Further Studies of the Role of a Heme-Protein in Oxygen Evolution

This study indicates the presence of a previously isolated heme-protein (Nakatani and Barber, 1981, Photobiochem. Photobiophys. 2, 69—78) in an O 2 -evolving photosystem II (PS-II) particle. This particle also contains cytochrome b-559 but in its low potential form. Although this low potential cyt. b-559 underwent light-induced redox changes at room temperature in the presence of potassium ferricyanide, no changes were observed when benzoquinone was used as an electron acceptor. Inhibition studies of O 2 evolution and chlorophyll fluorescence from the PS-II particles and inside-out thylakoids support the concept that the previously isolated catalase-type hemeprotein functions on the oxidising side of PS-II. Attempts to detect and isolate the manganese containing 65,000 MW protein of Spector and Winget (1980, Proc. Natl. Acad. Sci. U.S.A. 77, 957—959) have been unsuccessful as have experiments designed to reproduce the light-induced dimerisation effects presented by Winget and Spector at the 5th International Photosynthesis Congress, Halkidiki, Greece, 1980. The role of a catalase-type reaction involving a “crypto-hydroperoxy” intermediate in the water-splitting process is briefly discussed.