Question

How do the Km values of glucokinase and hexokinase reflect their roles in sugar metabolism ?

Answer 1

The K_m value of glucokinase and hexokinase reflect their selective role in sugar metabolisms. The hexokinase is an enzyme which adds a phosphate group to the hexose sugar. Most of the cases the glucose is the main substrate for the phosphorylation by it. The glucose converted into glucose 6-phosphate which can generally enter into glycolysis. Glycolysis is an essential pathway for energy production, and it occurs in nearly every cell. The km value of hexokinase is low and its affinity towards glucose is high so that it can easily start glycolysis. If the glycolysis is not needed by the cell, then the downstream pathway is regulated, so that the glucose 6-phosphate is accumulated, which leads to feedback inhibition of the enzyme.

Glucokinase is an isozyme of hexokinase, which has high k_m value, the affinity of this enzyme towards the glucose is very low. It phosphorylates only under high glucose concentration. In liver and hepatocytes, the glycogen synthesis takes place. Glucokinase localised to these tissues and allow glycogen synthesis occur only under high glucose concentration. Unlike hexokinases, the glucokinase is not inhibited by the glucose6-phosphate.