Two electron donation sites for exogenous reductants in chloroplast photosystem II.

Two sites are distinguished for the oxidation of exogenous donors by Photosystem II in non-oxygen evolving chloroplasts. In the presence of lipophilic donors (e.g. phenylenediamine, benzidine, diphenylcarbazide), the rate for Signal IIf rereduction following a flash increases as the concentration of exogenous reductant increases. There is a decrease (20-40%) in Signal IIf magnitude accompanying donor addition at low (smaller than 10(-%) M) concentrations, but the extent of the decrease does not change further with increasing donor concentrations. Complementary polarographic experiments monitoring donor (phenylenediamine) oxidation show an increase in oxidation rate with increasing donor concentration. In the presence of the hydrophilic donor, Mn-2+, the Signal IIf decay halftime remains constant with increasing Mn-2+ concentration. However, the flash-induced Signal IIf magnitude pregressively decreases with increasing Mn-2+ concentration. These results are interpreted in terms of two competing paths for the reduction of P680+. In one path P680+ reduction is accompanied by the appearance of Signal IIf, and lipophilic donors subsequently rereduce the Signal IIf species in a series reaction. This reduction follows pseudo-first order kinetics as a function of donor concentration. In the second path Mn-2+ reduces P680+ in a parallel reaction that competes with the formation of the Signal IIf species. This results in a decrease in the magnitude of Signal IIf, but no change in its decay time.