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

RNase Rs showed an approx. 2-fold increase in its activity when incubated i n the presence of 2 M urea at 37 degreesC. The increase in its activity, in the presence of urea, was comparable to the activity at its optimum temper ature, i.e. 45 degreesC. Compared to the native enzyme at 37 degreesC, the K-m and V-max of RNase Rs at 45 degreesC and in the presence of 2 M urea at 37 degreesC showed an increase while k(cat)/K-m decreased. Arrhenius plots in the presence and absence of urea showed a decrease in the activation en ergy in the presence of urea. Though there was no change in the secondary s tructure of the protein in the presence of urea, minor changes were observe d in the tertiary structure. Hence, the increase in the activity of RNase R s, in the presence of 2 M urea at 37 degreesC, is due to the lowering of th e activation energy as a result of changes in the microenvironment of the a ctive site. (C) 2001 Elsevier Science B.V. All rights reserved.