Glycolysis

Glycolysis Overview

• Glycolysis: A central metabolic pathway that converts glucose into pyruvate, producing ATP and NADH.

16.1 Glycolysis as an Energy-Conversion Pathway

• Process: One molecule of glucose is converted into two molecules of pyruvate with a net gain of 2 ATP.
• Enzyme Organization: Glycolytic enzymes may form complexes to enhance efficiency.

Stages of Glycolysis

• Stage 1 (Preparation Phase):

  • Traps glucose and modifies it to produce phosphorylated 3-carbon compounds (G3P and DHAP).
  • Key Enzymes:
  • Hexokinase: Converts glucose to glucose 6-phosphate (G6P).
  • Phosphoglucose isomerase: Converts G6P to fructose 6-phosphate (F6P).
  • Phosphofructokinase (PFK): Converts F6P to fructose 1,6-bisphosphate (F1,6BP).

• Stage 2 (Payoff Phase):

  • Converts the 3-carbon compounds into pyruvate and generates 4 ATP (net gain of 2 ATP).
  • Key Enzymes:
  • Glyceraldehyde 3-phosphate dehydrogenase: Converts G3P to 1,3-bisphosphoglycerate (1,3BPG).
  • Phosphoglycerate kinase: Converts 1,3BPG to 3-phosphoglycerate (3PG).
  • Pyruvate kinase: Converts phosphoenolpyruvate (PEP) to pyruvate, producing ATP.

16.2 NAD+ Regeneration from Pyruvate Metabolism

• Importance of NAD+: Essential for continuous ATP production; regenerated by:
  • Oxidation of pyruvate to CO2.
  • Conversion of pyruvate to ethanol or lactate.

Fermentation Processes

• Alcoholic Fermentation: Pyruvate to ethanol, regenerating NAD+.
• Lactic Acid Fermentation: Pyruvate to lactate, also regenerating NAD+.

16.3 Conversion of Fructose and Galactose to Glycolytic Intermediates

• Fructose: Processed via fructose 1-phosphate pathway or phosphorylated by hexokinase in different tissues.
• Galactose: Converted to glucose 6-phosphate through galactose-glucose interconversion pathway.

16.4 Regulation of Glycolysis

• Key Regulatory Enzymes: Hexokinase, phosphofructokinase (PFK), pyruvate kinase.
• Muscle Regulation:
  • PFK activity is influenced by ATP and AMP levels.
• Liver Regulation:
  • PFK regulated by fructose 2,6-bisphosphate and citrate.

16.5 Glycolysis in Pancreatic Beta Cells

• Insulin Secretion: Triggered by glucose metabolism, with increased ATP closing K+ channels and opening Ca2+ channels for insulin release.

Clinical Insights

• Triose Phosphate Isomerase Deficiency: Causes severe hemolytic anemia and neurodegeneration due to disrupted glycolysis.
• Milk Intolerance: Caused by lack of lactase, affecting lactose metabolism.
• Excessive Fructose Consumption: Linked to obesity, fatty liver.
• Cancer and Aerobic Glycolysis: Tumors may rely on glycolysis even in the presence of oxygen (Warburg effect).

Summary of Glycolytic Reaction Steps

Quick Quizzes

• Quiz 1: Differentiate between gross ATP yield (4 ATP) and net yield (2 ATP) in glycolysis.
• Quiz 2: Identify electron donors and acceptors in alcoholic and lactic acid fermentation.
• Quiz 3: Distinguish glycolysis roles in muscle vs liver conditions.