title: Comparative kinetics of D-xylose and D-glucose isomerase activities of the D-xylose isomerase from Thermus aquaticus HB8.
abstract: The D-xylose isomerase from T. aquaticus accepts, besides D-xylose, also D-glucose, and, with lower efficiency, D-ribose, and D-arabinose as alternative substrates. The activity of the enzyme is strictly dependent on divalent cations. Mn2+ is most effective in the D-xylose isomerase reaction and Co2+ in the D-glucose isomerization. Mg2+ is active in both reactions, Zn2+ only in the further one. The enzyme is strongly inhibited by Cu2+, and weakly by Ni2+, Fe2+, and Ca2+. A hyperbolic dependence of the reaction velocity of the D-xylose isomerase on the concentration of D-xylose xylose and of D-glucose was found, while biphasic saturation curves were obtained by variation of the metal ion concentrations. The D-glucose isomerization reaction shows normal behaviour with respect to the metal ions. A kinetic model was derived on the basis of the assumption of two binding sites for divalent cations, one cofactor site with higher affinity and a second, low affinity site, which modulates the activity of the enzyme.
authors.affiliation: Physiologisch-Chemisches Institut der Universität Tübingen.
authors.forename: A
authors.initials: A
authors.lastname: Lehmacher
authors.forename: H
authors.initials: H
authors.lastname: Bisswanger
mesh_terms: D019747:Aldose-Ketose Isomerases; D002238:Carbohydrate Epimerases / Q000378:metabolism*; D002412:Cations / Q000494:pharmacology; D007536:Isomerism; D007700:Kinetics; D013379:Substrate Specificity; D013824:Thermus / Q000187:drug effects / Q000201:enzymology*; D014994:Xylose / Q000494:pharmacology
chemical_list: D002412:Cations; D014994:Xylose; D002238:Carbohydrate Epimerases; D019747:Aldose-Ketose Isomerases; C023298:xylose isomerase