title: Kinetics of photo-induced electron transfer from high-potential iron-sulfur protein to the photosynthetic reaction center of the purple phototroph Rhodoferax fermentans.
abstract: The kinetics of photo-induced electrontransfer from high-potential iron-sulfur protein (HiPIP) to the photosynthetic reaction center (RC) of the purple phototroph Rhodoferarfermentans were studied. The rapid photooxidation of heme c-556 belonging to RC is followed, in the presence of HiPIP, by a slower reduction having a second-order rate constant of 4.8 x 10(7) M(-1) x s(-1). The limiting value of kobs at high HiPIP concentration is 95 s(-1). The amplitude of this slow process decreases with increasing HiPIP concentration. The amplitude of a faster phase, observed at 556 and 425 nm and involving heme c-556 reduction, increases proportionately. The rate constant of this fast phase, determined at 425 and 556 nm, is approximately 3 x 10(5) s(-1). This value is not dependent on HiPIP concentration, indicating that it is related to a first-order process. These observations are interpreted as evidence for the formation of a HiPIP-RC complex prior to the excitation flash, having a dissociation constant of -2.5 microM. The fast phase is absent at high ionic strength, indicating that the complex involves mainly electrostatic interactions. The ionic strength dependence of kobs for the slow phase yields a second-order rate constant at infinite ionic strength of 5.4 x 10(6) M(-1) x s(-1) and an electrostatic interaction energy of -2.1 kcal/mol (1 cal = 4.184 J). We conclude that Rhodoferar fermentans HiPIP is a very effective electron donor to the photosynthetic RC.
authors.affiliation: Department of Biology, University of Bologna, Italy.
authors.forename: A
authors.initials: A
authors.lastname: Hochkoeppler
authors.forename: D
authors.initials: D
authors.lastname: Zannoni
authors.forename: S
authors.initials: S
authors.lastname: Ciurli
authors.forename: T E
authors.initials: TE
authors.lastname: Meyer
authors.forename: M A
authors.initials: MA
authors.lastname: Cusanovich
authors.forename: G
authors.initials: G
authors.lastname: Tollin
mesh_terms: D004579:Electron Transport; D007506:Iron-Sulfur Proteins / Q000378:metabolism*; D007700:Kinetics; D008027:Light; D008433:Mathematics; D008962:Models, Theoretical; D010788:Photosynthesis; D045322:Photosynthetic Reaction Center Complex Proteins / Q000737:chemistry / Q000302:isolation & purification / Q000378:metabolism*; D012244:Rhodospirillaceae / Q000254:growth & development / Q000378:metabolism*; D013053:Spectrophotometry; D013997:Time Factors
chemical_list: D007506:Iron-Sulfur Proteins; D045322:Photosynthetic Reaction Center Complex Proteins