Enhanced activity and stability in the presence of organic solvents by increased active site polarity and stabilization of a surface loop in a metalloprotease

Salinivibrio zinc-metalloprotease (SVP) is an enzyme which was isolated from Salinivibrio proteolyticus, a moderately halophilic species from a hypersaline lake in Iran. A195E and G203D mutants were constructed to increase polarity near the active site in order to preserve the hydration layer against organic solvents [dimethylformamide (DMF), methanol, isopropanol and n-propanol]. A268P was constructed to stabilize a surface loop far from the active site and A195E/A268P was constructed to investigate the combined effects of these two mutations. Results showed that relative C₅₀ values of A195E increased to ~26 and 11% in DMF and methanol whereas an increase of ~32 and 41% was observed in the presence of isopropanol and n-propanol. The irreversible thermoinactivation rate (ki) for A195E was estimated to be 60 and 130 (x10⁻³ min⁻¹) in the presence of DMF and n-propanol, respectively, while ki for SVP was 90 and 190 (x10⁻³ min⁻¹). G203D exhibited similar ki as A195E in the presence of methanol and isopropanol, but the calculated ki in the presence of DMF and n-propanol was 70 and 160 (x10⁻³ min⁻¹), respectively. A268P and A268P/A195E variants marginally increased the thermoresistance of the enzyme in this condition.