Effects of Mutations of Thermolysin, Asn116 to Asp and Asp150 to Glu, on Salt-Induced Activation and Stabilization

Abstract

Neutral salts activate and stabilize thermolysin. We previously found that two single mutations, Asn116→Asp and Asp150→Glu, increase the activity of thermolysin. In the present study, we examined their effects on NaCl-induced activation and stabilization. In the hydrolysis of N-[3-(2-furyl)acryloyl]-glycyl-L-leucine amide, the relative activities (the ratios of the specificity constant, k_cat/K_m, at x M NaCl to that at 0 M NaCl) at 0.5–4.0 M NaCl of D150E and N116D/D150E were lower than those of wild-type thermolysin (WT) and N116D, respectively. In thermal inactivation at 70 °C, the relative stabilities (the ratios of the first-order rate constant, k_obs, at 0 M NaCl to that at x M NaCl) at 0.5–4.0 M NaCl of D150E and N116D/D150E were lower than those of WT and N116D, respectively. These results indicate that unlike Asn116→Asp, Asp150→Glu reduced NaCl-induced activation and stabilization, suggesting that the binding of ions with certain residues of thermolysin is involved in the activation and stabilization.