The role of active-site aromatic and polar residues in catalysis and substrate discrimination by xylose isomerase.

The functions of individual amino acid residues in the active site of Thermoanaerobacterium thermosulfurigenes D-xylose ketol-isomerase (EC 5.3.1.5) were studied by site-directed substitution. The role of aromatic residues in the active-site pocket was not limited to the creation of a hydrophobic environment. For example, Trp-188 provided for substrate binding and Trp-139 allowed for the discrimination between D-xylose and D-glucose. Substrate discrimination was accomplished by steric hindrance caused by the side chain of Trp-139 toward the larger glucose molecule. Preference of the enzyme for the alpha-anomer of glucose depended on the His-101/Asp-104 pair. Wide differences observed in the catalytic constant (kcat) for alpha- versus beta-glucose in the wild-type enzyme and the fact that only the kcat for alpha-glucose was changed in the His-101-->Asn mutants strongly suggest that the substrate molecule entering the hydride-shift step is still in the cyclic form. On the basis of these results a revised hypothesis for the catalytic mechanism of D-xylose isomerase has been proposed that involves His-101, Asp-104, and Asp-339 functioning as a catalytic triad.