Kinetic studies of Mg(2+)- Co(2+)- and Mn(2+)-activated D-xylose isomerases.

摘要

The kinetic parameters for the interconverting substrates D-xylose in equilibrium D-xylulose and D-glucose in equilibrium D-fructose were determined for several D-xylose isomerases, with Mg2+, Co2+ and Mn2+ as metal ion activators. The Km, kcat. and kcat./Km values are tabulated for the anomeric mixtures (observed parameters) as well as for the respective reactive species, i.e. the alpha-pyranose anomers of D-xylose and D-glucose and the alpha-furanose forms of D-xylulose and D-fructose (real parameters). The real Km values and catalytic efficiencies are more favourable for the ketose sugars (reverse reaction) than for the aldose sugars (forward reaction). Comparisons of the kinetic parameters further support the existence of two distinct groups of D-xylose isomerases. Inhibition constants for the cyclic substrate analogues 5-thio-alpha-D-xylopyranose and alpha-D-xylopyranosyl fluoride and for the acyclic substrate analogue xylitol and its dehydrated form 1,5-anhydroxylitol were determined and are discussed.