Kinetics of human erythrocyte hexokinase. Influence of temperature, ATP 4− and magnesium ions

Abstract

Human erythrocyte hexokinase (EC 2.7.1.1) is inhibited competitively with respect to Mg · ATP 2− by uncomplexed Mg 2+ ( K i = 16–18 mM) and ATP 4− ( K i = 1.6 mM ). No real activation by low concentrations of Mg 2+ could be detected and no allosteric behaviour was observed under the conditions tested. The temperature dependence of the enzyme was studied in relationship to the presence of Mg 2+ or ATP 4−. At equal concentrations of Mg 2+ and ATP 4− a break in the Arrhenius plot was observed at 27.5°C, the higher temperature form of the enzyme having the lower activation energy. This break point in the Arrhenius plot was shifted to 36°C in the presence of 5 mM Mg 2+. A straight-line relationship was observed in the presence of 2.5 mM ATP 4−. The K m for Mg · ATP 2− showed a linear increase at temperatures over about 36°C independent of the presence of Mg 2+ or ATP 4−. The nature of these phenomena is discussed.