Study Notes: Citric Acid Cycle (Lecture 18)

Lecture 18 - The Citric Acid Cycle

Overview

• Lecture Date: April 7th, 2026

• Focus on the pathways of the citric acid cycle, including:

  • Conversion of pyruvate to acetyl-CoA

  • The cycle's enzymes and degradation products

  • Control and regulation through allosteric inhibitors

• Note:

  • Skip the glyoxylate cycle (pages 443-445) as it's less relevant for human biology.

What is the Citric Acid Cycle?

• Known as the tricarboxylic acid cycle (TCA cycle) or Krebs cycle.

• Key Intermediate: Citric Acid

  • First intermediary created in the TCA pathway.

• Major Steps Post-Pyruvate:

  1. Conversion of pyruvate to acetyl-CoA:

  • Produces electrons for reduced electron carriers.

  1. Oxidation of carbon atoms from acetyl-CoA to form CO₂ and further reduce carriers.

  2. Electron transport for oxidative phosphorylation (Chapter 14).

Key Stages of Energy Production

• Stage 1: Pyruvate to Acetyl-CoA

• Stage 2: The Citric Acid Cycle

• Stage 3: Oxidative Phosphorylation

• Important note for smaller organisms:

  • ATP production occurs at the cell surface due to lack of mitochondria.

Mitochondrial Functions

• Eukaryotic cellular respiration occurs in mitochondria:

  • Unique structure: two membranes (outer and inner).

  • Role of membranes:

  • Convert ADP to ATP by pumping H⁺ ions into the intermembrane space.

  • Increased surface area due to folded cristae enhancing ATP production using ATP synthase.

• Gradient Formation:

  • Electrons from the citric acid cycle and glycolysis pass through inner membrane proteins and ultimately accepted by oxygen to form water.

Overview of the Citric Acid Cycle

Step-by-Step Reactions

1. Acetyl-CoA (2 carbons) forms citrate by attaching to oxaloacetate.

2. Citrate is rearranged to isocitrate through an initial water removal and re-addition.

3. Isocitrate oxidized to α-Ketoglutarate:

   • Produces NADH and CO₂.

4. α-Ketoglutarate oxidized to succinyl-CoA:

   • Produces NADH and CO₂.

5. Succinyl-CoA converts to succinate:

   • Produces GTP.

6. Succinate to fumarate:

   • Produces FADH₂.

7. Fumarate to malate:

   • Via hydration reaction.

8. Malate converts back to oxaloacetate:

   • Produces NADH and completes the cycle.

Memorization Aid

• Suggested mnemonic for intermediates: Curious Insects Keep Swirling Skies Fresh, Making Orbits.

Conversion of Pyruvate to Acetyl-CoA

Enzymatic Involvement

• Enzymes:

  1. Pyruvate Dehydrogenase

  • Decarboxylates pyruvate forming a hydroxyl ethyl group.

  1. Dihydrolipoamide Transacetylase

  • Transfers the hydroxyl ethyl group to acetyl-CoA.

  1. Dihydrolipoamide Dehydrogenase

  • Transfers electrons to NAD⁺, producing NADH.

• Pyruvate Dehydrogenase Complex (PDH complex) assembles these enzymes.

The Citric Acid Cycle Steps

Step 1: Citrate Formation

• Acetyl-CoA transfers acyl groups to oxaloacetate via citrate synthase.

Step 2: Isocitrate Formation

• A two-step process facilitated by aconitase (lyase):

  • Removal and rearrangement of hydroxyl groups.

Step 3: Production of α-Ketoglutarate

• Catalyzed by isocitrate dehydrogenase:

  • Produces NADH and releases CO₂.

Step 4: Conversion to Succinyl-CoA

• Catalyzed by α-ketoglutarate dehydrogenase:

  • Reduces NAD⁺ to NADH, releases CO₂, and utilizes CoA.

Step 5: GTP Production

• Succinyl-CoA synthetase catalyzes formation of GTP from succinyl.

Step 6: Fumarate Formation

• Succinate dehydrogenase converts succinate to fumarate, producing FADH₂.

Step 7: Hydration to Malate

• Fumarate hydratase utilizes water to form malate.

Step 8: Regeneration of Oxaloacetate

• Catalyzed by malate dehydrogenase:

  • Produces NADH and closes the cycle by reforming oxaloacetate.

Energetics and Stoichiometry of the Cycle

• Reactants:

  • Acetyl-CoA + 2H₂O + 3NAD⁺ + FAD + GDP + Pi

• Products:

  • 2CO₂ + 3NADH/H⁺ + FADH₂ + CoA + GTP

• Overall from glycolysis to the end of the pathway:
  $ext{Glucose} + 2H_2O + 10NAD^+ + 2FAD + 4ADP + 4P_i <br>ightarrow 6CO_2 + 10NADH/H^+ + 2FADH_2 + 4ATP$

Regulation of the Citric Acid Cycle

• Control points:

  1. Conversion of pyruvate to acetyl-CoA (most control).

  2. Conversion of acetyl-CoA to citrate.

  3. Conversion of isocitrate to α-Ketoglutarate.

  4. Conversion of α-Ketoglutarate to succinyl-CoA.

• Activation/Regulation Factors:

  1. Acetyl-CoA: inhibited by NADH and succinyl-CoA.

  2. Isocitrate: activated by ADP and Ca²⁺; inhibited by NADH and ATP.

  3. α-Ketoglutarate: activated by Ca²⁺; inhibited by succinyl-CoA and NADH.

Pyruvate Oxidation Control

• Mechanisms:

  1. Feedback Inhibition: Acetyl-CoA and NADH act as allosteric regulators for PDH complex.

  2. Phosphorylation Control: E1 requires phophate removal to function; high ATP levels inactivate it, while high calcium and magnesium levels stimulate it.

Lecture Summary

• Next sessions will review energy production and metabolism, concluding with oxidative phosphorylation, the electron transport chain, and mitochondrial diseases implications.

• Next Class: Continue with oxidative phosphorylation details and mitochondrial health implications.