Investigating the effect of substrate binding on the catalytic activity of xylanase.

XynAF1 from Aspergillus fumigatus Z5 is an efficient thermophilic xylanase belonging to glycoside hydrolase family 10 (GH10). The non-catalytic amino acids N179 and R246 in its catalytic center formed one and three intermolecular H-bonds with the substrate in the aglycone region, respectively. Here we purified XynAF1-N179S and XynAF1-R246K, and obtained the protein-product complex structures by X-ray diffraction. The snapshots indicated that mutations at N179 and R246 had decreased the substrate-binding ability in the aglycone region. XynAF1-N179S, XynAF1-R246K, and XynAF1-N179S-R246K lost one, three, and four H-bonds with the substrate in comparison with the wild-type XynAF1, respectively, but this had little influence on the protein structure. As expected, N179S, R246K, and N179S-R246K led to a gradual decrease of substrate affinity of XynAF1. Interestingly, the enzyme assay showed that N179S increased catalytic efficiency, while both R246K and N179S-R246K had decreased catalytic efficiency. KEY POINTS: • The non-catalytic amino acids of XynAF1 could form H-bonds with the substrate. • The protein-product complex structures were obtained by X-ray diffraction. • The enzyme-substrate-binding capacity could affect enzyme catalytic efficiency.