BIOL 4301: The Citric Acid Cycle

Overview of Food Catabolism

• Stages of Food Catabolism
  • Digestion: Breakdown of food via stomach acid and enzymes; absorption occurs in the gut.
  • Glycolysis: Occurs in the cytosol; converts glucose to pyruvate.
  • Citric Acid Cycle & Oxidative Phosphorylation: Takes place in mitochondria using the electron transport chain.

Glycolysis Summary

• Three Stages of Glycolysis:
  1. Energy Investment: Uses ATP to prepare glucose for breakdown.
  2. Splitting: Glucose is split into two 3-carbon molecules, glyceraldehyde 3-phosphate.
  3. Payoff: Production of ATP and NADH; results in 2 molecules of pyruvate.
• Anaerobic conditions result in:
  • Fermentation pathways that do not require (O_2).
  • Net yield: 2 ATP from each glucose molecule under anaerobic conditions (e.g., lactic acid and ethanol fermentation).

Aerobic vs. Anaerobic Respiration

• Key Differences:
  • Aerobic respiration uses oxygen, while anaerobic respiration does not.
  • In aerobic conditions, pyruvate is completely oxidized into carbon dioxide with a yield of 36 to 38 ATP.
  • Pyruvate is transported from the cytosol into the mitochondrial matrix, where it is converted to Acetyl-CoA.

Formation of Acetyl-CoA

• Conversion Process:
  • Catalyzed by Pyruvate Dehydrogenase (PDH) in the mitochondrial matrix.
  • Reaction: [ \text{Pyruvate} \rightarrow \text{Acetyl CoA} + \text{CO}_2 ]
  • Mechanism: Oxidative decarboxylation removes (CO_2), yielding Acetyl-CoA and NADH.

The Citric Acid Cycle (Krebs Cycle)

• Function: Oxidizes Acetyl-CoA to extract high-energy electrons.
• Cycle Objectives:
  • Generate ATP, GTP, NADH, and FADH2 while releasing CO2.
  • Integrates multiple metabolic pathways.
• Reaction Products Per Cycle:
  • 3 NADH
  • 1 GTP
  • 1 FADH2
  • 2 CO2
  • Regeneration of Oxaloacetate (4C).

Steps of the Citric Acid Cycle

1. Acetyl Group Addition: Acetyl group (2C) from Acetyl-CoA joins with oxaloacetate (4C) to form citrate (6C).
2. Decarboxylation: Two-carbon units are released as two CO2.
3. Regeneration of Oxaloacetate: The original 4C compound is regenerated, allowing for continuous cycling.

Energy Capture and Metabolic Efficiency

• Multiple Electron Carriers:
  • NADH & FADH2 efficiently capture and transfer energy to minimize heat loss.
• Chemical Energy Harnessing:
  • Energy from food oxidation is captured as ATP, GTP, NADH, or FADH2.