Kinetic Study of Yeast Hexokinase

Abstract

The ionic species ATP 4− and Mg 2+ , at high concentrations, decrease the rate of glucose phosphorylation catalyzed by yeast hexokinase. A theoretical study of all the possible mechanisms (271 models) of this inhibition has been made. The comparison of these mechanisms with the experimental results shows that only one model can explain the inhibition of the reaction by ATP, and that only one can also explain the effect of the high magnesium concentrations. It was thus possible to show that the ionic species ATP 4− can be bound on hexokinase and on the hexokinase‐glucose complex. Its affinity is much stronger for the second form of the enzyme than for the first. The complexes thus formed are devoid of any activity (“dead‐end” complexes). The inhibition of the hexokinase by the high nucleotide concentrations is therefore due to the formation of these complexes. A ternary chelate of the Mg(ATP) 2 6− type does not exist. Magnesium possesses no affinity for the enzyme. Its role in the catalysis must therefore by indirect. The results presented agree with the idea that the metal polarizes the phosphoryl bond of the ATP that is split when glucose‐6‐phosphate is formed. Magnesium can form with ATP a ternary chelate of the Mg 2 ATP type, unable to be bound on the hexokinase. The inhibiting effect of high concentrations of the metal can be explained by the formation of this inactive ternary chelate and by the decrease in concentration of the MgATP 2− complex.