Transient homodimer interactions studied using the electron self-exchange reaction

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 (k_ese) of four plastocyanins. The k _ese 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 k _ese for the almost neutral cyanobacterial plastocyanin from Anabaena variabilis exhibits very little dependence on ionic strength. The temperature dependence of the k_ese 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 k_ese 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 k_ese. 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 k _ese of spinach plastocyanin, which along with the known properties of the thylakoid lumen suggests a biological role for electron self-exchange.