Acceptor side effects on the electron transfer at cryogenic temperatures in intact photosystem II |
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Authors: | Han Bao Keisuke Kawakami Jian-Ren Shen |
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Institution: | a Laboratory of Photochemistry, Beijing National Laboratory of Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, China b Graduate School of Natural Science and Technology/Department of Biology, Faculty of Science, Okayama University, Tsushima-naka, Okayama 700-8530, Japan |
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Abstract: | In intact PSII, both the secondary electron donor (TyrZ) and side-path electron donors (Car/ChlZ/Cytb559) can be oxidized by P680+ at cryogenic temperatures. In this paper, the effects of acceptor side, especially the redox state of the non-heme iron, on the donor side electron transfer induced by visible light at cryogenic temperatures were studied by EPR spectroscopy. We found that the formation and decay of the S1TyrZ EPR signal were independent of the treatment of K3Fe(CN)6, whereas formation and decay of the Car+/ChlZ+ EPR signal correlated with the reduction and recovery of the Fe3+ EPR signal of the non-heme iron in K3Fe(CN)6 pre-treated PSII, respectively. Based on the observed correlation between Car/ChlZ oxidation and Fe3+ reduction, the oxidation of non-heme iron by K3Fe(CN)6 at 0 °C was quantified, which showed that around 50-60% fractions of the reaction centers gave rise to the Fe3+ EPR signal. In addition, we found that the presence of phenyl-p-benzoquinone significantly enhanced the yield of TyrZ oxidation. These results indicate that the electron transfer at the donor side can be significantly modified by changes at the acceptor side, and indicate that two types of reaction centers are present in intact PSII, namely, one contains unoxidizable non-heme iron and another one contains oxidizable non-heme iron. TyrZ oxidation and side-path reaction occur separately in these two types of reaction centers, instead of competition with each other in the same reaction centers. In addition, our results show that the non-heme iron has different properties in active and inactive PSII. The oxidation of non-heme iron by K3Fe(CN)6 takes place only in inactive PSII, which implies that the Fe3+ state is probably not the intermediate species for the turnover of quinone reduction. |
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Keywords: | Chl chlorophyll ChlZ side-path redox active Chl Car redox active β-carotene Cytb559 cytochrome b559 D1 D2 reaction center core proteins DMSO dimethyl sulfoxide EDTA ethylenediaminetetraacetic acid EPR electron paramagnetic resonance LBHB low-barrier hydrogen bond MES 4-morpholine ethanesulfonic acid P680 primary electron donor of PSII PD1 PD2 two monomeric Chls of P680 associated with D1 and D2 respectively Pheo pheophytin PPBQ phenyl-p-benzoquinone PSII photosystem II QA and QB primary and secondary quinone electron acceptors respectively TyrZ tyrosine 161 of the D1 protein TyrD tyrosine 160 of the D2 protein |
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