Arginine residues in the D2 polypeptide may stabilize bicarbonate binding in photosystem II of Synechocystis sp. PCC 6803

Bicarbonate (HCO₃^-) causes a significant and reversible stimulation of anion-inhibited electron flow in photosystem II of higher plants and cyanobacteria. To test if selected arginine (Arg) residues are involved in the binding of HCO₃^-, we utilized oligonucleotide-directed mutagenesis to construct Synechocystis sp. PCC 6803 mutants carrying mutations in Arg residues in the D2 protein. Measurements of oxygen evolution showed that the D2 mutants R233Q (arginine-233 → glutamine) and R251S (arginine-251 → serine) were 10-fold more sensitive to formate than the wild type. The formate concentration giving half-maximal inhibition of the steady-state oxygen evolution rate was 48 mM, 4.5 mM and 4 mM for the wild type, R233Q and R251S, respectively. Measurements of oxygen evolution in single-turnover flashes confirm that the mutants are more sensitive to formate than the wild type. Measurements of chlorophyll a fluorescence decay kinetics after the second saturating actinic flash indicated that, after formate treatment, the halftime of Q_A^- oxidation was decreased by approximately a factor of 2, 4 and 6 in the wild type, R251S and R233Q, respectively. The recombination rate between Q_A^- and S₂ was approx. 2-fold slower in R251S and R233Q than in the wild type. In the presence of 100 mM sodium formate, reactivation of the Hill reaction by bicarbonate showed that the wild type had an apparent K_m for bicarbonate of 0.5 mM, while the K_m values for R233Q and R251S were 1.4 and 1.5 mM, respectively. We suggest that Arg-233 and Arg-251 in the D2 polypeptide contribute to stabilization of HCO₃^- binding in Photosystem II.