Bicarbonate stimulates the electron donation from Mn2+ to [formula omitted] in isolated D1/D2/cytochrome b559 complex

Abstract

Influence of bicarbonate on the efficiency of the electron donation from Mn2+ to P680+ in isolated D1/D2/cytochrome b559 complex was investigated. All the experiments were carried out in a medium depleted of HCO3-/CO2. Kinetics of photoinduced absorbance changes (ΔA) at different wavelengths and decrease of chlorophyll fluorescence yield (−ΔF) related to photoaccumulation of reduced pheophytin, the intermediary electron acceptor of photosystem II (PSII), in the presence of Mn2+ under anaerobic conditions were measured. Addition of bicarbonate (1mM) increased the amplitude of these ΔA and −ΔF at least by a factor of 3. Measurements of the photoinduced ΔA, related to photooxidation of the primary electron donor of PSII, chlorophyll P680, were done in the presence of silicomolybdate as electron acceptor. These results show that the addition of 0.05mM Mn2+ alone or jointly with 1mM bicarbonate induces a 20% and 70%-decrease of the magnitude of the ΔA at 680nm. The effect of Mn2+ (in the presence and absence of bicarbonate) was completely eliminated by the addition of 12mM EDTA. All these bicarbonate effects were not observed if MgCl2 or formate were used instead of MnCl2 and bicarbonate, respectively. In the absence of Mn2+, bicarbonate induced none of the mentioned above effects (increase of photoaccumulation of reduced pheophytin and decrease of photooxidation of P680). The presented data suggest that bicarbonate stimulates the electron donation from Mn2+ to D1/D2/cyt b559 reaction center evidently due to formation of easily oxidizable Mn-bicarbonate complexes.