DIFFERENTIAL SCANNING CALORIMETRIC STUDY OF THE THERMAL-STABILITY OF XYLANASE FROM STREPTOMYCES-HALSTEDII-JM8

The thermal stability of two xylanases with molecular masses of 45 (Xy s1L) and 35 (Xys1S) kDa has been characterized thermodynamically by hi gh-sensitivity scanning microcalorimetry in the pH range 3.0-9.0. Ther mal denaturation of Xys1L reveals three thermodynamically independent domains, and that of Xys1S, which is a proteolytic fragment of Xys1L ( without a C-terminal part), reveals two thermodynamically independent domains, each of which follows a two-state thermal unfolding process u nder our experimental conditions. Nevertheless, the thermodynamic para meters of unfolding for each domain do not fit some of the correlation s obtained for most compact globular proteins. It is known that if Del ta H-res(T) and Delta S-res(T) are plotted against temperature for a n umber of water-soluble compact globular proteins, they all have a comm on value at approximately 110 degrees C (383 K). Calculation of the va riations in the enthalpy and entropy of unfolding per residue for each domain of xylanase with temperature gave us Delta H-res(383) and Delt a S-res(383) values of approximately 3 kcal/(mol of residue) and 9 cal /(K.mol of residue), respectively. This is practically 2-fold larger t han those apparent for most medium-sized globular protein values. Thes e descrepancies might be related to features of the folded and/or unfo lded states of the protein.