Scan‐rate dependence in protein calorimetry: The reversible transitions of Bacillus circulans xylanase and a disulfide‐bridge mutant

The stabilities of Bucillus circuluns xylanase and a disulfide‐bridge‐containing mutant (S100C/N148C) were investigated by differential scanning calorimetry (DSC) and thermal inactivation kinetics. The thermal denaturation of both proteins was found to be irreversible, and the apparent transition temperatures showed a considerable dependence upon scanning rate. In the presence of low (nondenaturing) concentrations of urea, calorimetric transitions were observed for both proteins in the second heating cycle, indicating reversible denaturation occurs under those conditions. However, even for these reversible processes, the DSC curves for the wild‐type protein showed a scan‐rate dependence that was similar to that in the absence of urea. Calorimetric thermograms for the disulfide mutant were significantly less scan‐rate dependent in the presence of urea than in the urea‐free buffer. The present data show that, just as for irreversible transitions, the apparent transition temperature for the reversible denaturation of proteins can be scan‐rate dependent, confirming the prediction of Lepock et al. (Lepock JR, Rithcie KP, Kolios MC, Rodahl AM, Heinz KA, Kruuf J, 1992, Biochemistry 31 :12706‐12712). The kinetic factors responsible for scan‐rate dependence may lead to significant distortions and asymmetry of endotherms, especially at higher scanning rates. This points to the need to check for scan‐rate dependence, even in the case of reversible denaturation, before any attempt is made to analyze asymmetric DSC curves by standard thermodynamic procedures. Experiments with the disulfide‐bridge‐containing mutant indicate that the introduction of the disulfide bond provides additional stabilization of xylanase by changing the rate‐limiting step on the thermal denaturation pathway.