Hexokinase magnesium ions

In most animal, plant, and microbial cells, the enzyme that phosphorylates glucose is hexokinase. Magnesium ion (Mg ) is required for this reaction, as for the other kinase enzymes in the glycolytic pathway. The true substrate for the hexokinase reaction is MgATP. The apparent K , for glucose of the animal... 

Another enzyme used for the measurement of glucose is hexokinase (EC 2.7.1.1) which catalyses the phosphorylation of glucose to produce glucose-6-phosphate with adenosine triphosphate as the phosphate donor and magnesium ions as an activator. The rate of formation of glucose-6-phosphate can be linked to the reduction of NADP by the enzyme glucose-6-phosphate dehydrogenase (EC 1.1.1.49). This indicator reaction can be monitored spectrophotometrically at 340 nm or fluorimetrically ... 

Yeast hexokinase (1-2 international units per mL reaction solution) acting for 20 min in the presence of 10 mM D-glucose has proven to be quite effective in depleting ATP at levels not exceeding 1 mM. At pH 7 in the presence of 1-2 mM uncomplexed magnesium ion, the equilibrium constant for the hexokinase reaction is about 1500 thus, one can anticipate substantial conversion of 1 mM ATP, as indicated by the following equation ... 

For example, Bachelard used [Mgtotai]/[ATPtotai ] = 1 in his rate studies, and he obtained a slightly sigmoidal plot of initial velocity versus substrate ATP concentration. This culminated in the erroneous proposal that brain hexokinase was allosterically activated by magnesium ions and by magnesium ion-adenosine triphosphate complex. Purich and Fromm demonstrated that failure to achieve adequate experimental control over the free magnesium ion concentration can wreak havoc on the examination of enzyme kinetic behavior. Indeed, these investigators were able to account fully for the effects obtained in the previous hexokinase study. ... 

Consider the case of glucose (Glc) and ATP interactions with hexokinase (HK) in the absence of catalysis (which can be accomplished by omitting magnesium ion). 

The process is catalysed by an enzyme called hexokinase. Before the ATP can be accepted by the enzyme, it must become complexed to a magnesium ion, Mg +, through oxygen atoms attached to phosphorus atoms 2 and 3 (Structure 9.9). The exact way in which the magnesium ion bridges the two terminal phosphates is... 

Exley et al. [61] found aluminum in practically all reagents used in a study of the inhibition of hexokinase activity by this element. The way to overcome the problem was cleaning the solutions using an aminophosphonate chelation resin. The procedure reduced the contamination of ATP and NADP to approximately 5% and 10% of their initial values, respectively, but the resin was ineffective in removing aluminum from magnesium acetate or the enzyme glucose 6-phosphate dehydrogenase. Probably the conditions were not favorable for the resin to pick up the aluminum ions from these solutions. It is important to remember that, if there is an affinity between aluminum and the species in solution, a competition between this species and the resin will take place. 

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