Poster abstracts

Poster number 4 submitted by Zoee Gokhale

Elucidating Mechanisms Of Primary Charge Separation In Photosystem II

Zoee Gokhale (Biophysics, The Ohio State University), Richard Sayre (Plant Cellular and Molecular Biology, The Ohio State University)

Abstract:
PSII RC, a quinone-type RC is unique in its capability of being able to generate a cation oxidizing enough to extract electrons from water, a feature that does not exist in other RC types. However, similar to other type II RCs only one potential electron transfer (ET) pathway (the active branch) is utilized upon photoexcitation. We have generated mutations in the RC D1 and D2 polypeptides that are hypothesized to alter the yield and/or directionality of charge transfer. The (D1-T179I) mutation eliminates a potential Mg ligand of the D1 monomeric chlorophyll (ChlD1) and the (D2-L209H) mutant is predicted to result in the substitution of a Chl for the active branch pheophytin (PheoD1) due to the introduction of a histidine ligand. The D1-T179I mutant is intended to address issues that relate the asymmetrical nature of the PSII RC (especially in identity of the ligands for the Chl monomers). The D2-L209H mutant mimics similar mutations introduced into the bacterial reaction center and will help us address the affects of altered cofactor symmetry on primary charge separation. The D1T179I mutant had reduced rates (45% of wild-type) of oxygen evolution, higher chlorophyll fluorescence yield relative to wild-type (WT) and a decreased amplitude for the life-time component (~5 ps) attributed to primary charge separation; leading to speculation that energy trapping by the primary donor has been impaired in the mutant. The D2-L209H mutant shows a severe block in PSII ET manifested as a poor rate of growth under photoautotrophic conditions, a lack of variable Chl fluorescence as well as extremely low rates of oxygen evolution when compared with WT. These results are different than those reported for the corresponding BRC mutation. Further studies are underway to characterize the biophysical basis for the observed phenotype.

Keywords: Primary charge separation, Reaction center, Electron transfer