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Plastocyanin (Pc) is a soluble copper protein that transfers electrons from cytochromeb6 fto photosystem I (PSI), two protein complexes that are localized in the thylakoid mem-branes in chloroplasts. The surface electrostatic potential distributionof Pc plays akey role in complex formationwith the membrane-bound partners. It is practically identical for Pcs fromplants and green algae, but is quite different for Pc fromferns. | Eur. J. Biochem. 271 3449-3456 2004 FEBS 2004 doi 10.1111 j.1432-1033.2004.04283.x Functional characterization of the evolutionarily divergent fern plastocyanin Jose A. Navarro1 Christian E. Lowe2 Reinout Amons3 Takamitsu Kohzuma4 Gerard W. Canters2 Miguel A. De la Rosa1 Marcellus Ubbink2 and Manuel Hervas1 1Instituto de Bioquimica Vegetal y Fotosintesis Centro de Investigaciones Cientificas Isla de la Cartuja Universidad de Sevilla y CSIC Spain 2Leiden Institute of Chemistry Leiden University the Netherlands 3Department of Molecular Cell Biology Leiden University Medical Center the Netherlands 4Faculty of Science Ibaraki University Mito Japan Plastocyanin Pc is a soluble copper protein that transfers electrons from cytochrome b6f to photosystem I PSI two protein complexes that are localized in the thylakoid membranes in chloroplasts. The surface electrostatic potential distribution of Pc plays a key role in complex formation with the membrane-bound partners. It is practically identical for Pcs from plants and green algae but is quite different for Pc from ferns. Here we report on a laser flash kinetic analysis of PSI reduction by Pc from various eukaryotic and prokaryotic organisms. The reaction of fern Pc with fern PSI fits a two-step kinetic model consisting of complex formation and electron transfer whereas other plant systems exhibit a mechanism that requires an additional intracomplex rearrangement step. The fern Pc interacts inefficiently with spinach PSI showing no detectable complex formation. This can be explained by assuming that the unusual surface charge distribution of fern Pc impairs the interaction. Fern PSI behaves in a similar way as spinach PSI in reaction with other Pcs. The reactivity of fern Pc towards several soluble c-type cytochromes including cytochrome f has been analysed by flavin-photosensitized laser flash photolysis demonstrating that the specific surface motifs for the interaction with cytochrome f are conserved in fern Pc. Keywords
