Studies on the Active Site of Yeast Hexokinase. Specific Phosphorylation of a Serine Residue Induced by d-Xylose and ATPMg

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Abstract
Yeast hexokinase A (ATP: d‐hexose 6‐phosphotransterase) is inactivated when incubated in the presence of xylose and ATPMg, or in the presence of d‐lyxose in a reaction medium in which ATPMg is being continuously regenerated (phosphoeno/pyruvate and pyruvate kinase). The inactivation is due to the phosphorylation of the protein. A linear relationship was observed between the inactivation and the incorporation of 32P from [y‐32P]ATP. All hexokinase and ATPase activity of the enzyme is lost when one phosphoryl group is incorporated per enzyme subunit (molecular weight 51000). The phosphoryl group is covalently bound by a ester linkage with a serine residue of the protein.