Kinetic and Magnetic Resonance Studies of the Mechanism of d-Xylose Isomerase

The interaction of Mn2+ with d-xylose isomerase from two species of bacteria has been studied by electron paramagnetic resonance and by the longitudinal proton relaxation rate of water solutions of the enzyme. Binary enzyme-Mn2+ complexes are detected with dissociation constants in order of magnitude agreement with kinetically determined activator constants. A 4-fold enhancement of the effect of d-xylose isomerase-Mn2+ on the proton relaxation rate of water was observed. This is decreased by substrates and inhibitors consistent with the replacement of water ligands of the enzyme-bound manganese by substrate oxygen functions. The dissociation constants of the ternary complexes of substrates and inhibitors are in agreement with their respective Km or Ki values. Differences in the enhancements of the ternary complexes of substrates and inhibitors indicate that α-anomeric forms have a greater effect than corresponding β-anomeric forms.