A binding site for chloride in photosystem II which specifically influences acceptor-side electron transfer and plastoquinone binding

Abstract

When photosystem II (PS II) membranes are exposed to a short xenon flash, charge separation occurs and reduction of plastoquinone follows. The re-oxidation of the reduced primary quinone, Q-A can be monitored by measuring the transient fluorescence decay. Millisecond transients represent electron transfer from, QA-, to the secondary plastoquinone, QB, and binding of plastoquinone to the empty QB site. To obtain chloride-free PS II membranes without loss of polypeptides the material was dialysed against chloride free media. The kinetics of induced fluorescence differs between chloride-containing and chloride-depleted PS II membranes. The effect of chloride is not uniform with regard to the choice of cryoprotectant. In chloride-depleted PS II membranes with sucrose in the buffer medium the acceptor-side electron transfer rates were lower and binding of plastoquinone to QB site was weakened. The effects of chloride depletion were reversed by re-addition of chloride. With glycerol as a cryoprotectant the kinetics were only marginally affected by chloride depletion. This indicates that glycerol can prevent the inhibition of fast electron transfer and the decrease in plastoquinone affinity caused by chloride removal. The affinity of the acceptor side-related Cl- was about 25 times weaker that of the Cl- ion previously shown to be associated with donor side function under similar conditions [Lindberg, K., Vanngard, T. and Andreasson, L.-E (1993) Photosynth. Res. 38, 401-408], with rapid exchange with the medium at all times. This shows that there are at least two functionally distinguishable chloride binding sites in PS II.