Kinetic and Thermodynamic Thermal Stabilities of Ribonuclease A and Ribonuclease B

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Abstract
The thermal stabilities of ribonuclease A (RNase A) and ribonuclease B (RNase B), which possess identical protein structures but differ by the presence of a carbohydrate chain attached to Asn34 in RNase B, were studied by proteolysis and UV spectroscopy at pH 8.0. Proteolysis was quantified by sodium dodecyl sulfate−polyacrylamide gel electrophoresis and densitometry. Increasing protease concentrations led to a hyperbolic increase of the rate constants of proteolysis. With thermolysin, which attacks the unfolded molecules only, the thermal unfolding constants were determined by extrapolating the rate constants of proteolysis to infinite concentration of protease. With trypsin, the unfolding constants of RNase A could be confirmed. Subtilisin attacked even the native RNases, where RNase B was more stable toward proteolytic degradation. Kinetic stabilities (ΔG⧧) calculated from the unfolding constants for temperatures between 52.5 and 65 °C revealed a higher kinetic stability of RNase B, which results from enthalpic effects only, whereas entropic effects counteract stabilization. ΔΔG⧧ at the transition temperature of RNase A (60.4 °C) was 2.2 ± 0.3 kJ mol-1. Thermodynamic stabilities (ΔG) were estimated from the thermal transition curves at 287 nm for the temperature range from 55 to 70 °C. For 17.5−25 °C, ΔG values were determined from transition curves of unfolding induced by guanidine hydrochloride and extrapolation of the free energy values to those in the absence of denaturant. At all temperatures, RNase B proved to be more stable than RNase A with essentially the same enthalpy and entropy of unfolding. ΔΔG was 2.5 ± 0.2 kJ mol-1 at 60.4 °C and 2.3 kJ mol-1 at 25 °C.