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The interactions between photosystem I and five charge mutants of plastocyanin from the cyanobacteriumPhormi-dium laminosumwere investigatedin vitro. The dependence of the overall rate constant of reaction,k2, on ionic strength was investigated using laser flash photolysis. The rate con-stant of the wild-type reaction increased with ionic strength, indicating repulsion between the reaction partners. Remov-inganegativechargeonplastocyanin(D44A) acceleratedthe reactionandmade it independentof ionic strength; removing a positive charge adjacent to D44 (K53A) had little effect | Eur. J. Biochem. 269 5893-5902 2002 FEBS 2002 doi 10.1046 j.1432-1033.2002.03314.x Role of electrostatics in the interaction between plastocyanin and photosystem I of the cyanobacterium Phormidium laminosum Beatrix G. Schlarb-Ridlev1. Jose A Navarro2 Matthew Spencer1 Derek S. Bendall1. Manuel Hervas2 . . . Christopher J. Howe1 and Miguel A. De la Rosa2 1Department of Biochemistry and Cambridge Centre for Molecular Recognition University of Cambridge UK 2Instituto de Bioquimica Vegetal y Fotosintesis Centro de Investigaciones Cientlficas Isla de la Cartuja Universidad de Sevilla y CSIC Spain The interactions between photosystem I and five charge mutants of plastocyanin from the cyanobacterium Phormi-dium laminosum were investigated in vitro. The dependence of the overall rate constant of reaction k2 on ionic strength was investigated using laser flash photolysis. The rate constant of the wild-type reaction increased with ionic strength indicating repulsion between the reaction partners. Removing a negative charge on plastocyanin D44A accelerated the reaction and made it independent of ionic strength removing a positive charge adjacent to D44 K53A had little effect. Neutralizing and inverting the charge on R93 slowed the reaction down and increased the repulsion. Specific effects of MgCl2 were observed for mutants K53A R93Q and R93E. Thermodynamic analysis of the transition state revealed positive activation entropies suggesting partial desolvation of the interface in the transition state. In comparison with plants plastocyanin and photosystem I of Phormidium laminosum react slowly at low ionic strength whereas the two systems have similar rates in the range of physiological salt concentrations. We conclude that in P. laminosum in contrast with plants in vitro hydrophobic interactions are more important than electrostatics for the reactions of plastocya-nin both with photosystem I this paper and with cytochrome f Schlarb-Ridley B.G. Bendall D.S. Howe C.J. 2002 .
