Rapid formation of the stable tyrosyl radical in photosystem II

Two symmetrically positioned redox active tyrosine residues are present in the photosystem II (PSII) reaction center. One of them, TyrZ, is oxidized in the ns–μs time scale by P680(+) and reduced rapidly (μs to ms) by electrons from the Mn complex. The other one, TyrD, is stable in its oxidized form and seems to play no direct role in enzyme function. Here, we have studied electron donation from these tyrosines to the chlorophyll cation (P680(+)) in Mn-depleted PSII from plants and cyanobacteria. In particular, a mutant lacking TyrZ was used to investigate electron donation from TyrD. By using EPR and time-resolved absorption spectroscopy, we show that reduced TyrD is capable of donating an electron to P680(+) with t(1/2) ≈ 190 ns at pH 8.5 in approximately half of the centers. This rate is ≈10(5) times faster than was previously thought and similar to the TyrZ donation rate in Mn-depleted wild-type PSII (pH 8.5). Some earlier arguments put forward to rationalize the supposedly slow electron donation from TyrD (compared with that from TyrZ) can be reassessed. At pH 6.5, TyrZ (t(1/2) = 2–10 μs) donates much faster to P680(+) than does TyrD (t(1/2) > 150 μs). These different rates may reflect the different fates of the proton released from the respective tyrosines upon oxidation. The rapid rate of electron donation from TyrD requires at least partial localization of P680(+) on the chlorophyll (P(D2)) that is located on the D2 side of the reaction center.