The role of four aromatic residues (W85, Y172, W266 and W274) in the structure-function relationship in xylanase A from Streptomyces lividans (XlnA) was investigated by site-directed mutagenesis where each residue was subjected to three substitutions (W85A/H/F; W266A/H/F; W274A/H/F and Y172A/F/S). These four amino acids are highly conserved among family 10 xylanases and structural data have implicated them in substrate binding at the active site. Far-UV circular dichroism spectroscopy was used to show that the overall structure of XlnA was not affected by any of these mutations. High-performance liquid chromatographic analysis of the hydrolysis products of birchwood xylan and xylopentaose showed that mutation of these aromatic residues did not alter the enzyme's mode of action. As expected, though, it did reduce the affinity of XlnA for birchwood xylan. A comparison of the kinetic parameters of different mutants at the same position demonstrated the importance of the aromatic nature of W85, Y172 and W274 in substrate binding. Replacement of these residues by a phenylalanine resulted in mutant proteins with a K(M) closer to that of the wild-type protein in comparison with the other mutations analyzed. The kinetic analysis of the mutant proteins at position W266 indicated that this amino acid is important for both substrate binding and efficient catalysis by XlnA. These studies also demonstrated the crucial role of these active site aromatic residues for the thermal stability of XlnA. Study holds ProTherm entries: 12868, 12869, 12870, 12871, 12872, 12873, 12874, 12876, 12878, 12879, 12880, 12881, 12882, 12883, 12884, 12885, 12886, 12887, 12888, 12889, 12890, 12891, 12892, 12893 Extra Details: family 10 hydrolase; glycosyl hydrolase; site-directed mutagenesis;,structure-function; xylanase
Submitter: Connie Wang
Submission Date: April 24, 2018, 8:44 p.m.
|Number of data points||47|
|Proteins||Endo-1,4-beta-xylanase A ; Endo-1,4-beta-xylanase A|
|Assays/Quantities/Protocols||Experimental Assay: Tm ; Experimental Assay: activity_kcat ; Experimental Assay: activity_km ; Derived Quantity: dTm|
|Libraries||Mutations for sequence AESTLGAAAAQSGRYFGTAIASGRLSDSTYTSIAGREFNMVTAENEMKIDATEPQRGQFNFSSADRVYNWAVQNGKQVRGHTLAWHSQQPGWMQSLSGRPLRQAMIDHINGVMAHYKGKIVQWDVVNEAFADGSSGARRDSNLQRSGNDWIEVAFRTARAADPSAKLCYNDYNVENWTWAKTQAMYNMVRDFKQRGVPIDCVGFQSHFNSGSPYNSNFRTTLQNFAALGVDVAITELDIQGAPASTYANVTNDCLAVSRCLGITVWGVRDSDSWRSEQTPLLFNNDGSKKAAYTAVLDA|