Involvement of bicarbonate in the protonation of the secondary quinone electron acceptor of photosystem II via the non-haem iron of the quinone-iron complex

The mechanism of the bicarbonate effect was investigated by monitoring flash-induced pH changes. In control chloroplasts the proton yields exhibit a binary oscillation with a period of four. In CO2-depleted chloroplasts the binary oscillation disappears and only the period four pattern remains, which can be described by proton liberation in the water-oxidizing system. It is concluded that bicarbonate is involved in the protonation of Q2− B. The affinity of bicarbonate to its binding site is much lower in the presence of dithionite. It is suggested that bicarbonate exerts its influence through being a ligand for the non-haem iron between QA and QB.