NSC 408: Beta-Oxidation of Fatty Acids

Beta-Oxidation of Fatty Acids

Overview of Beta-Oxidation

  • Definition: Beta-oxidation is a metabolic process that breaks down fatty acids into Acetyl-CoA. This occurs in the inner mitochondrial matrix.

  • Purpose: To generate energy in the form of ATP from fatty acids.

  • Inputs: Acyl-CoA (fatty acid activated with coenzyme A).

  • Outputs per cycle: Acetyl-CoA, NADH, FADH2\text{2}.

  • Integration with other pathways: The Acetyl-CoA produced enters the Krebs Cycle (TCA cycle), while NADH and FADH2\text{2} feed into the electron transport chain (ETC) to produce ATP.

  • Location: The reactions occur within the inner mitochondrial matrix.

Carnitine Shuttle: Fatty Acid Entry into Mitochondria

To be oxidized, long-chain fatty acids must first be transported from the cytosol into the mitochondrial matrix. This is facilitated by the carnitine shuttle system.

  • Key Molecules & Enzymes Involved:

    • Acyl-CoA: A long-chain fatty acid esterified with CoA.

    • AMP: Adenosine Monophosphate.

    • ATP: Adenosine Triphosphate.

    • LACS (Long Chain Fatty Acyl-CoA Synthetase or LC-FACS): Activates fatty acids in the cytosol by forming Acyl-CoA.

      • This reaction requires energy derived from the hydrolysis of ATP to AMP and pyrophosphate (PPi\text{PPi}), which is equivalent to consuming 22 ATP molecules.

      • Reaction: Fatty Acid+CoA+ATP→LACSAcyl-CoA+AMP+2Pi\text{Fatty Acid} + \text{CoA} + \text{ATP} \xrightarrow{\text{LACS}} \text{Acyl-CoA} + \text{AMP} + \text{2Pi}

    • Carnitine Palmitoyl Transferase I (CPTI): Located on the outer mitochondrial membrane. Transfers the acyl group from Acyl-CoA to carnitine, forming Acylcarnitine. CoA is released.

    • Carnitine-Acylcarnitine Translocase (CACT): Embedded in the inner mitochondrial membrane. Transports Acylcarnitine into the mitochondrial matrix while simultaneously transporting free carnitine out.

    • Carnitine Palmitoyl Transferase II (CPTII): Located on the inner mitochondrial membrane (matrix side). Transfers the acyl group from Acylcarnitine back to CoA, regenerating Acyl-CoA within the matrix. Carnitine is released.

  • Carnitine Synthesis: Carnitine itself is synthesized from the amino acids lysine and methionine.

Detailed Steps of Beta-Oxidation

Each cycle of beta-oxidation involves four enzymatic reactions, systematically shortening the fatty acyl-CoA chain by two carbon atoms, producing one Acetyl-CoA, one FADH2\text{2}, and one NADH.

  1. Dehydrogenation (FAD-dependent)

    • Enzyme: Acyl-CoA Dehydrogenase

    • Reaction: Removes hydrogen atoms from the α\alpha and β\beta carbons (carbons 2 and 3) of Acyl-CoA, forming a trans double bond between them. This produces trans-Δ2\Delta^{2} -Enoyl-CoA.

    • Coenzyme: FAD is reduced to FADH2\text{2}. The electrons from FADH\text{2}} are passed to the electron transport chain via the Electron Transfer Flavoprotein (ETF) and ETF-ubiquinone oxidoreductase (ETF-UQO).

    • Mnemonic: "Dehydrogenation creates a double bond."

  2. Hydration

    • Enzyme: Enoyl-CoA Hydratase

    • Reaction: Adds a molecule of water across the trans double bond of trans-Δ2\Delta^{2} -Enoyl-CoA, producing L-β\beta -Hydroxyacyl-CoA.

    • Input: H2O\text{H}_2\text{O}

    • Mnemonic: "Hydration adds water."

  3. Dehydrogenation (NAD+\mathbf{+}-dependent)

    • Enzyme: L-Hydroxyacyl-CoA Dehydrogenase

    • Reaction: Oxidizes the hydroxyl group (-OH\text{-OH}) at the β\beta carbon of L-β\beta -Hydroxyacyl-CoA to a ketone group, creating β\beta -Ketoacyl-CoA.

    • Coenzyme: NAD+\text{+} is reduced to NADH + H+\text{+}. The electrons from NADH also enter the electron transport chain.

    • Mnemonic: "Another dehydrogenation, another double bond (implied keto group), another reduced coenzyme."

  4. Thiolysis (Cleavage)

    • Enzyme: Thiolase

    • Reaction: Cleaves the β\beta -Ketoacyl-CoA. A molecule of Coenzyme A (CoA-SH) attacks the β\beta -keto group, resulting in the release of one molecule of Acetyl-CoA and a fatty acyl-CoA molecule shortened by two carbons.

    • Inputs: CoA-SH

    • Outputs: Acetyl-CoA and a new Acyl-CoA (shorter by 22 carbons).

    • Mnemonic: "Cleavage breaks the chain, releasing Acetyl-CoA."

  • Repeat Cycle: The shortened acyl-CoA then re-enters the beta-oxidation pathway, repeating the cycle until the entire fatty acid chain is converted into Acetyl-CoA units.

    • For an even-number carbon fatty acid of n\text{n} carbons, the number of beta-oxidation cycles is n/2−1\text{n}/2 - 1. The last cycle yields two Acetyl-CoA molecules directly.

    • The total number of Acetyl-CoA molecules produced is n/2\text{n}/2.

Calculation of ATP from an 18-Carbon Fatty Acid

Let's calculate the net ATP yield from the complete oxidation of an 18-carbon saturated fatty acid (e.g., Stearic acid):

  1. Number of Acetyl-CoA molecules: An 18-carbon fatty acid yields 182=9\frac{18}{2} = 9 Acetyl-CoA molecules.

  2. Number of Beta-Oxidation cycles: There are 182−1=8\frac{18}{2} - 1 = 8 cycles of beta-oxidation.

  • ATP from Beta-Oxidation: Each cycle produces 11 FADH2\text{2} and 11 NADH.

    • Total FADH2\text{2} from beta-oxidation: 8×1=88 \times 1 = 8

    • Total NADH from beta-oxidation: 8×1=88 \times 1 = 8

  • ATP from Acetyl-CoA (TCA Cycle): Each Acetyl-CoA molecule entering the TCA cycle produces 33 NADH, 11 FADH2\text{2}, and 11 GTP.

    • Total NADH from TCA: 9 Acetyl-CoA×3 NADH/Acetyl-CoA=279 \text{ Acetyl-CoA} \times 3 \text{ NADH/Acetyl-CoA} = 27 NADH

    • Total FADH2\text{2} from TCA: 9 Acetyl-CoA×1 FADH2/Acetyl-CoA=99 \text{ Acetyl-CoA} \times 1 \text{ FADH}_2\text{/Acetyl-CoA} = 9 FADH2\text{2}

    • Total GTP from TCA: 9 Acetyl-CoA×1 GTP/Acetyl-CoA=99 \text{ Acetyl-CoA} \times 1 \text{ GTP/Acetyl-CoA} = 9 GTP

  • Total Reduced Coenzymes and GTP:

    • Total NADH: 8 (beta-oxidation)+27 (TCA)=358 \text{ (beta-oxidation)} + 27 \text{ (TCA)} = 35 NADH

    • Total FADH2\text{2}: 8 (beta-oxidation)+9 (TCA)=178 \text{ (beta-oxidation)} + 9 \text{ (TCA)} = 17 FADH2\text{2}

    • Total GTP: 99 GTP

  • ATP Equivalents (using standard conversion factors: 2.5 ATP/NADH, 1.5 ATP/FADH2\text{2}, 1 ATP/GTP):

    • ATP from NADH: 35 NADH×2.5 ATP/NADH=87.5 ATP35 \text{ NADH} \times 2.5 \text{ ATP/NADH} = 87.5 \text{ ATP}

    • ATP from FADH2\text{2}: 17 FADH<em>2×1.5 ATP/FADH</em>2=25.5 ATP17 \text{ FADH}<em>2 \times 1.5 \text{ ATP/FADH}</em>2 = 25.5 \text{ ATP}

    • ATP from GTP: 9 GTP×1 ATP/GTP=9 ATP9 \text{ GTP} \times 1 \text{ ATP/GTP} = 9 \text{ ATP}

  • Gross Total ATP: 87.5+25.5+9=122 ATP87.5 + 25.5 + 9 = 122 \text{ ATP}

  • Energy Cost for Activation: The initial activation of the fatty acid to Acyl-CoA requires 2 ATP2 \text{ ATP} equivalents (ATP →\rightarrow AMP + PPi).

  • Net ATP from 18-Carbon Fatty Acid: 122 ATP (gross)−2 ATP (activation)=120 Net ATP122 \text{ ATP (gross)} - 2 \text{ ATP (activation)} = 120 \text{ Net ATP}

Regulation of Beta-Oxidation

Beta-oxidation is tightly regulated to meet the cell's energy demands and to prevent futile cycling with fatty acid synthesis.

  • Product Inhibition: The products of each reaction can directly inhibit the enzymes involved in the pathway.

  • NADH/NAD+\mathbf{+} Ratio: A high ratio of NADH to NAD+\text{+} (indicating high energy state) inhibits the NAD+\text{+}-dependent dehydrogenation step (L-Hydroxyacyl-CoA Dehydrogenase). This prevents further breakdown when energy is already abundant.

  • Acetyl-CoA/CoA Ratio: A high ratio of Acetyl-CoA to free CoA (also indicating high energy state and abundant substrate for TCA) inhibits Thiolase, the last enzyme of the beta-oxidation cycle. This also reduces flux through the pathway.

  • PPAR Transcription Factors (Peroxisome Proliferator-Activated Receptors): These are nuclear receptor proteins that regulate the expression of genes involved in fatty acid metabolism, including those encoding beta-oxidation enzymes. They act as sensors for fatty acids and their derivatives, upregulating beta-oxidation when fatty acid levels are high to promote their catabolism.