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 i n the ns-mus time scale by P680(+) and reduced rapidly (As to ms) by electr ons from the Mn complex. The other one, TyrD, is stable in its oxidized for m and seems to play no direct role in enzyme function. Here, we have studie d 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 t1/2 approximate to 190 ns a t pH 8.5 in approximately half of the centers. This rate is approximate to 10(5) times faster than was previously thought and similar to the TyrZ dona tion rate in Mn-depleted wild-type PSII (pH 8.5). Some earlier arguments pu t forward to rationalize the supposedly slow electron donation from TyrD (c ompared with that from TyrZ) can be reassessed. At pH 6.5, TyrZ (t(1/2) = 2 -10 mus) donates much faster to P680(+) than does TyrD (t(1/2) > 150 mus). These different rates may reflect the different fates of the proton release d from the respective tyrosines upon oxidation. The rapid rate of electron donation from TyrD requires at least partial localization of P680+ on the c hlorophyll (P-D2) that is located on the D2 side of the reaction center.