The functional properties of extremophilic Dictyoglomus thermophilum xylanase (XYNB) and the N-terminal disulphide-bridge mutant (XYNB-DS) were studied at high pressure and temperature. The enzymes were quite stable even at the pressure of 500 MPa at 80 degrees C. The half-life of inactivation in these conditions was over 30 h. The inactivation at 80 degrees C in atmospheric pressure was only 3-times slower. The increase of pressure up to 500 MPa at 80 degrees C decreased only slightly the enzyme's stability, whereas in 500 MPa the increase of temperature from 22 to 80 degrees C decreased significantly more the enzyme's stability. While the high temperature (80-100 degrees C) decreased the enzyme reaction with short xylooligosaccharides (xylotetraose and xylotriose), the high pressure (100-300 MPa) had an opposite effect. The temperature of 100 degrees C strongly increased the K-m but did not affect the k(cat) to the same extent, thus indicating that the interaction of the substrate with the active site suffers before the catalytic reaction begins to decrease as the temperature rises. Circular dichroism spectroscopy showed the high structural stability of XYNB and XYNB-DS at 93 degrees C. (C) 2014 Elsevier Inc. All rights reserved.