Substrate Binding Site of Streptomyces Xylose Isomerase, Studied by the Fluorescence Spectrophotometry Using Xylose and Xylitol

Binding of a substrate xylose to Streptomyces xylose isomerase (XIase) was observed by using a change (decrease) in fluorescence intensity (based on tryptophan residue) as a probe, and the binding parameters Kb and ΔFmax were evaluated for the xylose-XIase complex formation. An analogue xylitol was found not to produce the change in fluorescence intensity ; never theless, it competitively inhibits the XIase-catalyzed reaction for a substrate xylose . These findings suggest that the tryptophan residue (s) is located at the binding site of xylose to interact with some group of the xylose molecule, of which group is missing in xylitol . eq-Glucose, α-glucose and fructose were confirmed to be bound into XIase as a substrate, whereas IS-glucose was certified not to be a substrate of the enzyme.