Mutation of a putative ligand to the non-heme iron in Photosystem II: implications for Q_A reactivity, electron transfer, and herbicide binding

In Photosystem II (PS II), a non-heme iron is present near the electron-accepting quinones Q_A and Q_B. A putative ligand to this non-heme iron, His-268 in the D2 protein, was mutated to Gln in the cyanobacterium Synechocystis sp. PCC 6803. The resulting mutant H268Q has lost photoautotrophic capacity and shows large alterations in the properties both of Q_A and of the Q_B pocket. In the mutant, the quantum efficiency of Q_A reduction is decreased by approximately 50-fold, electron transfer from Q_A to the plastoquinone pool is blocked, Q_A apparently can be displaced by exogenous quinones, and the stability of reduced Q_A is increased by more than an order of magnitude. Also the affinity of the PS II-directed herbicide diuron to the PS II complex is decreased to undetectable levels. We suggest that these mutation-induced changes in the properties of the acceptor side of PS II are caused by a functional loss of the non-heme iron. This would imply that the non-heme iron in PS II plays a functionally more important role than observed in reaction centers from purple bacteria, and has drastic effects on the properties of Q_A. Moreover, the results obtained on the D2 mutant H268Q illustrate that the D2 protein can have a pronounced influence on the properties of the Q_B/herbicide-binding environment, which is associated mostly with the D1 protein. Thus, the non-heme iron in PS II appears to both affect Q_A reactivity and alter the properties of the Q_B pocket.