Abstract
The light-induced oxidation of Tyrosine-D in Photosystem II has been studied by time-resolved measurements of the EPR Signal II slow at room temperature. When induced with single turnover flashes, the oxidation of Tyrosine-D undergoes a period-four oscillation as a function of flash number, showing Tyrosine-D + formation in the S 2 and S 3 oxidation states of the water-oxidizing complex. The kinetics of Tyrosine-D oxidation by the S 2 and S 3 states are almost identical in the pH range of 4.5 to 7.8, and show the same pH dependence for the S 3 state as has previously been observed for the S 2 state (Vass and Styring (1991) Biochemistry 30, 830–839). It is concluded from the pH-dependent oxidation kinetics that a proton binding with a p K around 7.0–7.2 retards electron transfer from Tyrosine-D to the water-oxidizing complex both in the S 2 and in the S 3 states. In addition, our results imply that the S 2/S 1 and S 3/S 2 redox couples have about the same redox potential relative to that of the Tyrosine-D +/Tyrosine-D couple. Removal of chloride from Photosystem II induced an approximately 10-times slowdown in the Tyrosine-D oxidation kinetics by the S 2 state. This result indicates that Tyrosine-D can interact with the S 2 state in the absence of chloride. The retarded oxidation kinetics observed under these conditions are consistent with the previously demonstrated stabilization of the chloride-free S 2 state. We also observed the flash-induced oxidation of Tyrosine-Z in a large fraction of the chloride depleted Photosystem II centers. In this system Tyr-Z + was abnormally stable and decayed biphasically with 500 ms and 12–15 s half-times.
Published Version
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