Glycolysis in different tissues

Glycolysis Regulation - The On/Off Switches

Key irreversible enzymes serve as control points:

• Hexokinase/Glucokinase:
  • Hexokinase (most tissues): Inhibited by its product, Glucose-6-P.
  • Glucokinase (liver, pancreas): Induced by insulin.
• PFK-1 (Rate-Limiting Step):
  • Activators: AMP, Fructose-2,6-bisphosphate.
  • Inhibitors: ATP, Citrate.
• Pyruvate Kinase:
  • Activator: Fructose-1,6-bisphosphate (feed-forward).
  • Inhibitors: ATP, Alanine.

⭐ Citrate, a TCA cycle intermediate, allosterically inhibits PFK-1. This signals that the cell has abundant energy, halting glycolysis.

Liver vs. Muscle - A Tale of Two Tissues

• Primary Goal & Location:
  • Liver (Hepatocytes): Clear blood glucose post-meal for storage (glycogen, fatty acids). Acts only when glucose is high.
  • Muscle (Myocytes): Provide ATP for contraction, ensuring glucose uptake even when blood levels are low.

⭐ In the liver, insulin activates the PFK-2 part of the bifunctional enzyme, producing Fructose-2,6-bisphosphate (F-2,6-BP). F-2,6-BP is a potent allosteric activator of PFK-1, committing glucose to glycolysis over gluconeogenesis.

RBCs, Brain & Cancer - The Specialists

• Red Blood Cells (RBCs):

  • Rely exclusively on anaerobic glycolysis for all ATP (net 2 ATP/glucose).
  • Lack mitochondria, preventing oxidative phosphorylation.
  • A shunt produces 2,3-BPG, which decreases hemoglobin's $O_2$ affinity, aiding oxygen release to tissues.

• Brain:

  • Primarily fueled by glucose; glycolysis is the mandatory first step before aerobic respiration.
  • Maintains a very high, constant glucose demand.

• Cancer Cells (Warburg Effect):

  • Exhibit "aerobic glycolysis": a high rate of glycolysis even when $O_2$ is plentiful.
  • Generates lactate and provides biosynthetic intermediates to support rapid proliferation.

⭐ PET Scans exploit the Warburg effect. They use a glucose analog, Fluorodeoxyglucose (FDG), to visualize tumors, which have characteristically high glucose uptake.

• Red blood cells depend exclusively on anaerobic glycolysis for their ATP supply.
• The liver uses glucokinase (a high-Km isozyme) to handle high glucose loads after meals.
• In muscle, hexokinase (low-Km) ensures rapid glucose uptake for energy during contraction.
• The brain has a constant, insulin-independent uptake of glucose to fuel its high metabolic rate.
• Pyruvate kinase deficiency leads to hemolytic anemia by disrupting RBC metabolism.
• The Warburg effect describes the reliance of cancer cells on aerobic glycolysis.