Photoelectric responses of oxygen-evolving complexes of photosystem II

The generation of a transmembrane electric potential difference induced by a series of laser flashes was studied by the direct electrometrical method in proteoliposomes containing oxygen-evolving particles of photosystem II. In addition to the fast stage of generation of the membrane potential, which is due to electron transfer from the redox active tyrosine residue Tyr-161 (YZ) to the primary quinone acceptor QA, electrogenic stages corresponding to the S1 --> S2 (tau = 30 &mgr;sec), S2 --> S3 (tau = 240 &mgr;sec), and S3 --> S4 --> S0 (tau = 6.2 msec) transitions of the oxygen-evolving complex (OEC) were observed. The amplitudes of the photoelectric responses show that the contribution of the OEC to the overall electrogenicity is small. The parameters of the electrogenic reactions of the OEC as measured in photosystem II preparations containing the peripheral proteins of 23 and 17 kD were similar to those of photosystem II preparations devoid of these peptides. It is concluded that neither the 23- nor the 17-kD proteins are involved in the electrogenic reactions of the OEC.