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

Abstract

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 CO 2 ‐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 Q 2− 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 Q A and Q B .