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

Abstract

Sodium formate at concentration of 5–20 μM 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 acceptors) and oxygen evolution and the increase of absorbance changes related to photooxidation of P₆₈₀, 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 μM 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 μM 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.