Transition in the Subunit of Streptomyces Xylose Isomerase : Its Relation with the Catalytic Activity and Substrate Binding

Abstract

Xylose isomerase (XIase, from Streptomyces phaeochromogenus)-catalyzed activity and the enzyme Rf value on gel-filtration chromatograms were found to be very dependent on the concentration of NaCl. As one of the most likely explanations for this, these findings suggest that XIase forms a larger subunit (tetramer) without NaCI, whereas at concentrations higher than 0.13 M NaCI the enzyme forms a smaller subunit (dimer), the specific activity of which is almost twice that of the tetramer . The dissociation constant Ka of the xylose-subunit complex was evaluated; the Kd's were almost identical (0.3 mM) for both the tetramer and the dimer. Mg2+ ions may be essential for the binding of substrate . In conclusion, the dimer appears to be more useful than the tetramer for the XIase-catalyzed conversion of glucose to fructose.