Comparison of the amino acid sequences of two families of glycosyl hydrolases reveals that they are related in a region in the central part of the sequences. One of these families (GH family 68) includes levansucrases and the other one (glycosyl hydrolase family 43) includes bifunctional beta-xylosidases and alpha-L-arabinofuranosidases. The similarity of the primary structure of proteins from these families allows us to consider the invariant glutamate residue as a component of their active center. It is shown for the first time that glycosyl hydrolases recognizing different glycofuranoside residues can have a common sequence motif.