Nugent J H
Department of Biology, Darwin Building, University College London, Gower Street, WC1E 6BT, London, UK.
Biochim Biophys Acta. 2001 Apr 2;1504(2-3):288-98. doi: 10.1016/s0005-2728(00)00257-7.
An electron paramagnetic resonance (EPR) signal near g=6 in Photosystem II (PSII) membranes has been assigned to a high spin form of cytochrome (Cyt) b(559) (R. Fiege, U. Schreiber, G. Renger, W. Lubitz, V.A. Shuvalov, FEBS Lett. 377 (1995) 325-329). Here we have further investigated the origin of this signal. A slow formation of the signal during storage in the dark is observed in oxygen-evolving PSII membranes, which correlate with the oxidation of Fe(2+) by plastosemiquinone or oxygen. Removal of oxygen inhibits formation of the high spin iron signal. The g=6 EPR signal is photoreduced at cryogenic temperatures and is restored slowly by subsequent dark storage at 77 K. The amplitude of the photoreduced signal increases as the pH is lowered, which shows that the origin is not the hydroxyl ligated Cyt b(559) species proposed previously. Different cryoprotectants also influence the amplitude and lineshape of the high spin iron signal in a manner suggesting that smaller cryoprotectants can penetrate the iron environment. A correlation between the high spin iron and g=1.6 EPR signal assigned to an interaction involving the semiquinones of Qa and Qb is shown. It is concluded that the appearance of the high spin iron signal in oxygen-evolving PSII membranes involves reduced PSII electron acceptors and oxygen and suggests that the signal is from the non-haem iron of PSII.
在光系统II（PSII）膜中，g = 6附近的电子顺磁共振（EPR）信号已被归因于细胞色素（Cyt）b(559)的高自旋形式（R. Fiege、U. Schreiber、G. Renger、W. Lubitz、V.A. Shuvalov，《欧洲生物化学学会联合会快报》377 (1995) 325 - 329）。在此，我们进一步研究了该信号的起源。在放氧的PSII膜中，黑暗储存期间信号缓慢形成，这与亚铁离子被质体半醌或氧气氧化相关。去除氧气会抑制高自旋铁信号的形成。g = 6的EPR信号在低温下被光还原，并在随后77 K的黑暗储存中缓慢恢复。随着pH降低，光还原信号的幅度增加，这表明其起源并非先前提出的羟基连接的Cyt b(559)物种。不同的冷冻保护剂也以一种表明较小的冷冻保护剂可穿透铁环境的方式影响高自旋铁信号的幅度和线形。显示了高自旋铁与g = 1.6的EPR信号之间的相关性，后者归因于涉及Qa和Qb半醌的相互作用。得出的结论是，放氧的PSII膜中高自旋铁信号的出现涉及还原的PSII电子受体和氧气，并表明该信号来自PSII的非血红素铁。
