Hexokinase

  • 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

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