title: 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 degrees C. The increase in its activity, in the presence of urea, was comparable to the activity at its optimum temperature, i.e. 45 degrees C. Compared to the native enzyme at 37 degrees C, the K(m) and V(max) of RNase Rs at 45 degrees C and in the presence of 2 M urea at 37 degrees C showed an increase while k(cat)/K(m) 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 degrees C, is due to the lowering of the activation energy as a result of changes in the microenvironment of the active site.
authors.affiliation: Division of Biochemical Sciences, National Chemical Laboratory, Pune-411 008, India.
authors.forename: R A
authors.initials: RA
authors.lastname: Deshpande
authors.forename: A R
authors.initials: AR
authors.lastname: Kumar
authors.forename: M I
authors.initials: MI
authors.lastname: Khan
authors.forename: V
authors.initials: V
authors.lastname: Shankar
mesh_terms: D001665:Binding Sites / Q000187:drug effects; D002384:Catalysis / Q000187:drug effects; D005656:Fungal Proteins / Q000737:chemistry / Q000187:drug effects* / Q000378:metabolism; D019791:Guanidine / Q000494:pharmacology; D006146:Guanidines / Q000494:pharmacology; D007700:Kinetics; D011489:Protein Denaturation; D017433:Protein Structure, Secondary / Q000187:drug effects; D017434:Protein Structure, Tertiary / Q000187:drug effects; D012233:Rhizopus / Q000201:enzymology*; D012260:Ribonucleases / Q000737:chemistry / Q000187:drug effects* / Q000378:metabolism; D013696:Temperature; D013861:Thiocyanates / Q000494:pharmacology; D014508:Urea / Q000494:pharmacology*
chemical_list: D005656:Fungal Proteins; D006146:Guanidines; D013861:Thiocyanates; C054436:guanidine thiocyanate; D014508:Urea; D012260:Ribonucleases; C427217:ribonuclease Rs; D019791:Guanidine; C009941:potassium thiocyanate