Characterization of the active site and thermostability regions of endoxylanase from Thermoanaerobacterium saccharolyticum B6A-RI.

Deletion mutants were constructed from pZEP12, which contained the intact Thermoanaerobacterium saccharolyticum endoxylanase gene (xynA). Deletion of 1.75 kb from the N-terminal end of xynA resulted in a mutant enzyme that retained activity but lost thermostability. Deletion of 1.05 kb from the C terminus did not alter thermostability or activity. The deduced amino acid sequence of T. saccharolyticum B6A-RI endoxylanase XynA was aligned with five other family F beta-glycanases by using the PILEUP program of the Genetics Computer Group package. This multiple alignment of amino acid sequences revealed six highly conserved motifs which included the consensus sequence consisting of a hydrophobic amino acid, Ser or Thr, Glu, a hydrophobic amino acid, Asp, and a hydrophobic amino acid in the catalytic domain. Endoxylanase was inhibited by EDAC [1-(3-dimethylamino propenyl)-3-ethylcarbodiimide hydrochloride], suggesting that Asp and/or Glu was involved in catalysis. Three aspartic acids, two glutamic acids, and one histidine were conserved in all six enzymes aligned. Hydrophobic cluster analysis revealed that two Asp and one Glu occur in the same hydrophobic clusters in T. saccharolyticum B6A-RI endoxylanase and two other enzymes belonging to family F beta-glycanases and suggests their involvement in a catalytic triad. These two Asp and one Glu in XynA from T. saccharolyticum were targeted for analysis by site-specific mutagenesis. Substitution of Asp-537 and Asp-602 by Asn and Glu-600 by Gln completely destroyed endoxylanase activity. These results suggest that these three amino acids form a catalytic triad that functions in a general acid catalysis mechanism.