Oxygenic photosystem II: The mutation D1-D61N in Synechocystis sp. PCC 6803 retards S-state transitions without affecting electron transfer from Y-Z to P-680(+)

Abstract

Photosynthetic oxygen evolution is powered by photosystem II (PSII), in particular by the oxidized chl a-aggregate P-680(+), and catalyzed by the oxygen-evolving complex (Mn4X-entity) as well as a tyrosine residue (Yz). The role of particular amino acids as cofactors of electron and proton transfer or as modulators of the activity is still ill-defined. The effects of single-site mutations at the donor side of PSII on the partial reactions of water oxidation have been primarily studied in whale cells. Because of better signal-to-noise in oxygen-evolving core preparations more detailed information on the electronic, protonic, and electrostatic events is expected from studies with such material. We investigated cells and oxygen-evolving core preparations from the wildtype of Synechocystis sp. PCC 6803 and point-mutants of D1-D61. In cells, oxygen-release was slowed drastically in D61A (8-fold) and D61N (10-fold) compared to WT, whereas it remained unchanged in D61E within the time resolution of the measurements. In core preparations, the S-1 double right arrow S-2 and S-2 double right arrow S-3 transitions were slowed approximately 2-fold in D61N compared to WT. However, the nanosecond components of electron transfer from Y-Z to P-680(+) were unchanged in the same mutant, We conclude that substitution of a neutral residue for D1-D61 selectively affects electron-transfer events on the donor side of Y-2.