Miqyass Mohamed, Marosvölgyi Marcell A, Nagel Zachary, Yocum Charles F, van Gorkom Hans J
Department of Biophysics, Huygens Laboratory, Leiden University, P.O. Box 9504, NL-2300 RA Leiden, The Netherlands.
Biochemistry. 2008 Jul 29;47(30):7915-24. doi: 10.1021/bi8006059. Epub 2008 Jul 2.
The functional role of the Ca (2+) ion in the oxygen-evolving complex of photosystem II is not yet clear. Current models explain why the redox cycle of the complex would be interrupted after the S 3 state without Ca (2+), but the literature shows that it is interrupted after the S 2 state. Reinterpretation of the literature on methods of Ca (2+) depletion [Miqyass, M., van Gorkom, H. J., and Yocum, C. F. (2007) Photosynth. Res. 92, 275-287] led us to propose that all S-state transitions require Ca (2+). Here we confirm that interpretation by measurements of flash-induced S-state transitions in UV absorbance. The results are explained by a cation exchange at the Ca (2+) binding site that, in the absence of the extrinsic PsbP and PsbQ polypeptides, can occur in minutes in low S-states and in seconds in high S-states, depending on the concentration of the substituting cation. In the S 2(K (+)) or S 2(Na (+)) state a slow conformational change occurs that prevents recovery of the slow-exchange situation on return to a lower S-state but does not inhibit the S-state cycle in the presence of Ca (2+). The ratio of binding affinities for monovalent vs divalent cations increases dramatically in the higher S-states. With the possible exception of S 0 to S 1, all S-state transitions specifically require Ca (2+), suggesting that Ca (2+)-bound H 2O plays an essential role in a H (+) transfer network required for H (+)-coupled electron transfer from the Mn cluster to tyrosine Z.
钙离子在光系统II放氧复合体中的功能作用尚不清楚。目前的模型解释了在没有钙离子的情况下，该复合体的氧化还原循环为何在S3状态后会中断，但文献表明它在S2状态后就被中断了。对有关钙离子耗尽方法的文献[Miqyass, M., van Gorkom, H. J., and Yocum, C. F. (2007) Photosynth. Res. 92, 275 - 287]进行重新解读后，我们提出所有S态转换都需要钙离子。在此，我们通过测量紫外吸收中闪光诱导的S态转换来证实这一解释。结果可以通过钙离子结合位点的阳离子交换来解释，在没有外在的PsbP和PsbQ多肽的情况下，根据替代阳离子的浓度，这种交换在低S态下几分钟内发生，在高S态下几秒钟内发生。在S2(K+)或S2(Na+)状态下会发生缓慢的构象变化，这种变化会阻止回到较低S态时缓慢交换情况的恢复，但在有钙离子存在时不会抑制S态循环。在较高S态下，单价阳离子与二价阳离子的结合亲和力之比会急剧增加。除了可能从S0到S1的转换外，所有S态转换都特别需要钙离子，这表明结合了钙离子的水在从锰簇到酪氨酸Z的质子耦合电子转移所需的质子转移网络中起着至关重要的作用。
