Glycolysis Regulation

Glycolysis Regulation

• Hexokinase/Glucokinase is inhibited by its product Glucose-6-Phosphate and by Fructose-6-Phosphate.
• Phosphofructokinase-1 (PFK-1) is the rate-limiting step of glycolysis:
  • PFK-1 is activated by Fructose-2,6-bisphosphate, AMP, and high levels of ADP while it is inhibited by ATP, citrate, and low levels of ADP.
• Phosphoglycerate kinase is reversible meaning it can operate in both directions of the reaction.
• Pyruvate kinase is the last step in glycolysis and is irreversible:
  • It is activated by Fructose-1,6-bisphosphate which is a product of the previous step.
  • It is inhibited by Glucagon, Alanine, and low levels of ATP.

Glycolysis Regulation

• Hexokinase/Glucokinase catalyzes the first step of glycolysis, converting glucose to glucose-6-phosphate.
  • Regulation is allosterically inhibited by glucose-6-phosphate.
  • Glucokinase in the liver is also regulated by fructose-6-phosphate.
• The second step of glycolysis, where fructose-6-phosphate is converted to fructose-1,6-bisphosphate, is catalyzed by Phosphofructokinase-1 (PFK-1).
  • This is the rate-limiting step of glycolysis.
  • Regulation is complex, involving several factors:
    • Fructose-2,6-bisphosphate: Stimulates PFK-1 activity.
    • AMP: Stimulates PFK-1 activity, indicating low energy levels.
    • Citrate: Inhibits PFK-1 activity, indicating high energy levels.
    • ATP: Inhibits PFK-1 activity, indicating high energy levels.
• Phosphoglycerate kinase catalyzes the conversion of 1,3-bisphosphoglycerate to 3-phosphoglycerate in a reversible reaction.
• Pyruvate kinase, in the final step, converts phosphoenolpyruvate to pyruvate in an irreversible reaction.
  • Regulation is influenced by various factors:
    • Fructose-1,6-bisphosphate: Stimulates pyruvate kinase activity.
    • Glucagon: Inhibits pyruvate kinase activity, promoting gluconeogenesis.
    • Alanine: Inhibits pyruvate kinase activity.
    • ATP: Inhibits pyruvate kinase activity, indicating high energy levels.

Glycolysis Regulation

• Hexokinase/Glucokinase catalyzes the first step in glycolysis, the phosphorylation of glucose to glucose-6-phosphate
  • Regulation: Glucose-6-phosphate is an allosteric inhibitor of hexokinase, while fructose-6-phosphate inhibits glucokinase
• Phosphofructokinase-1 (PFK-1) is the rate-limiting enzyme in glycolysis, catalyzing the conversion of fructose-6-phosphate to fructose-1,6-bisphosphate
  • Regulation:
    • Fructose-2,6-bisphosphate activates PFK-1, promoting glycolysis
    • AMP activates PFK-1, indicating low energy levels
    • Citrate inhibits PFK-1, signaling high energy levels
    • Low ATP increases the activity of PFK-1
• Phosphoglycerate Kinase catalyzes the conversion of 1,3-bisphosphoglycerate to 3-phosphoglycerate, generating ATP
  • Regulation: Reversible, meaning it can work in both directions based on substrate availability
• Pyruvate Kinase catalyzes the irreversible conversion of phosphoenolpyruvate to pyruvate, generating ATP
  • Regulation:
    • Fructose-1,6-bisphosphate activates pyruvate kinase, a feed-forward mechanism to speed up glycolysis
    • Glucagon, a hormone released during low blood sugar, inhibits pyruvate kinase
    • Alanine inhibits pyruvate kinase, reflecting high amino acid levels
    • Low ATP increases the activity of pyruvate kinase

Hexokinase / Glucokinase:

• Hexokinase: All tissues (except liver), low Km (high affinity), high Vmax, inhibited by glucose-6-phosphate
• Glucokinase: Liver, high Km (low affinity), low Vmax, not inhibited by glucose-6-phosphate, activated by fructose-6-phosphate
• Both catalyze phosphorylation of glucose to glucose-6-phosphate
• Regulation: Product inhibition by glucose-6-phosphate for hexokinase but not for glucokinase

Phosphofructokinase-1 (PFK-1):

• Catalyzes the conversion of fructose-6-phosphate to fructose-1,6-bisphosphate (committed step of glycolysis)
• Rate-limiting step: Controls the overall flux of glycolysis
• Regulation:
  • Positive regulators: Fructose-2,6-bisphosphate (most potent activator), AMP (indicates low energy status)
  • Negative regulators: Citrate (indicate high energy status), ATP (high concentration indicates high energy status)

Phosphoglycerate Kinase:

• Catalyzes the phosphorylation of 1,3-bisphosphoglycerate to 3-phosphoglycerate
• Reversible reaction
• Regulation: Not directly regulated

Pyruvate Kinase:

• Catalyzes the conversion of phosphoenolpyruvate to pyruvate, which is the final step of glycolysis
• Irreversible reaction
• Regulation:
  • Positive regulators: Fructose-1,6-bisphosphate (feed-forward activation)
  • Negative regulators: Glucagon, alanine, low ATP levels (indicate high energy status)