Glycolysis and Its Regulation

Overview of Glycolysis and Regulation

• Glycolysis is a series of reactions that convert glucose into pyruvate, releasing energy stored as ATP.
• Total Delta G for glycolysis: approximately -102.9 kJ/mol, indicating that the pathway is thermodynamically favorable.

Key Steps of Glycolysis

• Steps 6 and 7: Example of coupled reactions where an unfavorable step (step 6 with a positive ext{ΔG} ) is coupled with a favorable reaction (step 7) to drive the overall process forward.
• Main Outputs: 2 molecules of pyruvate and generation of energy-rich metabolites.

Important Enzymes and Their Functions

• Hexokinase: Converts glucose to glucose-6-phosphate (G6P), trapping glucose in the cell and making it available for glycolysis or storage (glycogen).
• Phosphofructokinase (PFK): A key regulatory enzyme that is often a bottleneck in glycolysis and subject to allosteric regulation.
  • Its activity is influenced by ATP (inhibitor) and ADP/AMP (activators).
• Pyruvate Kinase: Converts phosphoenolpyruvate to pyruvate, also under regulatory control influenced by fructose-1,6-bisphosphate (activator) and ATP (inhibitor).

Metabolic Pathway Characteristics

• Substrate Level Phosphorylation: Formation of ATP directly from metabolic intermediates in glycolysis without the need for oxygen.
• Oxidative Phosphorylation: A process that occurs after glycolysis where electrons from pyruvate are used to generate ATP through an electrochemical gradient in the mitochondria, requiring oxygen.

Role of NAD and Lactate Production

• NAD/NADH Balance: Essential for glycolysis continuation. Lactate dehydrogenase converts pyruvate to lactate during anaerobic respiration to regenerate NAD, allowing glycolysis to proceed.

Historical Insight

• Louis Pasteur's Experiment: Found that yeast metabolism halts under aerobic conditions, indicating that oxygen inhibits certain steps of glycolysis, particularly at the phosphofructokinase level.

Regulation of Glycolysis

• Rate-Limiting Steps: The three primary steps regulated are catalyzed by hexokinase, PFK, and pyruvate kinase.
• Negative Feedback: Hexokinase is inhibited by its product, G6P, preventing overproduction when there’s an excess of the product.
• Allosteric Regulation of PFK: Influenced by multiple metabolites, with ATP acting as an inhibitor signaling the cell to stop glycolysis when energy levels are high, while ADP and AMP activate it when energy is needed.

Enzyme Activity and Regulation Mechanisms

• PFK Activation and Inhibition:
  • High ATP levels inhibit PFK, preventing further breakdown of glucose when energy is sufficient.
  • Fructose-2,6-bisphosphate (F2,6BP) activates PFK, enhancing glycolysis when glucose is abundant.
• Feedforward Activation: A term used to describe how fructose-1,6-bisphosphate activates pyruvate kinase to continue downstream processing of glycolysis.

Summary of Enzyme Interactions

• Hexokinase: Inhibited by product G6P
• PFK: Inhibited by ATP, activated by ADP/AMP and F2,6BP
• Pyruvate Kinase: Activated by F1,6BP, inhibited by ATP and acetyl CoA

Metabolite Functionality

• Acetyl CoA: Indicates sufficient energy stores, inhibits glycolysis but promotes gluconeogenesis, signaling the need to form glucose from other substrates.
• Alanine: Serves as a precursor for gluconeogenesis, indicating a need to switch from glycolysis.

Concluding Thoughts

• Glycolysis features intricate regulation that allows for flexible metabolic responses based on the cell's energy needs and substrate availability.
• The interplay between glycolysis and gluconeogenesis illustrates the complexity of metabolic control mechanisms within cells.