Spectroscopic and functional characterization of a highly oxygen evolving Photosystem II reaction center complex from spinach

An oxygen evolving Photosystem II (PS II) reaction center complex was isolated with sucrose gradient centrifugation of PS II membranes solubilized with the detergent dodecybnaltoside in the presence of Ca 2+ and Cl - . The maximum oxygen evolution was about 1300 μ mol O 2 /mg Chi per hour. The quantum yield at low light intensities was similar in PS II membranes and the reaction center complex which indicates that all centers in the reaction center preparation still evolved oxygen. The complex which had a molecular weight of about 350 kDa was highly enriched in the intrinsic PS II reaction center polypeptides and the extrinsic 33 kDa protein from the oxygen evolving system. It contained only trace amounts of the light harvesting complex which was disconnected from the reaction centers. It contained 4 Mn-atoms, 1 low potential cytochrome b -559,1 dark-stable EPR Signal II and 1 primary acceptor Q A per 66 chlorophyll molecules. A normal EPR spectrum from the S 2 -state (“multiline signal”) indicates that essential structural elements on the donor side of PS II remained intact during the detergent treatment. The acceptor side was modified as shown by an increased accessibility of Q A , a slow electron transfer between Q A and the secondary acceptor , Q B , and an altered EPR signal from the reduced Q A Fe complex.