Noguchi T, Inoue Y, Tang X S
Photosynthesis Research Laboratory, The Institute of Physical and Chemical Research (RIKEN), Saitama, Japan.
Biochemistry. 1999 Jan 5;38(1):399-403. doi: 10.1021/bi982294v.
Interactions of the primary quinone acceptor QA of photosystem II (PS II) with surrounding amino acid residues were studied by analysis of FTIR difference spectra of QA upon its photoreduction (QA-/QA). Structural coupling with a His side chain was revealed by identifying the imidazole bands in the QA-/QA spectrum using the PS II core complexes from Synechocystis PCC 6803 in which both of the two imodazole nitrogens of His side chains were specifically labeled with 15N. Strong hydrogen bonding of the imidazole NH was shown by (i) the presence of several peaks at 2600-3000 cm-1, which arise from Fermi resonance of harmonics or combinations of imidazole ring modes with the hydrogen bonding NH stretching vibration, and (ii) the 1179 cm-1 band, which can be assigned to the mode including NH deformation, is at a frequency significantly higher than the corresponding 1151 cm-1 band of model compounds 4- and 5-methylimidazole in aqueous solution. Also, the presence of the bands specific to the Npi-protonated state at 1109/1102/1090 and 1359 cm-1 suggests that the QA-coupled His is protonated at the Npi site. These results are in good agreement with the model of QA interaction in which His215 (D2), which coordinates to the non-heme iron at Ntau, is hydrogen bonded to the QA carbonyl through the Npi-H bond. In contrast, no bands of Trp side chains were detected in the QA-/QA spectrum upon labeling of the indole ring of Trp residues with indole-d5. This result indicates that Trp254 (D2), which corresponds to Trp252 (M) of the bacterial reaction center that is located in van der Waals contact with QA, is not strongly coupled with QA in PS II. Probably, the predicted pi-pi interaction is not strong enough to influence the vibrations of the indole ring of Trp upon QA reduction, or Trp254 (D2) is located rather far from QA in PS II.
通过分析光系统II（PS II）的初级醌受体QA光还原时（QA-/QA）的傅里叶变换红外差谱，研究了QA与周围氨基酸残基的相互作用。使用来自集胞藻PCC 6803的PS II核心复合物，通过在QA-/QA谱中识别咪唑带，揭示了与组氨酸侧链的结构偶联，其中组氨酸侧链的两个咪唑氮均用15N特异性标记。咪唑NH的强氢键作用表现为：（i）在2600 - 3000 cm-1处存在几个峰，这是由于咪唑环模式的谐波或组合与氢键NH伸缩振动的费米共振产生的；（ii）1179 cm-1的谱带，可归属于包括NH变形的模式，其频率明显高于水溶液中模型化合物4-和5-甲基咪唑相应的1151 cm-1谱带。此外，在1109/1102/1090和1359 cm-1处存在特定于Npi-质子化状态的谱带，表明与QA偶联的组氨酸在Npi位点质子化。这些结果与QA相互作用模型高度一致，在该模型中，在Ntau与非血红素铁配位的His215（D2）通过Npi-H键与QA羰基形成氢键。相反，在用吲哚-d5标记色氨酸残基的吲哚环后，在QA-/QA谱中未检测到色氨酸侧链的谱带。该结果表明，对应于细菌反应中心中与QA处于范德华接触的Trp252（M）的Trp254（D2），在PS II中与QA的偶联不强。可能是预测的π-π相互作用不够强，不足以影响QA还原时色氨酸吲哚环的振动，或者在PS II中Trp254（D2）离QA较远。
