Solution structure of 5-keto-D-fructose: relevance to the specificity of hexose kinases

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Abstract
5-Keto-D-fructose (5KF) is isolated from cultures of Gluconobacter cerinus growing on D-fructose as the sole C source. 5KF is a substrate for hexokinase, fructokinase and several polyol dehydrogenases. 1H and 13C NMR studies show that 5KF exists in different forms in anhydrous dimethyl-d6 sulfoxide and D2O. In dimethyl-d6 sulfoxide, 5KF exists as a spirane dimer with linked furanose and pyranose rings, similar to the structure reported for crystalline 5KF (Hassen, L. et al., 1976). In D2O, 5KF exists predominantly (> 95%) in a .beta.-pyranose form with the 5-keto group hydrated to form a gem-diol. 13C-1H coupling patterns, 13C relaxation measurements, and 13C deuterium-induced differential isotope shifts confirm this structure of 5KF. The phosphorylation of 5KF by fructokinase can be accounted for by an .apprx. 2% proportion of the .beta.-furanose form in solution at 25.degree. C. Both the .beta.-pyranose and .beta.-furanose forms of 5KF are proposed to be substrates for yeast hexokinase.