Lecture on the Citric Acid Cycle

Jul 9, 2024

Lecture Notes on the Citric Acid Cycle

Introduction

  • Chapter 13 covers the Citric Acid Cycle.
  • Transition from glycolysis to the citric acid cycle involves transforming pyruvate into acetyl CoA.
  • Acetyl CoA feeds into the citric acid cycle.
  • The cycle produces carbon dioxide and high-energy molecules like ATP and electron carriers for the electron transport chain.
  • This process is a form of catabolism.

Detailed Steps

Stage 1: Conversion to Acetyl CoA

  • Pyruvate, the end product of glycolysis, is converted to acetyl CoA.
  • This step is essential before the citric acid cycle begins.
  • Amino acids and fatty acids can also feed into the citric acid cycle.

Stage 2: Citric Acid Cycle

  • Acetyl CoA is oxidized in a series of reactions.
  • Produces:
    • 2 CO₂
    • 1 ATP
    • Electron carriers for the electron transport chain.

Stage 3: Electron Transport and Oxidative Phosphorylation

  • Will be covered in later lectures but involves the usage of electron carriers to produce ATP.

Cellular Location

  • In bacterial cells: occurs in the cytosol (as they lack mitochondria).
  • In mammalian cells: occurs in the mitochondria.
  • The mitochondria is termed the 'powerhouse of the cell' due to these processes.

Citric Acid Cycle Steps and Enzymes

Initial Reaction

  • Acetyl CoA + Oxaloacetate → Citrate.

Intermediate Reactions

  • Citrate → Isocitrate: involves dehydration and rehydration.
  • Isocitrate → α-Ketoglutarate: involves decarboxylation.
  • α-Ketoglutarate → Succinyl CoA: another decarboxylation step.
  • Succinyl CoA → Succinate: involves removal of CoA and formation of GTP (equivalent to ATP).
  • Succinate → Fumarate: involves removal of hydrogens and formation of FADH₂.
  • Fumarate → Malate: hydration reaction.
  • Malate → Oxaloacetate: oxidation reaction producing NADH.

Regeneration and Energy Yield

  • The cycle regenerates oxaloacetate to begin the process again.
  • Total yield per cycle: 3 NADH, 1 FADH₂, 1 ATP (or equivalent).
  • One glucose molecule results in twice the cycle yield because glycolysis produces two pyruvate molecules.

Energy Equivalent Calculations

  • Glycolysis: 2 ATP + 2 NADH (Total: 7 ATP equivalents).
  • Pyruvate to Acetyl CoA: 2 NADH (5 ATP equivalents).
  • Citric Acid Cycle (per glucose molecule):
    • 3 NADH × 2 = 6 NADH
    • 1 FADH₂ × 2 = 2 FADH₂
    • 1 ATP × 2 = 2 ATP
    • Total ATP from the cycle: 20 ATP equivalents.
  • Total from one glucose molecule: 32 ATP equivalents.

Regulation

  • Enzymatic steps are highly regulated (positive and negative) based on concentrations of reactants/products.
  • Pyruvate to Acetyl CoA is regulated by molecules such as pyruvate and insulin.
  • Citric acid cycle intermediates are also involved in other biosynthetic processes (e.g., amino acids, fatty acids).
  • Intermediates may exit the cycle, requiring replenishment pathways.

Summary

  • Understanding the steps, regulation, and energy yield of the citric acid cycle is critical for comprehending cellular metabolism.
  • Important to visualize the cycle for clarity.

Tip: Keep a diagram of the citric acid cycle handy for reference while studying individual reactions.