Substitution of chloride by bromide modifies the low-temperature tyrosine Z oxidation in active photosystem II

Chloride is an essential cofactor for photosynthetic water oxidation. However, its location and functional roles in active photosystem II are still a matter of debate. We have investigated this issue by studying the effects of Cl− replacement by Br− in active PSII. In Br− substituted samples, Cl− is effectively replaced by Br− in the presence of 1.2 M NaBr under room light with protection of anaerobic atmosphere followed by dialysis. The following results have been obtained. i) The oxygen-evolving activities of the Br−-PSII samples are significantly lower than that of the Cl−-PSII samples; ii) The same S2 multiline EPR signals are observed in both Br− and Cl−-PSII samples; iii) The amplitudes of the visible light induced S1TyrZ
• and S2TyrZ
• EPR signals are significantly decreased after Br− substitution; the S1TyrZ
• EPR signal is up-shifted about 8 G, whereas the S2TyrZ
• signal is down-shifted about 12 G after Br− substitution. These results imply that the redox properties of TyrZ and spin interactions between TyrZ
• and Mn-cluster could be significantly modified due to Br− substitution. It is suggested that Cl−/Br− probably coordinates to the Ca2+ ion of the Mn-cluster in active photosystem II.