard Sayre^*,1, ¹ Department of Plant Cellular and Molecular Biology, Columbus, Ohio, USA² 2National Renewable Energy Laboratory, Golden, Colorado, USA³ Department of Chemistry, Evanston, Illinois, USA⁴ Department of Molecular and Cellular Biochemistry, Columbus, Ohio, USA

ABSTRACT- All photosynthetic reaction centers (RCs) have two parallel sets of electron transfer cofactors that cross the membrane. In quinone-type RCs (including photosystem II (PSII)), however, only one pathway (the active branch) is used for electron transfer. Since the electron transfer cofactors of each pathway have nearly identical distance and orientation relationships, it is assumed that local differences in protein environment determine the directionality of electron transfer. To characterize the energy transfer dynamics of PSII RCs we replaced the inactive-branch pheophytin (Pheo) with a chlorophyll (Chl). We mutated he D1-L210 residue to a histidine (D1-L210H) to provide a Mg-ligand for Chl. Analyses of the pigment composition of D1-L210H RCs indicated that the inactive-branch Pheo had been replaced by a Chl. Comparisons of WT and D1-L210 transient absorption spectra confirmed that the red-shifted Pheo Qx absorption band (543.5 nm) belonged to the active-branch Pheo. Surprisingly, intact D1-L210H PSII complexes were unable to evolve oxygen, lacked Chl variable fluorescence, (following a flash), and were unable to photo-accumulate reduced Qa; indicating that electron transfer in D1-L210H PSII complexes was severely perturbed. The kinetics of primary charge separation, however, were not substantially altered in D1-L210H RCs indicating that the Chl substitution had not perturbed the energetics of the primary electron donor/acceptor pair. Significantly, intact D1-L210H PSII core complexes had a substantially increased and red-shifted Chl fluorescence emission band (attributed to Chls of the distal antennae complex) as well as a blue-shifted fluorescence emission peak (attributed to Chls of the proximal antennae complex) (77 K). These results are interpreted in terms of a redistribution of the excited state energy among the pigments of the RC multimer, leading to loss of the excited state via fluorescence in the D1-L210H mutant.

KEY WORDS: chlorophyll, pheophytin, photosystem II, Chlamydomonas