Effects of bicarbonate and formate on the donor side of Photosystem 2

Sodium formate at concentration of 5-20 mu M suppresses electron flow on the donor side of Photosystem 2 (PS 2) in pea subchloroplast membranes (DT-20) which is revealed by inhibition of photoinduced changes of chlorophyll fluorescence yield related to photoreduction of Q(A) and pheophytin (the primary and intermediary electron accepters) and oxygen evolution and the increase of absorbance changes related to photooxidation of P-680, the primary electron donor, under continuous illumination. These activities are also inhibited upon partial depletion of bicarbonate in the medium and restored by the addition of 0.1-10 mM NaHCO3. At concentrations higher than 20 mu M formate induces the known bicarbonate effect on the acceptor side of PS 2 which dominates at millimolar concentrations of the agent. In Tris-treated (Mn-depleted) DT-20 the restoration of electron flow with 0.2 mu M MnCl2 (4 Mn atoms per one PS 2 reaction center) in the medium depleted of bicarbonate is efficient only after the addition of 5 mM NaHCO3. The restoration in the presence of NaHCO3 is accompanied by an increased functional binding of Mn2+ to PS 2 membranes which is confirmed by experiments on removal of added Mn2+ by either sedimentation or the addition of EDTA. Pre-illumination increases the Mn binding in the presence of bicarbonate. The data show that the bicarbonate effect on the donor side of PS 2 is related to a relatively low-affinity bound pool of bicarbonate. It is suggested that bicarbonate takes part in the formation of the Mn-cluster capable of water oxidation as an obligatory ligand or through modification of the binding site(s) of Mn.