Acceptor side effects on the electron transfer at cryogenic temperatures in intact photosystem II

Abstract

In intact PSII, both the secondary electron donor (Tyr Z) and side-path electron donors (Car/Chl Z/Cyt b 559) can be oxidized by P 680 + 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 S 1Tyr Z EPR signal were independent of the treatment of K 3Fe(CN) 6, whereas formation and decay of the Car + /Chl Z + EPR signal correlated with the reduction and recovery of the Fe 3+ EPR signal of the non-heme iron in K 3Fe(CN) 6 pre-treated PSII, respectively. Based on the observed correlation between Car/Chl Z oxidation and Fe 3+ reduction, the oxidation of non-heme iron by K 3Fe(CN) 6 at 0 °C was quantified, which showed that around 50–60% fractions of the reaction centers gave rise to the Fe 3+ EPR signal. In addition, we found that the presence of phenyl- p-benzoquinone significantly enhanced the yield of Tyr Z 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. Tyr Z 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 K 3Fe(CN) 6 takes place only in inactive PSII, which implies that the Fe 3+ state is probably not the intermediate species for the turnover of quinone reduction.