Thermal denaturation of ribonuclease T1. A DSC study

The thermal denaturation of microbial Ribonuclease T1 (RNAase T1) as a function of pH, was studied by means of DSC microcalorimetry. The midpoint denaturation temperatures, enthalpy changes and heat capacity changes of Ribonuclease T1 were compared with those obtained for pancreatic Ribonuclease A (RNAase A). It was found that the microbial T1 protein undergoes a more complex conformational transition than the simple two-state transition shown by Ribonuclease A. The hypothesis of the presence of a 'molten globule' form is discussed. The conformational stability of RNAase T1 is lower than that of RNAase A at high pH values. Indeed, the maximum stability of RNAase T1 occurs at pH ≈ 5, whereas that of RNAase A occurs at pH ≈ 8. At pH=3.7 an irreversible aggregation phenomenon was indicated by the existence of a reproducible exothermic peak. The conformational transition of RNAase T1 is reversible in the range of pH 4.5-7.0, whereas it becomes irreversible at pH≥8.0 as for RNAase