Hexokinase catalyzes the first step within the glycolytic sequence.
In mammalian tissues four different isoenzymes of hexokinase are found: type I – type IV

Metabolic scheme

The isoenzymes type I – III are characterized by high affinities to their substrate glucose.

All three isoenzymes have a molecular weight of 100 kDa and are evolved by duplication and fusion of one gene encoding an ancestral hexokinase protein with a molecular weight of 50 kDa.

In the case of the hexokinase types I and III only the C-terminal halves have catalytic activity whereas the N-terminal site has regulatory functions.

Hexokinase type II has two catalytic sites.

Hexokinase isoenzymes type I and II are associated with the outer mitochondrial membrane.
The binding partner is porine, a channel-protein responsible for the transport of metabolites to the outer mitochondrial membrane.
The physiological function of this association with mitochondria is the coordination of the ATP dependent first step of the glycolytic sequence with the ATP producing tricarbolic cycle and oxidative phosphorylation.

The mitochondrial bound hexokinase is not inhibited by its product glucose 6-P, thereby allowing the accumulation of glycolytic phosphometabolites.

The hexokinase isoenzyme type III is associated with the periphery of the nucleus.
Furthermore, with the yeast two hybrid technique five totally different proteins have been found to bind to the regulatory N-terminal site of hexokinase type III :

      Insulin-like growth factor binding protein type 4
      MIZ-1
      Leptin
      Lipocalin-type Prostaglandin D synthase
      Granulin Precursor

Each of the five proteins binds to a distinct region of the isoenzyme and all five proteins can bind simultaneously to the enzyme to form a macromolecular complex. 

The physiological function of this complex is at yet not known.

Hexokinase type 4, also called glucokinase, is mainly expressed in liver and pancreatic beta cells and is characterized by a low affinity to its substrate glucose. Changes in the rate of glucose phosphorylation are linked to changes in the rate of insulin secretion by the pancreatic beta cells.

References

Glycolysis : isoenzyme equipment