• Catalyzes phosphorylation of glucose to form glucose-6P
• Traps glucose inside the cell
• 1st regulated enzymes in glycolysis – catalyze an irreversible reaction
• 4 isozymes – I, II, III, and IV (glucokinase)

HEXOKINASE (I, II & III) vs. GLUCOKINASE (IV)

• Tissue distribution
• Kinetics (Km and Vmax)
• Regulation (allosteric vs hormonal)

Hexokinase

• Ubiquitous in mammals
• Low Km & low Vmax
• Allosteric regulation – inhibited by glucose-6P

Glucokinase

• Liver & pancreatic beta cells
• High Km & high Vmax: spares glucose for brain, muscle & other tissues (glucose sensor)
• Hormonal regulation: inhibited by glucagon, activated by insulin
• Glucokinase regulatory protein (GKRP): nuclear protein that reversibly binds/inactivates glucokinase
• High [glucose] inhibits GKRP & promotes glucokinase release
• Fructose-6P (glycolytic intermediate in equilibrium with glucose-6P): promotes GKRP-GK binding
• Liver glucose-6P shunts into one of three pathways: glycolysis, glycogen or fatty acid synthesis