title: A high-potential redox component located within cyanobacterial photosystem II.
abstract: The calcium-dependent oxygen evolution activity of preparations of Phormidium luridum shows a marked selectivity in favor of ferricyanide over benzoquinone as Hill oxidant. In addition, the rate of oxygen evolution increases with increasing solution redox potential over the range +350 to +550 mV vs. the standard hydrogen electrode. These properties pertain to both 3-(3,4-dichlorophenyl)-1,1-dimethylurea-sensitive and -insensitive fractions of the total oxygen evolution activity. Neither changes in solution potential nor use of oxidants other than ferricyanide obviate the need for added Ca(2+). To explain these observations, two models are proposed, each of which invokes the existence of a redox component located within Photosystem II and having a midpoint potential greater than +450 mV. In one model, the postulated species is a donor which competes with water for oxidizing equivalents generated by System II. In the other model, the 450 mV species is a high-potential primary acceptor of System II electrons.
mesh_terms: D002118:Calcium / Q000494:pharmacology; D000458:Cyanobacteria / Q000378:metabolism*; D004579:Electron Transport; D005292:Ferricyanides / Q000494:pharmacology; D008956:Models, Chemical; D010084:Oxidation-Reduction; D010100:Oxygen / Q000378:metabolism; D010788:Photosynthesis / Q000187:drug effects
chemical_list: D005292:Ferricyanides; D010100:Oxygen; D002118:Calcium