Hoganson C W, Babcock G T
Department of Chemistry, Michigan State University, East Lansing 48824-1322.
Biochemistry. 1992 Dec 1;31(47):11874-80. doi: 10.1021/bi00162a028.
The stable tyrosine radical in photosystem II, YD*, has been studied by ESR and ENDOR spectroscopies to obtain proton hyperfine coupling constants from which the electron spin density distribution can be deduced. Simulations of six previously published ESR spectra of PSII (one at Q band; five at X band, of which two were after specific deuteration and two others were of oriented membranes) can be achieved by using a single set of magnetic parameters that includes anisotropic proton hyperfine tensors, an anisotropic g tensor, and noncoincident axis systems for the g and A tensors. From the spectral simulation of the oriented samples, the orientation of the phenol head group of YD* with respect to the membrane plane has been determined. A similar orientation for YZ*, the redox-active tyrosine in PSII that mediates electron transfer between P680 and the oxygen-evolving complex, is expected. ENDOR spectra of YD* in PSII preparations from spinach and Synechocystis support the set of hyperfine coupling constants but indicate that small differences between the two species exist. Comparison with the results of spectral simulations for tyrosyl radicals in ribonucleotide reductase from prokaryotes or eukaryotes and with in vitro radicals indicates that the spin density distribution remains that of an odd-alternant radical but that interactions with the protein can shift spin density within this basic pattern. The largest changes in spin density occur at the tyrosine phenol oxygen and at the ring carbon para to the oxygen, which indicates that mechanisms exist in the protein environment for fine-tuning the chemical and redox properties of the radical species.
通过电子自旋共振（ESR）和电子核双共振（ENDOR）光谱对光系统II中稳定的酪氨酸自由基YD进行了研究，以获得质子超精细耦合常数，由此可以推导出电子自旋密度分布。利用一组单一的磁参数，包括各向异性质子超精细张量、各向异性g张量以及g和A张量的非重合轴系，能够模拟之前发表的6个光系统II的ESR光谱（1个在Q波段；5个在X波段，其中2个是特定氘化后的光谱，另外2个是取向膜的光谱）。从取向样品的光谱模拟中，确定了YD的酚羟基头基相对于膜平面的取向。预计光系统II中介导P680与放氧复合体之间电子转移的氧化还原活性酪氨酸YZ也有类似的取向。菠菜和集胞藻的光系统II制剂中YD的ENDOR光谱支持了这组超精细耦合常数，但表明这两个物种之间存在细微差异。与原核生物或真核生物核糖核苷酸还原酶中酪氨酸自由基的光谱模拟结果以及体外自由基的结果进行比较表明，自旋密度分布仍然是奇交替自由基的分布，但与蛋白质的相互作用可以在这个基本模式内转移自旋密度。自旋密度的最大变化发生在酪氨酸酚氧和与氧对位的环碳上，这表明在蛋白质环境中存在微调自由基物种化学和氧化还原性质的机制。
