Fatty Acid Metabolism Study Notes

Fatty Acid Metabolism Overview

  • Fatty acids are crucial energy sources and metabolic intermediates in the body.

Fatty Acid Release and Transport

  • Fatty acids are released from adipocytes (fat cells) into the bloodstream.

    • Adipocytes: Specialized cells that store fat.

  • Released fatty acids bind to the protein serum albumin in the bloodstream, which serves as a carrier.

    • Serum Albumin:

    • Represents approximately 50% of serum protein.

    • Capable of binding up to ten fatty acid molecules.

    • Acts as a transport mechanism to deliver fatty acids to tissues that require fuel.

Activation of Fatty Acids

  • The initial activation of fatty acids is necessary before they enter the mitochondria for oxidation.

    • This activation involves conversion to fatty acyl-CoA via the enzyme fatty acyl-CoA synthetase.

    • The overall reaction can be summarized as follows:
      extFA+extATP+extCoA<br>ightarrowextFA−CoA+extAMP+extPPiext{FA} + ext{ATP} + ext{CoA} <br>ightarrow ext{FA-CoA} + ext{AMP} + ext{PPi}

    • The standard change in free energy (extAGext{AG}) for this reaction is approximately -34 kJ/mol.

Transport into Mitochondria

  • Neither CoA nor acetyl-CoA can directly cross the mitochondrial membrane.

  • Carnitine Shuttle facilitates transport of fatty acyl-CoA into the mitochondria.

    • Step 1: Fatty acyl-CoA is converted to fatty acylcarnitine:

    • extFA−CoA+extCarnitine<br>ightarrowextFA−Carnitine+extCoAext{FA-CoA} + ext{Carnitine} <br>ightarrow ext{FA-Carnitine} + ext{CoA}

    • This reaction is catalyzed by carnitine acyltransferase I.

    • Step 2: The fatty acylcarnitine is shuttled across the inner mitochondrial membrane into the matrix via a specific transporter.

    • Step 3: Inside the mitochondria, the fatty acylcarnitine is converted back to fatty acyl-CoA:

    • extFA−Carnitine+extCoA<br>ightarrowextFA−CoA+extCarnitineext{FA-Carnitine} + ext{CoA} <br>ightarrow ext{FA-CoA} + ext{Carnitine}

    • This reaction is catalyzed by carnitine acyltransferase II.

    • Step 4: Carnitine is then returned to the intermembrane space using the same transporter.

Beta-Oxidation of Fatty Acids

  • Beta-Oxidation is the process by which saturated fatty acyl-CoA is degraded via four enzymatic reactions:

    1. Oxidation by FAD

    2. Hydration

    3. Oxidation by NAD+

    4. Thiolysis

    • The result of beta-oxidation is a fatty acid that has been shortened by two carbons and generates the following products:

    • Acetyl-CoA

    • FADH2

    • NADH

  • Example of the reaction steps:

    1. Oxidation of acyl-CoA:

    • Converting acyl-CoA into enoyl-CoA:

    • extR−SCoA<br>ightarrowexttrans−A−enoyl−CoA+extFADH2ext{R-SCoA} <br>ightarrow ext{trans-A-enoyl-CoA} + ext{FADH2}

    1. Hydration step:

    • The hydration of double bonds in enoyl-CoA:

    • exttrans−A−enoyl−CoA+extH2O<br>ightarrowextL−B−hydroxyacyl−CoAext{trans-A-enoyl-CoA} + ext{H2O} <br>ightarrow ext{L-B-hydroxyacyl-CoA}

    1. Oxidation of Hydroxy Group:

    • Conversion of the hydroxy group to a carbonyl group using NAD+.

    • extL−B−hydroxyacyl−CoA<br>ightarrowextB−ketoacyl−CoA+extNADH+extH+ext{L-B-hydroxyacyl-CoA} <br>ightarrow ext{B-ketoacyl-CoA} + ext{NADH} + ext{H+}

    1. Thiolytic Cleavage:

    • Cleavage of B-ketoacyl-CoA yields a fatty acyl-CoA that is two carbons shorter:

    • extB−ketoacyl−CoA+extCoA−SH<br>ightarrowextFA−CoA+extAcetyl−CoAext{B-ketoacyl-CoA} + ext{CoA-SH} <br>ightarrow ext{FA-CoA} + ext{Acetyl-CoA}

Yield and Calculation

  • Subsequent rounds of beta-oxidation continue to yield acetyl-CoA until the fatty acid is completely degraded.

  • The net yield for the complete oxidation of palmitoyl-CoA (a 16-carbon saturated fatty acid) is 108 ATP.

  • Important to understand the calculation of ATP yield from beta-oxidation.

Variations in Beta-Oxidation

  • Additional steps are required for unsaturated fatty acids, which undergo specific alterations due to the presence of double bonds.

  • Fatty acids with an odd number of carbons yield one succinyl-CoA via propionyl-CoA during metabolism.

  • Beta-oxidation must also account for branched fatty acids, which require distinct pathways for degradation.

Implications and Applications

  • Understanding fatty acid metabolism provides insights into energy homeostasis, the role of different substrates in metabolism, and links to metabolic disorders.

  • Fatty acid oxidation is critical in various physiological conditions including fasting, exercise, and metabolic diseases.