This subproject is one of many research subprojects utilizing theresources provided by a Center grant funded by NIH/NCRR. The subproject andinvestigator (PI) may have received primary funding from another NIH source,and thus could be represented in other CRISP entries. The institution listed isfor the Center, which is not necessarily the institution for the investigator.The rates of electron transfer in photosynthetic reaction centers (RC) are affected by binding of transition metals. After screening different metal ions we found that electron transfer rates were also affected by addition of SrCl2 and CaCl2. To determine the binding site, 2.4 A resolution x-ray diffraction data were collected on tetragonal crystals of Rb. sphaeroides RCs incubated with 0.3M SrCl2. Electron density maps revealed a 17 sigma difference density peak at the glutamic acid cluster of H43, H79, and H81 and the carbonyl backbone oxygen of Ser L4. Anomalous difference maps from data collected near the strontium absorption edge showed this peak was due to strontium. X-ray data collected to 2.4 A on RC crystals incubated with 0.3M CaCl2, resulted in an 8 sigma difference density peak at the same site, about 17 A from QA on the cytoplasmic surface of the RC. Due to the proximity of the site to QA, metal binding is expected to modulate the redox potential of QA and affect its electron transfer reactions. Addition of Sr2+ and Ca2+ increased the recombination rates for the reaction D+QA---> DQA by 20% and 10%, with KD of 2 mM and 10 mM respectively. The increase in rate is opposite to the effect expected from an electrostatic stabilization of QA- by the bound metal and may be due to the specific binding of Ca2+ and Sr2+ to this site producing conformational changes that destabilize the charge separated state. Efforts are now underway to refine the crystal structures of the Sr and Ca-bound RC's to identify this proposed conformational change.
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