Polyethylene glycol-induced stabilization of subtilisin

Methoxypolyethylene glycol(PEG, molecular weight 5,000), activated by cyanuric chloride or nitrophenol carbonate, was used to modify subtilisin Carlsberg. The thermostabilities of both the native and PEG-modified enzymes were studied via activity assays and differential scanning calorimetry, The two PEG-modified enzymes were significantly more stable against both temperature and pH, but the thermal transition temperature of the enzyme was not greatly changed. The kinetics of enzyme deactivation for each enzyme indicate that the increased stability upon PEG modification is probably the result of a decrease in the rate of autolysis rather than an increase in intrinsic stability.