Open AccessRole of electrostatics in the interaction between plastocyanin and photosystem I of the cyanobacterium Phormidium laminosum
Author(s) -
SchlarbRidley Beatrix G.,
Navarro José A.,
Spencer Matthew,
Bendall Derek S.,
Hervás Manuel,
Howe Christopher J.,
De la Rosa Miguel A.
Publication year - 2002
Publication title -
european journal of biochemistry
Language(s) - English
Resource type - Journals
eISSN - 1432-1033
pISSN - 0014-2956
DOI - 10.1046/j.1432-1033.2002.03314.x
Subject(s) - plastocyanin , ionic strength , photosystem i , photosystem ii , chemistry , photosynthetic reaction centre , photochemistry , reaction rate constant , photosynthesis , electron transfer , biochemistry , kinetics , physics , aqueous solution , quantum mechanics
The interactions between photosystem I and five charge mutants of plastocyanin from the cyanobacterium Phormidium laminosum were investigated in vitro . The dependence of the overall rate constant of reaction, k 2 , 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 MgCl 2 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 plastocyanin, both with photosystem I (this paper) and with cytochrome f [Schlarb‐Ridley, B.G., Bendall, D.S. & Howe, C.J. (2002) Biochemistry 41 , 3279–3285]. We discuss the implications of this conclusion for the divergent evolution of cyanobacterial and plant plastocyanins.