The family 52 β-xylosidase from Geobacillus stearothermophilus is a dimer: Structural and biophysical characterization of a glycoside hydrolase

Xylans are the most abundant polysaccharides forming the plant cell wall hemicelluloses, and they are degraded, among other proteins, by β-xylosidase enzymes. In this work, the structural and biophysical properties of the family 52 β-xylosidase from Geobacillus stearothermophilus, XynB2, are described. Size exclusion chromatography, analytical centrifugation, ITC, CD, fluorescence (steady state and ANS-binding) and FTIR were used to obtain the structure, the oligomerization state and the conformational changes of XynB2, as pH, chemical denaturants or temperature were modified. This report describes the first extensive conformational characterization of a family 52 β-xylosidase. The active protein was a highly hydrated dimer, whose active site was formed by the two protomers, and it probably involved aromatic residues. At low pH, the protein was not active and it populated a monomeric molten-globule-like species, which had a conformational transition with a pKa of ~ 4.0. Thermal and chemical-denaturations of the native protein showed hysteresis behaviour. The protein at physiological pH was formed by α-helix (30%) and β-sheet (30%), as shown by CD and FTIR. Comparison with other xylosidases of the same family indicates that the percentages of secondary structure seem to be conserved among the members of the family.