From the Biophysics of Photosynthesis, Institute for Biology, Humboldt-Universität zu Berlin, Philippstrasse 13, 10115 Berlin, Germany,
From the Biophysics of Photosynthesis, Institute for Biology, Humboldt-Universität zu Berlin, Philippstrasse 13, 10115 Berlin, Germany.
J Biol Chem. 2018 Jun 8;293(23):9090-9100. doi: 10.1074/jbc.RA117.000953. Epub 2018 Apr 25.
The binding of photosystem I (PS I) from to the native cytochrome (cyt) and cyt from horse heart (cyt ) was analyzed by oxygen consumption measurements, isothermal titration calorimetry (ITC), and rigid body docking combined with electrostatic computations of binding energies. Although PS I has a higher affinity for cyt than for cyt , the influence of ionic strength and pH on binding is different in the two cases. ITC and theoretical computations revealed the existence of unspecific binding sites for cyt besides one specific binding site close to P Binding to PS I was found to be the same for reduced and oxidized cyt Based on this information, suitable conditions for cocrystallization of cyt with PS I were found, resulting in crystals with a PS I:cyt ratio of 1:1. A crystal structure at 3.4-Å resolution was obtained, but cyt cannot be identified in the electron density map because of unspecific binding sites and/or high flexibility at the specific binding site. Modeling the binding of cyt to PS I revealed a specific binding site where the distance and orientation of cyt relative to P are comparable with cyt from purple bacteria relative to P This work provides new insights into the binding modes of different cytochromes to PS I, thus facilitating steps toward solving the PS I-cyt costructure and a more detailed understanding of natural electron transport processes.
通过氧耗测量、等温热滴定（ITC）和刚性对接结合静电计算结合能，分析了来源于 和天然细胞色素（cyt）以及来源于马心的细胞色素（cyt）与光系统 I（PSI）的结合。尽管 PSI 与 cyt 相比对 cyt 具有更高的亲和力，但在这两种情况下，离子强度和 pH 对结合的影响是不同的。ITC 和理论计算揭示了 cyt 除了靠近 P 结合位点的一个特异性结合位点外，还存在非特异性结合位点。研究发现 PSI 与还原型和氧化型 cyt 的结合是相同的。基于这些信息，找到了与 PSI 共结晶 cyt 的合适条件，得到了 PSI:cyt 的比例为 1:1 的晶体。获得了分辨率为 3.4-Å 的晶体结构，但由于非特异性结合位点和/或特异性结合位点的高灵活性，无法在电子密度图中识别 cyt。对 cyt 与 PSI 结合的建模表明，存在一个特异性结合位点，其中 cyt 相对于 P 的距离和取向与紫细菌中的 cyt 相对于 P 的距离和取向相当。这项工作为不同细胞色素与 PSI 的结合模式提供了新的见解，从而有助于解决 PSI-cyt 共结构和更详细地了解天然电子传递过程的步骤。
