Hexokinase

Hexokinase catalyzes the ATP-dependent phosphorylation of a broad spectrum of 6-carbon sugars. In the reaction involving glucose and ATP, glucose-6-phosphate (G6P) and ADP are formed.

> D-Glucose + ATP <=> G6P + ADP + H+

(see ). This reaction constitutes the first step of the process of glycolysis and is energetically favorable ( = -16.7 kJ/mol).

Hexokinase requires Mg2+. The enzyme has a low K M for the sugar substrate ( about 0.1 mM). Hexokinase is inhibited by the product of its reaction, G6P.

Intracellular levels of glucose are usually far higher than the K M value for hexokinase, meaning that the enzyme functions in vivo at saturating substrate concentrations. A liver form of the enzyme called glucokinase has the same substrate specificity, but a significantly higher K M (about 10mM).

Hexokinase is not involved in the process of gluconeogenesis. Instead, the enzyme glucose-6-phosphatase catalyzes the hydrolysis of G6P to glucose and Pi, bypassing the large of the hexokinase-catalyzed reaction.

Next step of glycolysis

See also: Enzymes/Energies of Glycolysis, Reaction Picture

INTERNET LINKS:

> 1. Rasmol Image of Hexokinase (slow)

> 2. Glycolysis/Gluconeogenesis