Figure 6.

S195 mutants of thrombin.A, rate of relaxation for Na⁺ binding to thrombin wild-type (black) and mutants S195T (white), C42A/C58A (green), C42A/C58A/S195T (shaded green), and S195C (red). Continuous lines were drawn as for the data in Figure 2B, with best-fit parameter values listed in Table 1. Experimental conditions are: 400 mM ChCl, 50 mM Tris, 0.1% PEG8000, pH 8.0, at 15 °C. B, M⁺ activation profile for thrombin wild-type and mutants S195T, C42A/C58A (CC), C42A/C58A/S195T (S195T/CC) and S195C. Data depict values of the specificity constant $s=k_{\text{cat}}/K_m$ for the hydrolysis of chromogenic substrate FPR (bottom panel) relative to the value measured in the presence of the inert cation Ch⁺ (top panel). M⁺s refer to Li⁺ (red), Na⁺ (orange), K⁺ (yellow), Rb⁺ (cyan), Cs⁺ (blue). Experimental conditions are: 5 mM Tris, 0.1% PEG8000, pH 8.0 at 25 °C, in the presence of 400 mM M⁺Cl⁻ salt as indicated.