Menaquinone as the Secondary Electron Acceptor in the Type I Homodimeric Photosynthetic Reaction Center of Heliobacterium modesticaldum

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• 作者：Toru Kondo ; Shigeru Itoh ; Masahiro Matsuoka ; Chihiro Azai ; Hirozo Oh-oka
• 刊名：Journal of Physical Chemistry B
• 出版年：2015
• 出版时间：July 9, 2015
• 年：2015
• 卷：119
• 期：27
• 页码：8480-8489
• 全文大小：470K
• ISSN：1520-5207

文摘

The type I photosynthetic reaction center (RC) of heliobacteria (hRC) is a homodimer containing cofactors almost analogous to those in the plant photosystem I (PS I). However, its three-dimensional structure is not yet clear. PS I uses phylloquinone (PhyQ) as a secondary electron acceptor (A₁), while the available evidence has suggested that menaquinone (MQ) in hRC has no function as A₁. The present study identified a new transient electron spin-polarized electron paramagnetic resonance (ESP-EPR) signal, arising from the radical pair of the oxidized electron donor and the reduced electron acceptor (P800⁺MQ^{鈥?/sup>), in the hRC core complex and membranes from Heliobacterium modesticaldum. The ESP signal could be detected at 5鈥?0 K upon flash excitation only after prereduction of the iron鈥搒ulfur center, F_X, and was selectively lost by extraction of MQ with diethyl ether. MQ was suggested to be located closer to F_X than PhyQ in PS I based on the simulation of the unique A/E (A, absorption; E, emission) ESP pattern, the reduction/oxidation rates of MQ, and the power saturation property of the static MQ鈥?/sup> signal. The result revealed the quinone usage as the secondary electron acceptor in hRC, as in the case of PS I.}