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Abstract
Summary: Three glucose-phosphorylating enzymes were separated from cell-free extracts of Saccharomyces cerevisiae by hydroxylapatite chromatography. Variations in the amounts of these enzymes in cells growing on glucose and on ethanol showed that hexokinase PI was a constitutive enzyme, whereas synthesis of hexokinase PII and glucokinase were regulated by the carbon source used. Glucokinase proved to be a glucomannokinase with K m values of 0.04 mm for both glucose and mannose. D-Xylose produced an irreversible inactivation of the three glucose-phosphorylating enzymes depending on the presence or absence of ATP. Hexokinase PI inactivation required ATP, while hexokinase PII was inactivated by D-xylose without ATP in the reaction mixture. Glucokinase was protected by ATP from this inactivation. D-Xylose acted as a competitive inhibitor of hexokinase PI and glucokinase and as a non-competitive inhibitor of hexokinase PII.
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