U01.01.095 Hexokinase vs glucokinase

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Learning Objectives

  • Compare the kinetic properties (K_m and V_{max}) of Hexokinase and Glucokinase.
  • Identify the tissue distribution and metabolic purpose of each enzyme.
  • Understand how insulin and feedback mechanisms regulate glucose trapping.

1. Glucose Trapping Mechanism

The phosphorylation of glucose to Glucose-6-Phosphate (G6P) is the first step of glycolysis. This step “traps” glucose inside the cell, as G6P cannot cross the plasma membrane.

Feature Hexokinase Glucokinase (Hexokinase IV)
Location Most tissues (except liver/β cells) Liver and β cells of the pancreas
Affinity ($Latex K_m$) Lower $Latex K_m$ (Higher affinity) Higher $Latex K_m$ (Lower affinity)
Capacity ($LatexV_{max}$) Lower $Latex V_{max}$ Higher $Latex V_{max}$
Insulin Induction No Yes
Feedback Inhibition Glucose-6-phosphate Fructose-6-phosphate

Biochemical Correlation: K_m and Physiological Roles

Hexokinase’s low K_m allows tissues to sequester glucose even when blood levels are low (ensuring energy for the brain). Glucokinase’s high K_m and $V_{max}$ allow the liver to quickly clear large amounts of glucose after a meal to store it as glycogen.

Clinical Correlate: Glucokinase Deficiency

Deficiency in glucokinase raises the threshold for glucose-stimulated insulin release. This is a primary cause of Maturity-Onset Diabetes of the Young (MODY) and Gestational Diabetes, as β cells require higher glucose concentrations to trigger insulin secretion.

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