Structural Insights into the β-Xylosidase from Trichoderma reesei Obtained by Synchrotron Small-Angle X-ray Scattering and Circular Dichroism Spectroscopy†

The enzyme β-xylosidase from Trichoderma reesei, a member of glycosil hydrolase family 3 (GH3), is a glycoside hydrolase which acts at the glycosidic linkages of 1,4-β-xylooligosaccharides and that also exhibits α-l-arabinofuranosidase activity on 4-nitrophenyl α-l-arabinofuranoside. In this work, we show that the enzyme forms monomers in solution and derive the low-resolution molecular envelope of the β-xylosidase from small-angle X-ray scattering (SAXS) data using the ab initio simulated annealing algorithm. The radius of gyration and the maximum dimension of the β-xylosidase are 30.3 ± 0.2 and 90 ± 5 A, respectively. In contrast to the fold of the only two structurally characterized members of GH3, the barley β-d-glucan exohydrolase and β-hexosaminidase from Vibrio cholerae, which have respectively two or one distinct domains, the shape of the β-xylosidase indicates the presence of three distinct structural modules. Domain recognition algorithms were used to show that the C-terminal part of the amino a...