54 The reaction catalyzed by the enzyme hexokinase is MgATP

5.4. The reaction catalyzed by the enzyme hexokinase is MgATP+ GMgADP + G6P where MgATP is magnesium adenosine S\'-triphosphate, G is glucose, MgADP is magne- sium adenosine 5\'-diphosphate, and G6P is glucose 6-phosphate. What mechanisms are consistent with the initial-rate steady-state data given in the following table for yeast hex- okinase at pH 8,25°C, and 0.3 M (CH),NCI [G. G. Hammes and D. Kochavi,J.Am. Chem Soc. 84, 2069,2073,2076 (1962)-

Solution

The mechanism proceeds through the formation of quarternary intermediate complex produced by reaction of an enzyme-glucose complex with MgATP.  The hexokinase binds first glucose and then the chelate MgATP^2-. The latter substrate is not able to form directly the first complex with the phosphotransferase. The ionic species Mg²⁺ and ATP^4- have only a negligible affinity for the enzyme, or the enzyme-glucose complex.

Glucose-6-phosphate and ADP appear in the medium following the decomposition of the enzyme-glucose-magnesium-ATP complex. This decomposition is also controlled by an ordered mechanism, MgADP^- being formed first, and glucose-6-phosphate afterwards. MgADP^- is able not only to fix itself on the enzyme-glucose-6-phosphate complex but also on the enzyme-glucose complex.