Evidence for two tightly bound iron-quinones in the electron acceptor complex of photosystem II

AbstractThe EPR signal of the iron-quinone electron acceptor of photosystem II in higher plant chloroplasts is normally difficult to observe. If the preparation is washed with formate to remove bound CO2 the signal becomes larger. Using formate-washed photosystem II particles from peas the redox potential of the ironquinone complex has been determined. Two waves are observed in the titrations at Em ~0 and ~ −250 mV. The ability to reduce the pheophytin intermediary electron acceptor by illumination at 200 K is largely associated with the −250 mV step. It is suggested that there are two tightly bound quinones in the reaction centre and that the low-potential component is Qa. It is unlikely that the high-potential component is the gating quinone (Qb) as it is not displaced by DCMU and is present in Chlamydomonas reinhardtii preparations which lack the gating quinone