Thermodynamic stability of two variants of xylanase (Xys1) from Streptomyces halstedii JM8.

In a continuation of our earlier study [Ruiz-Arribas, A., Santamaría, R.I., Zhadan, G. G., Villar, E. & Shnyrov, V. L. (1994) Differential scanning calorimetric study of the thermal stability of xylanase from Streptomyces halstedii JM8, Biochemistry 33, 13787-13791], we used high-sensitivity differential scanning microcalorimetry, intrinsic tryptophan fluorescence and far-ultraviolet circular dichroism to study the effect of regional sequence differences on the thermodynamic stability of xylanase (Xys1) from Streptomyces halstedii JM8 (1,4-beta-D-xylanohydrolase, EC 3.2.1.8). Thermal transitions were measured for original xylanase (Xys1S) and two variants. Thermal denaturation of all the xylanases studied revealed two structural domains, each of which, despite its partial irreversibility, follows a two-state thermal unfolding process under our experimental conditions. Both variants were found to exhibit slightly decreased stability, possessing the same activity as the original. The unfolding parameters for each domain of both variants, unlike the situation with wild-type xylanase (see our previous report), fit some correlations obtained for the most compact globular proteins. The values of enthalpy and entropy of unfolding/residue at 383 K were found to be inversely proportional to residual, well-regulated structures in unfolded states.