Plastocyanin binding to photosystem I as a function of the charge state ofthe metal ion: Effect of metal site conformation

The binding of Ag- and Cd-substituted plastocyanin to reduced photosystem 1 of spinach has been studied through the rotational correlation time of pla stocyanin measured by the technique of perturbed angular correlation of gam ma-rays (PAC), Ag and Cd are used as models for native Cu(I) and Cu(II), re spectively, A dissociation constant of 5 mu M was found for Ag-plastocyanin , whereas the dissociation constant was at least 24 times higher for Cd-pla stocyanin. PAC was further used to characterize the structure of the metal site of Cd- and Ag-plastocyanin. The Cd spectra are characteristic of a pla nar configuration of one cysteine and two histidines, However, the spectra show an unusual peak broadening and a high degree of internal motion, inter preted as motion of one of the histidines within the plane. Ag-111 decays t o Cd-111, followed by the emission of two gamma-rays used for the PAC exper iment. The Ag-111 PAC spectra indicate that one of the coordinating histidi nes has a different position in the Ag protein than in the Cd protein but t hat the decay of Ag to Cd causes a relaxation of the position of this histi dine to the position in the Cd protein within 20 ns, Binding of Ag-plastocy anin to photosystem I stabilized the Ag metal site structure so that no rel axation was observed on a time scale of 100 ns. This stabilization of the A g structure upon binding indicates that the metal site structure is involve d in regulating how the dissociation constant for plastocyanin depends on t he charge of the metal ion.