Competitive inhibition of electron donation to photosystem 1 by metal-substituted plastocyanin.
The electron transfer from wild-type spinach plastocyanin (Pc) to photosystem 1 has been studied by flash-induced absorption changes at 830 nm. The decay kinetics of photo-oxidized P700 are drastically slower in the presence of Ag(I)-substituted Pc, while addition of Zn(II)-substituted Pc has a weaker effect. The metal-substituted forms of Pc act as competitive inhibitors of the reaction between normal, Cu-containing, Pc and P700. The inhibition constants obtained from an analysis of the kinetic data were 30 and 410 microM for Ag(I)- and Zn(II)-substituted Pc, respectively. When the Gly8Asp mutant form of Pc was used instead of the wild-type form, the corresponding values were found to be 77 and 442 microM. If the Ag- and Zn-derivatives can be considered as structural mimics of reduced and oxidized CuPc, respectively, our results imply that there is a redox-induced decrease in the affinity between Pc and photosystem 1 that follows the electron donation to P700. Our data also imply that the Gly8Asp mutation can diminish the magnitude of this change. The findings reported here are consistent with a reaction mechanism where the electron transfer in the complex between Pc and photosystem 1 is assumed to be reversible.
Jansson
Hanna
H
Department of Chemistry, University of Gothenburg, PO Box 462, SE-405 30 Gothenburg, Sweden.
Hansson
Orjan
O
D003300:Copper; D004583:Electrons; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D008670:Metals / Q000378:metabolism*; D050505:Mutant Proteins / Q000737:chemistry; D010084:Oxidation-Reduction; D010782:Photolysis; D045331:Photosystem I Protein Complex / Q000378:metabolism*; D010970:Plastocyanin / Q000737:chemistry / Q000302:isolation & purification / Q000378:metabolism*; D017433:Protein Structure, Secondary; D012834:Silver; D013057:Spectrum Analysis; D018724:Spinacia oleracea / Q000378:metabolism*; D015032:Zinc
D008670:Metals; D050505:Mutant Proteins; D045331:Photosystem I Protein Complex; D012834:Silver; D003300:Copper; D010970:Plastocyanin; D015032:Zinc