This invention provides use of a series of recombinant Thermoanaerobacterium saccharolyticum glucose isomerases with improved catalytic activity obtained by using recombinant techniques. These mutants comprise at least one amino acid variation at position 87, position 139, position 182, position 187, position 217, position 260, position 276, or position 299, and can be used in the conversion of hemicellulose to ethanol.