Ribonuclease Rs from Rhizopus stolonifer: lowering of optimum temperature in the presence of urea

Abstract

RNase Rs showed an approx. 2-fold increase in its activity when incubated in the presence of 2 M urea at 37°C. The increase in its activity, in the presence of urea, was comparable to the activity at its optimum temperature, i.e. 45°C. Compared to the native enzyme at 37°C, the Km and Vmax of RNase Rs at 45°C and in the presence of 2 M urea at 37°C showed an increase while kcat/Km decreased. Arrhenius plots in the presence and absence of urea showed a decrease in the activation energy in the presence of urea. Though there was no change in the secondary structure of the protein in the presence of urea, minor changes were observed in the tertiary structure. Hence, the increase in the activity of RNase Rs, in the presence of 2 M urea at 37°C, is due to the lowering of the activation energy as a result of changes in the microenvironment of the active site.