Yeast Hexokinase and Its Substrates d-Fructofuranose and d-Glucose

Abstract

ON solution in water, beta d-fructopyranose, the only crystalline form of d-fructose, exhibits mutarotation which mainly consists in a pyranose ⇌ furanose interconversion^1,2. In experiments at 0° C. and pH. = 4·5, under which conditions mutarotation proceeds the slowest, it was previously³ shown that beta d-fructopyranose added to a suspension of baker's yeast in 0·1 M KH₂PO₄ is unfermentable, whereas alpha d-glucose is fermented at the expected rate. Since in any quantitative consideration of the composition of a d-fructose solution at equilibrium (pH. = 4·5) open-chain forms (keto-form, enol-form) can be neglected, d-fructofuranose, chiefly present in the beta configuration, remains as the only fermentable isomer. The non-fermentability of d-fructopyranose—obviously due to lack of an OH-group at C₆—rendered possible the determination of the proportion of the furanose form in an equilibrium mixture of d-fructose at 0° C.; this proportion was found⁴ to be 12 per cent. From this figure and the known differences in mutarotation of d-fructopyranose at 0° C. and at 25° C., it can be calculated that an equilibrated d-fructose solution at 25° C. contains approximately 22 per cent of the sugar in the furanose modification; it is in this way that the concentrations of fructofuranose given in this report have been derived.