Bicarbonate binding and the properties of photosystem II electron acceptors

AbstractWe have investigated the quinone-binding region of PS II using exchange reactions to replace key components. Bicarbonate was replaced by formate and QB by exogenous quinones or herbicide. The effects of these changes were monitored using the ESR signals from the QA iron-semiquinone and Fe3+ non-haem iron components. We observed that QB binding caused characteristic changes in the Q−A ESR signal and confirmed that the characteristics of Q−A depend on bicarbonate binding. The non-haem iron was oxidised only under conditions where bicarbonate was bound. The characteristics of bicarbonate binding were observed in Phormidium laminosum PS II, showing that the bicarbonate effect occurs in cyanobacteria. The results support a hypothesis which gives a central role to bicarbonate in providing the conditions for electron transfer in both cyanobacteria and higher plant PS II. It is suggested that bicarbonate binds at or close to the non-haem iron and influences the characteristics of QA, QB and the non-haem iron