Rapid electron donation from tyrosineD to P680+ in photosystem II

Abstract

Electron donation from the two redox active tyrosines to the chlorophyll cation (P680+) in Mn-depleted PSII from plants and cyanobacterial was studied. In particular a mutant protein lacking the TyrZ was used to investigate electron donation from TyrD using EPR and time-resolved absorption spectroscopy. Tyr D donates an electron to the P680+ with a half time of 190 ns at pH 8.5 in around half the centers. This rate is around 105 times faster than was previously thought. Earlier arguments put forward to rationalize the supposedly slow electron donation from TyrD compared to that from TyrZ can be eliminated. At lower pH values this rapid donation from TyrD does not occur. The literature shows that the donation rate of TyrZ slows by a factor of ~100 as the pH is decreased and this was related to the protonation state either to the Tyr itself or to a neighbouring histidine. In contrast, the electron donation rate of TyrD to P680+ seems to slow by at least 4 orders of magnitude over this range. This may reflect different situations in terms of the amino acids acting as bases for the respective tyrosines, with TyrZ being at the start of a H-bonded proton chain, while TyrD may have only one base in its environment. Given current structural models, the rapid rate of electron donation from TyrD suggests that the cation corresponding to P680+ under these conditions is on the chlorophyll (PD2) which is located on the D2 side of the reaction center.