Localization of the essential histidine and carboxylate group in d-xylose isomerases

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Abstract
D-Xylose isomerases from different bacterial strains were chemically modified with histidine and carboxylate-specific reagents. The active-site residues were identified by amino acid sequence analysis of peptides recognized by differential peptide mapping on ligand-protected and unprotected derivatized enzyme. Both types of modified residues were found to cluster in a region with consensus sequence: Phe-His-Asp-Xaa-Asp-Xaa-Xaa-Pro-Xaa-Gly, conserved in all D-xylose isomerases studied so far. These results are consistent with the recently published X-ray data of the enzyme active centre from Streptomyces rubiginosus showing hydrogen bond formation between Asp-57 and His-54 which locks the latter in one tautomeric form. A study of the pH-dependence of the kinetic parameters suggests the participation of a histidine group in the substrate-binding but not in the isomerization process. Comparison of the N-terminal amino acid sequences of several D-xylose isomerases further revealed a striking homology among the Actinomycetaceae enzymes and identifies them as a specific class of D-xylose isomerases.