The stable tyrosyl radical in Photosystem II: why D?

Two redox-active tyrosines are present in Photosysytem II, the water-oxidizing enzyme. While the tyrosine that is kinetically competent in electron transfer, TyrZ, may also have a role in the enzyme mechanism, the second tyrosine, TyrD, has a stable radical and is not directly involved in the redox chemistry associated with enzyme function. Nevertheless, reasonable mechanistic roles for TyrD have been postulated that satisfy desires to rationalise the presence of this cofactor, or, in English, we think we know what it does. First, the TyrD radical acts an oxidant of the Mn cluster in the lowest state of the redox accumulation cycle (i.e., S-0), providing potential benefits in maintaining the cluster in the more stable higher valence states. This redox role may also be important during Mn assembly and indeed overreduced forms of the Mn cluster appear to be oxidised by TyrD(.). Second, the proton generated by the TyrD radical is thought to remain in its vicinity having an electrostatic influence on the location and potential of the chlorophyll cation, P+. This effect may be important for the kinetics of TyrZ oxidation and may provide a significant thermodynamic boost to the enzyme. In addition, through its electrostatic influence, TyrD(.)(H+) may confine the highly oxidising cation P+ to the chlorophyll nearest to TyrZ, thereby accelerating TyrZ oxidation and restricting the potentially damaging redox chemistry to one side of the reaction centre: the disposable D1 side. This second role, evidence for which is beginning to emerge, constitutes a new role for a redox-active tyrosine in biology: as a positive charge generator in a hydrophobic environment. In this short review, we focus on work relevant to these two roles. (C) 2004 Elsevier B.V. All rights reserved.