Mutations D1-E189K, R and Q of Synechocystis sp. PCC6803 are without influence on ns-to-ms electron transfer between OEC-YZ-P680 in photosystem II

Abstract

The oxygen-evolving manganese cluster (OEC) of photosynthesis is oxidised by the photochemically generated primary oxidant (P680(+) of photosystem II (PSII) via a tyrosine residue known as YZ (Tyr161 on the D1 subunit). The redox span between these components is rather small and probably tuned by protonic equilibria. The very efficient electron transfer from YZ to P680(+ in nanoseconds requires the intactness of a hydrogen bonded network presumably involving the OEC, YZ, D1-His190, and neighbouring residues (1). Recent data suggest that D1-Glu189 plays a critical role in maintaining this hydrogen bonded network and may help to position a group that accepts a proton from D1-His190 when YZ is oxidised by P680(+ (2). To further examine the role of Glu189, we measured the rates of electron transfer from the OEC to YZ(and from YZ to P680(+ in the mutants D1-E189Q, D1-E189R and D1-E189K of Synechocystis sp. PCC 6803. Surprisingly, neither the electron transfer from the OEC to YZox nor the electron transfer from YZ to P680(+ was affected to any significant extent by any of these mutations. The missing electrostatic effect of the mutations of D1-Glu189 may have two reasons. First, D1-Glu189 could be far apart from the manganese cluster and YZ and/or second, D1-Glu189 could be located in a very hydrophobic environment so that all residues, namely E, K, R and Q, are uncharged. According to these data D1-Glu189 plays no direct role as a proton-acceptor/donor for the proton coupled electron transfer between YZ and P680(+. 1. Haumann, M., Mulkidjanian, A., and Junge, W. (1999) Biochemistry 38, 1258-1267. 2. Debus, R. J., Campbell, K. A., Pham, D. P., Hays, A. M. A., and Britt, R. D. (2000) Biochemistry 39, 6275-6287.