title: Redox and acid-base characterization of cytochrome b-559 in photosystem II particles.
abstract: The redox and acid/base states and midpoint potentials of cytochrome b-559 have been determined in oxygen-evolving photosystem II (PS II) particles at room temperature in the pH range from 6.5 to 8.5. At pH 7.5 the fresh PS II particles present about 2/3 of their cytochrome b-559 in its reduced and protonated (non-auto-oxidizable) high-potential form and about 1/3 in its oxidized and non-protonated low-potential form. Potentiometric reductive titration shows that the protonated high-potential couple is pH-independent (E'0, + 380 mV), whereas the low-potential couple is non-protonated and pH-independent above pH 7.6 (E'0, pH greater than 7.6, + 140 mV), but becomes pH-dependent below this pH, with a slope of -72 mV/pH unit. Moreover, evidence is presented that in PS II particles cytochrome b-559 can cycle, according to its established redox and acid/base properties, as an energy transducer at two alternate midpoint potentials and at two alternate pKa values. Red light absorbed by PS II induces reduction of cytochrome b-559 in these particles at room temperature, the reaction being completely blocked by dichlorophenyldimethylurea.
authors.affiliation: Instituto de Bioquímica Vegetal y Fotosíntesis, Facultad de Biología, Universidad de Sevilla, Spain.
authors.forename: J M
authors.initials: JM
authors.lastname: Ortega
authors.forename: M
authors.initials: M
authors.lastname: Hervás
authors.forename: M
authors.initials: M
authors.lastname: Losada
mesh_terms: D002734:Chlorophyll / Q000032:analysis*; D003573:Cytochrome b Group / Q000032:analysis*; D004735:Energy Transfer; D006863:Hydrogen-Ion Concentration; D008027:Light; D045342:Light-Harvesting Protein Complexes; D010084:Oxidation-Reduction; D045322:Photosynthetic Reaction Center Complex Proteins; D045332:Photosystem II Protein Complex; D010940:Plant Proteins / Q000032:analysis*; D013696:Temperature
chemical_list: D003573:Cytochrome b Group; D045342:Light-Harvesting Protein Complexes; D045322:Photosynthetic Reaction Center Complex Proteins; D045332:Photosystem II Protein Complex; D010940:Plant Proteins; D002734:Chlorophyll; C019313:cytochrome b559