ELECTRON TRANSFER IN THE PHOTOSYNTHETIC REACTION CENTER

这个子项目是许多研究子项目中利用 资源由NIH/NCRR资助的中心拨款提供。子项目和 调查员(PI)可能从NIH的另一个来源获得了主要资金， 并因此可以在其他清晰的条目中表示。列出的机构是 该中心不一定是调查人员的机构。 光合作用反应中心(RC)的电子传递速率受过渡金属结合的影响。在筛选不同的金属离子后，我们发现电子转移速率也受到SrCl2和CaCl2添加的影响。为了确定Rb的结合位置，在Rb的四方晶体上收集了2.4A分辨率的X射线衍射数据。球藻RCS与0.3MSrCl2孵育。电子密度图显示在H43、H79和H81的谷氨酸簇和Ser L4的羰基主链氧上有一个17sigma的密度差峰。从锶吸收边附近收集的数据得到的异常差值图显示，这个峰是由于锶引起的。在0.3M CaCl2孵育的RC晶体上收集到2.4A的X射线数据，在同一位置产生了一个8西格玛差密度峰，与RC细胞质表面的QA相差约17A。由于位置靠近QA，金属结合有望调节QA的氧化还原电位并影响其电子转移反应。Sr2+和Ca2+的加入使D+QA-&GT；DQA反应的重组率分别提高了20%和10%，Kd分别为2 mM和10 mM。这种速率的增加与结合金属对QA-的静电稳定的预期效果相反，可能是由于Ca~(2+)和Sr~(2+)与这个位置的特异结合，产生了破坏电荷分离态稳定的构象变化。目前正在努力改进锶和钙结合的RC的晶体结构，以确定这一拟议的构象变化。

This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for 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.