Salt dependence of the thrombin-protein C interaction. The value of k_cat/K_m for protein C activation by thrombin under experimental conditions of 50mM Tris, pH 7.4, 5mM CaCl₂, 0.1% PEG8000 at 37°C, was measured as a function of salt concentration in the range 200 to 800mM. The slope in the log-log plot, Γ, gives a measure of the electrostatic coupling (ie, charges involved or ions exchanged) on formation of the complex. Symbols refer to protein C wild-type (circles) or mutant EDD (squares) in the presence of thrombin wild-type (closed symbols) or mutant E192Q (open symbols). Values of Γ are as follows: −0.7 ± 0.1 (closed circles, thrombin-protein C), −1.4 ± 0.1 (closed squares, thrombin-protein C EDD), −0.5 ± 0.1 (open circles, thrombin E192Q-protein C), and −1.1 ± 0.1 (open squares, thrombin E192Q-protein C EDD). These values prove that there is no electrostatic clash between thrombin and protein C. The thrombin-protein C interaction is actually favored by electrostatic coupling that increases with the EDD mutation of protein C but is not affected by the E192Q mutation of thrombin.