Transient homodimer protein interactions have been investigated by analyzing the influence of ionic strength (NaCl) on the electron self-exchange (the bimolecular reaction whereby the two oxidation states of a redox protein interconvert) rate constant (kese) of four plastocyanins. The kese values for the plastocyanins from spinach, Dryopteris crassirhizoma (a fern), and the green alga Ulva pertusa, which possess acidic patches of varying size and locations, increase 190-, 29-, and 21-fold, respectively, at elevated ionic strength (I = 2.03 m). In contrast, the kese for the almost neutral cyanobacterial plastocyanin from Anabaena variabilis exhibits very little dependence on ionic strength. The temperature dependence of the kese for spinach plastocyanin (I = 0.28 m) provides evidence for poor packing at the homodimer interface. Representative structures of the transient homodimers involved in electron self-exchange, which are consistent with fits of the ionic strength dependence of kese to van Leeuwen theory, have been obtained from protein modeling and docking simulations. The Coulombic energy of the docked homodimers follows the order spinach > D. crassirhizoma > U. pertusa > A. variabilis, which matches that of the overall influence of ionic strength on kese. Analysis of the homodimer structures indicates that poor packing and high planarity are features of the interface that favor transient interactions. The physiologically relevant Mg2+ ion has a much more pronounced influence on the kese of spinach plastocyanin, which along with the known properties of the thylakoid lumen suggests a biological role for electron self-exchange.
