BICARBONATE IS AN ESSENTIAL CONSTITUENT OF THE WATER-OXIDIZING COMPLEX OF PHOTOSYSTEM-II

It is shown that restoration of photoinduced electron flow and O-2 evo lution with Mn2+ in Mn-depleted photosystem II (PSII) membrane fragmen ts isolated from spinach chloroplasts is considerably increased with b icarbonate in the region pH 5.0-8.0 in bicarbonate-depleted medium. In buffered solutions equilibrated with the atmosphere (nondepleted of b icarbonate), the bicarbonate effect is observed only at pH lower than the pK of H2CO3 dissociation (6.4), which indicates that HCO3- is the essential species for the restoration effect, The addition of just 2 M n2+ atoms per one PSII reaction center is enough for the maximal react ivation when bicarbonate is present in the medium, Analysis of bicarbo nate concentration dependence of the restoration effect reveals two bi nding sites for bicarbonate with apparent dissociation constant (K-d) of approximate to 2.5 mu M and 20-34 mu M when 2,6-dichloro-p-benzoqui none is used as electron acceptor, while in the presence of silicomoly bdate only the latter one remains, Similar bicarbonate concentration d ependence of O-2 evolution was obtained in untreated Mn-containing PSI I membrane fragments. It is suggested that the K-d of 20-34 mu M is as sociated with the donor side of PSII while the location of the lower K -d binding site is not quite clear. The conclusion is made that bicarb onate is an essential constituent of the water-oxidizing complex of PS II, important for its assembly and maintenance in the functionally act ive state.