̣-oxidation of Fatty Acids and Ketone Bodies

• Definition: beta-oxidation is the primary metabolic pathway responsible for the breakdown of fatty acids to produce cellular energy.

• Location: The process occurs predominantly within the mitochondrial matrix.

• Core Mechanics: Each full cycle of beta-oxidation involves the following:

  • The fatty acid chain is shortened by exactly two carbon atoms per cycle.

  • The production of one molecule of Acetyl-CoA.

  • The generation of reducing equivalents: NADH and FADH₂.

• Metabolic Integration: The products generated feed into secondary pathways:

  • Acetyl-CoA enters the Citric Acid Cycle (TCA cycle).

  • NADH and FADH₂ enter the respiratory (electron-transfer) chain for oxidative phosphorylation.

  • The final output of these integrated processes is the production of ATP and H₂O.

The Four-Step β-oxidation Pathway

• Stage 1: Step One - Oxidation (Desaturation):

  • Enzyme: Acyl-CoA dehydrogenase.

  • Reaction: Catalyzes the oxidation of the acyl-CoA ester by forming a double bond between the α and β carbons (carbons 2 and 3).

  • Cofactor: FAD is reduced to FADH₂.

  • Product: trans-Δ²-Enoyl-CoA.

  • Significance: This step initiates the extraction of energy from the fatty acid chain.

• Stage 2: Step Two - Hydration:

  • Enzyme: Enoyl-CoA hydratase.

  • Reaction: H₂O is added across the double bond formed in the previous step.

  • Product: L-β-Hydroxy-acyl-CoA.

  • Structural Change: A hydroxyl group is formed on the β-carbon.

• Stage 3: Step Three - Oxidation (Oxidation of Alcohol):

  • Enzyme: β-Hydroxyacyl-CoA dehydrogenase.

  • Reaction: The hydroxyl group on the β-carbon is oxidized to a ketone group.

  • Cofactor: NAD⁺ is reduced to NADH + H⁺.

  • Product: β-Ketoacyl-CoA.

• Stage 4: Step Four - Thiolytic Cleavage:

  • Enzyme: Acyl-CoA acetyltransferase (commonly known as Thiolase).

  • Reaction: CoA-SH attacking the β-carbon to cleave the bond between the α and β carbons.

  • Products:

    1. One molecule of Acetyl-CoA (2 carbons).

    2. An Acyl-CoA chain that is now two carbons shorter than the original (e.g., if starting with C₁₆ Palmitoyl-CoA, the product is C₁₄ Myristoyl-CoA).

  • Outcome: The shortened fatty acyl-CoA enters the cycle again until the entire chain is converted to Acetyl-CoA.

ATP Yield and Energy Calculations for Palmitate (C₁₆:0)

• Energy Yield per Single Cycle:

  • 1 × FADH₂ (via Acyl-CoA oxidase/dehydrogenase): 1.5 ATP

  • 1 × NADH (via 3-hydroxyacyl-CoA dehydrogenase): 2.5 ATP

  • Oxidation of 1 × Acetyl-CoA via TCA Cycle:

    • 3 × NADH = 7.5 ATP

    • 1 × FADH₂ = 1.5 ATP

    • 1 × GTP (ATP equivalent) = 1.0 ATP

  • Total ATP per cycle: 14 ATP.

• Total Yield for Palmitate (C₁₆):

  • A C₁₆ fatty acid undergoes 7 cycles of β-oxidation.

  • Production Totals:

    • 8 × Acetyl-CoA

    • 7 × NADH (from β-oxidation)

    • 7 × FADH₂ (from β-oxidation)

  • Entering TCA Cycle (8 × Acetyl-CoA):

    • 8 × 3 = 24 NADH

    • 8 × 1 = 8 FADH₂

    • 8 × 1 = 8 GTP

  • Combined Totals:

    • Total NADH + H⁺ = 24 + 7 = 31

    • Total FADH₂ = 8 + 7 = 15

    • Total GTP = 8

• Calculated ATP (Measured/Practical Yields):

  • 31 × 2.5 = 77.5 ATP

  • 15 × 1.5 = 22.5 ATP

  • 8 × 1 = 8 ATP

  • Subtotal: 108 ATP

  • Activation Cost: -2 ATP (needed to form Palmitoyl-CoA from Palmitate and CoA).

  • Final Net Yield: 106 ATP (Note: Theoretical yield based on older factors of 3.0 and 2.0 would result in 129 ATP).

Energy Efficiency and Comparisons

• Efficiency Analysis:

  • Standard free energy of oxidation of palmitate: -9790 kJ/mol.

  • Energy captured (as ATP): 129 × (-31 kJ/mole) = -3999 kJ/mole.

  • Proportion Captured: -3999/-9790 ≈ 40%.

  • Heat Generation: The remaining 60% of energy is lost as heat, assisting in maintaining body temperature.

• Energy Density Comparison:

  • Palmitate: Yields ≈ 8.2 ATP per carbon atom oxidized.

  • Glucose: Yields ≈ 6.3 ATP per carbon atom oxidized (30 ATP total for a C₆ molecule).

  • Conclusion: Lipids are significantly more energy-dense fuel molecules than carbohydrates.

Ketone Body Synthesis (Ketogenesis)

• Context: Under conditions such as fasting, starvation, prolonged exercise, or low carbohydrate intake, oxaloacetate is diverted from the TCA cycle to be used in gluconeogenesis.

• Consequence: Without sufficient oxaloacetate, acetyl-CoA cannot enter the TCA cycle and instead builds up.

• Product: Excess acetyl-CoA is converted into ketone bodies in the liver mitochondria.

• Primary Ketone Bodies:

  • Acetoacetate

  • D-β-hydroxybutyrate (also written as D-3-hydroxybutyrate)

  • Acetone (produced via spontaneous or enzymatic decarboxylation of acetoacetate).

• Properties: Ketone bodies are water-soluble and energy-rich, allowing for easy transport in the blood without specialized carriers.

Biochemical Pathway of Ketogenesis

1. Condensation: 2 × Acetyl-CoA → Acetoacetyl-CoA + CoA-SH.

2. HMG-CoA Formation: Acetoacetyl-CoA + Acetyl-CoA + H₂O → β-Hydroxy-β-methylglutaryl-CoA (HMG-CoA) + CoA-SH.

3. Cleavage: HMG-CoA → Acetoacetate + Acetyl-CoA.

   • Note: HMG-CoA lyase is a key enzyme primarily located in the liver.

4. Conversion:

   • Acetoacetate → D-β-Hydroxybutyrate (requires NADH + H⁺).

   • Acetoacetate → Acetone + CO₂.

Ketone Body Utilisation

• Users: Heart muscle, kidney cortex, and the brain (specifically during prolonged fasting).

• Reversion Process:

  1. D-β-Hydroxybutyrate is oxidized back to Acetoacetate via D-β-hydroxybutyrate dehydrogenase (producing NADH + H⁺).

  2. Acetoacetate + Succinyl-CoA → Acetoacetyl-CoA + Succinate.

  3. Acetoacetyl-CoA + CoA-SH → 2 × Acetyl-CoA.

• Outcome: The resulting Acetyl-CoA enters the local TCA cycle of the consuming tissue to produce ATP.

Metabolic Integration Summary

• Lipolysis: Occurs in adipocytes to release free fatty acids.

• Liver Function: Central hub where β-oxidation, gluconeogenesis, and ketogenesis are coordinated.

• Fate of Acetyl-CoA:

  • TCA Cycle: High carbohydrate availability.

  • Ketone Bodies: Low carbohydrate/Fasting state.

  • Biosynthesis: Can also be used for fatty acids and sterols.