Low-temperature electron transfer suggests two types of QA in intact photosystem II

The correlation between the reduction of Q(A) and the oxidation of Tyr(z) or Car/Chl(z)/Cyt(b559) in spinach PSII enriched membranes induced by visible light at 10 K is studied by using electron paramagnetic resonance spectroscopy. Similar g = 1.95-1.86 Q(A)(-center dot)EPR signals are observed in both Mn-depleted and intact samples, and both signals are long lived at low temperatures. The presence of PPBQ significantly diminished the light induced EPR signals from Q(A)(-center dot), Car(+center dot)/Chl(+center dot) and oxidized Cyt(b559), while enhancing the amplitude of the S(1)Tyr(z)(center dot) EPR signal in the intact PSII sample. The quantification and stability of the g = 1.95-1.86 EPR signal and signals arising from the oxidized Tyrz and the side-path electron donors, respectively, indicate that the EPR-detectable g = 1.95-1.86 Q(A)(-center dot) signal is only correlated to reaction centers undergoing oxidation of the side-path electron donors (Car/Chl(z)/Cyt(b559)), but not of Tyr(z). These results imply that two types of Q(A)(-center dot) probably exist in the intact PSII sample. The structural difference and possible function of the two types of QA are discussed. (C) 2009 Elsevier B.V. All rights reserved.