Specific Inactivation of Yeast Hexokinase Induced by Xylose in the Presence of a Phosphoryl Donor Substrate

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Abstract
Yeast hexokinase can be inactivated by xylose in the presence of MgATP. The effect is highly specific for xylose. The nucleotide and divalent metal specificities are similar to those of the hexokinase reaction. The dissociation constants, as deduced from the effect of the concentration of the inactivating agents on the rate of inactivation, are about 0.2 mM for MgATP and about 10 mM for xylose. These values are similar to the Michaelis constant and Ki value for these compounds as substrate and competitive inhibitor, respectively, in the hexokinase reaction. Sugars capable of binding at the sugar substrate subsite protect against xylose‐induced inactivation with efficiencies closely related to their respective Ks or Ki values. The dissociation constant of glucose, as deduced from its protective effect, is about 0.04 mM.When resting yeast is incubated aerobically with 0.1 M xylose and 1% ethanol, a marked loss of hexokinase activity takes place.Among the possible mechanisms of this irreversible inactivation there is evidence against the involvement of phosphorylation or glycosylation of the protein as well as of a change in its degree of polymerization.