Regulatory properties of the constitutive hexose transport in saccharomyces cerevisiae
Saturation curves for the initial rates of uptake of non-fermentable sugars and for fermentable ones in normal and iodoacetate-treated cells have been obtained with baker's yeast harvested in log phase. The Km values for each of the sugars tested were found to be 2 to 10 times lower in the presence of fermentation than in its absence. The same effect has been observed in efflux measurements.
The kinetic properties of transport under conditions in which the latter is largely inactivated by uranyl ions are similar to those in untreated cells in the case of glucose and fructose and markedly different in the case of mannose.
The saturation curves appear biphasic in double reciprocal plots under certain conditions (for instance, cells treated with iodoacetic acid and uranyl ions, or cells washed a few seconds after their contact with sugars). Under these conditions two Km values have been calculated for glucose, mannose and xylose.
In steady fermentation of mannose, whose transport seems to be in large potential excess over phosphorylation, the intracellular concentration of free sugar is nevertheless much lower than that corresponding to equilibration by an excess of transport over phosphorylation. In cells in which hexokinase activity is depleted by treatment with xylose, mannose accumulates to near equilibrium levels with the outside sugar.
The kinetics of aerobic fermentation of glucose in respiring cells show an apparent Km one order of magnitude higher than that corresponding to anaerobic fermentation.
The above observations are interpreted as indicating that the constitutive transport system for sugars can exist in two states showing different affinities for sugars. In the absence of fermentation or in aerobic fermentation in respiring cells the state of higher Km prevails, while in the presence of anaerobic fermentation the state of lower Km prevails. Under certain conditions the two states can coexist to the point of giving rise to biphasic saturation curves. The evidence for two affinity states in the carrier and the fact that mannose is fermented at the same maximal rate as glucose or fructose, in spite of the marked differences in their kinetic parameters of transport and phosphorylation, are interpreted as consistent with the hypothesis of the existence of a regulatory feed-back mechanism responsive to the level of an intermediary metabolite of glycolysis.
KeywordsXylose Fructose Mannose Hexokinase Saturation Curve
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