REDOX INTERACTION OF TYROSINE-D WITH THE S-STATES OF THE WATER-OXIDIZING COMPLEX IN INTACT AND CHLORIDE-DEPLETED PHOTOSYSTEM-II

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 S2 and S3 oxidation states of the water-oxidizing complex. The kinetics of Tyrosine-D oxidation by the S2 and S3 States are almost identical in the pH range of 4.5 to 7.8, and show the same pH dependence for the S3 state as has previously been observed for the S2 state (Vass and Styring (1991) Biochemistry 30, 830-839). It is concluded from the pH-dependent oxidation kinetics that a proton binding with a pK around 7.0-7.2 retards electron transfer from Tyrosine-D to the water-oxidizing complex both in the S2 and in the S3 states. In addition, our results imply that the S2/S1 and S3/S2 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 S2 state. This result indicates that Tyrosine-D can interact with the S2 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 S2 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.