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α-Oxidation

• Definition: Refers to the oxidation process that targets branched fatty acids (FAs), primarily very-long-chain fatty acids (VLCFAs) such as those containing 22 or more carbon atoms. This process is crucial for the metabolism of certain fatty acids that cannot be directly degraded by β-oxidation due to their branched structure.

• Chemical Structure Example:

  • General structure of branched fatty acid:

  • $ext{CH}_3 - ext{CH}_2 - ext{CH}_2 - ext{…} - ext{CH}_2 - ext{CH}_2 - ext{COOH}$

• ω-oxidation: This involves oxidation at the ω-carbon, which is the terminal carbon of the fatty acid chain. It serves as an alternative pathway for fatty acid oxidation when β-oxidation is impaired, particularly in the liver.

• Example of Substrates: Common substrates include medium-chain fatty acids such as C10 (capric acid) and C12 (lauric acid), which can undergo ω-oxidation.

• β-Oxidation: Refers to the oxidation occurring at the β-carbon of the fatty acids, which is critical for the degradation of saturated and unsaturated fatty acids into two-carbon units.

β-Oxidation Introduction

• Entry Mechanism:

  • Fatty acids enter target cells and are activated to acyl-CoA through the action of acyl-CoA synthetase, an enzyme that catalyzes the conversion of free fatty acids to acyl-CoA. This activation is an ATP-dependent process.

  • The transfer of acyl-CoA into mitochondria, where β-oxidation occurs, happens via the carnitine shuttle system, which helps transport long-chain acyl-CoA esters across the mitochondrial membranes.

• Key Function:

  • β-Oxidation serves as a major energy source for tissues, especially during periods of fasting, prolong exercise, and in situations where glucose availability is low. It provides energy by generating high-energy molecules like ATP, NADH, and FADH2.

• Process of Fatty Acid Degradation:

  • Fatty acids are released from triacylglycerols stored in adipose tissue into the bloodstream.

  • In the blood, fatty acids bind to albumin, which facilitates their transport to tissues where they are utilized as energy substrates.

β-Oxidation Details

• Location:

  • Primarily occurs in muscle tissue and the mitochondrial matrix. β-oxidation also takes place in peroxisomes, particularly for the oxidation of VLCFAs that cannot be efficiently processed in mitochondria.

• Key Enzymes:

  • The β-oxidation pathway involves essential enzymes including:

    • Acyl-CoA synthetase

    • Carnitine palmitoyl transferase I and II

    • Carnitine acylcarnitine translocase

    • Dehydrogenase (FAD-dependent and NAD+-dependent types)

    • Hydratase

    • Thiolase

• Substrate: The main substrate for β-oxidation is acyl-CoA, derived from fatty acids upon their activation.

• Final Products: The primary products include acetyl-CoA, which enters the citric acid cycle, and propionyl-CoA in certain fatty acids degradation contexts.

Mechanism of β-Oxidation

• Process:

  • β-Oxidation involves repeated cycles of shortening fatty acid chains by two carbons, resulting in the cleavage of two carbon atoms from the fatty acid as acetyl-CoA.

  • Each cycle continues until all carbon atoms are converted into acetyl-CoA, allowing them to enter the citric acid cycle for further energy extraction.

• Example:

  • For instance, the complete oxidation of one molecule of palmitoyl-CoA (C16:0) generates 8 molecules of acetyl-CoA.

  • Other metabolic products of this pathway include reducing equivalents in the form of NADH and FADH2, which are utilized in the electron transport chain to produce ATP, contributing to a high ATP yield from fatty acid oxidation.

Activation of Fatty Acids (FA)

• Process:

  • Fatty acids undergo activation through the action of acyl-CoA synthetase, which utilizes ATP for the conversion. This activation is essential for the further utilization and catabolism of fatty acids.

• Reaction:

  • The reaction can be summarized as:

  • $ext{Fatty acid} + ext{ATP} + ext{CoA-SH} \ <br>ightarrow ext{acyl-CoA} + ext{AMP} + ext{PPi}$

• Role of Enzymes:

  • Acyl-CoA synthetase is the key enzyme catalyzing this reaction.

  • Additionally, pyrophosphatase hydrolyzes pyrophosphate (PPi) into two inorganic phosphates (2Pi), facilitating the activation process:

  • $ext{PPi} + ext{H}_2 ext{O} \ <br>ightarrow 2 ext{Pi}$

Carnitine and Transport into Mitochondria

• Role of Carnitine:

  • Carnitine plays a pivotal role in the transport of long-chain fatty acids (LCFAs) into mitochondria for β-oxidation. Without carnitine, LCFAs cannot efficiently cross the inner mitochondrial membrane.

• Key Enzymes in Transport:

  • Carnitine Palmitoyl Transferase I (CPT I): This enzyme catalyzes the transfer of acyl groups from CoA to carnitine, forming acyl-carnitine, thus facilitating its transport.

  • Carnitine Acylcarnitine Translocase: This enzyme mediates the transport of acyl-carnitine into the mitochondrial matrix and carnitine back to the cytosol.

  • Carnitine Palmitoyl Transferase II (CPT II): This catalyzes the transfer of acyl groups from acyl-carnitine back to CoA, readying them for β-oxidation within the mitochondrial matrix.

Source and Deficiency of Carnitine

• Sources:

  • Exogenous Sources: Significant dietary sources of carnitine include meat and dairy products, which contribute to overall carnitine levels in the body.

  • Endogenous Production: The body can also synthesize carnitine endogenously from amino acids, particularly lysine and methionine, through biochemical pathways involving various enzymes and cofactors.

• Cell Transport Mechanism:

  • Carnitine is transported into cells via a specific transporter system functioning to maintain adequate intracellular levels for fatty acid metabolism.

• Consequences of Carnitine Deficiency:

  • A deficiency in carnitine can lead to impaired transport of acyl-CoA into mitochondria, resulting in lipid accumulation in tissues, potential heart damage (myocardial damage), and skeletal muscle weakness.

  • It may also cause hypoglycemia due to increased reliance on glucose metabolism in the absence of efficient fatty acid oxidation.

  • Symptoms often mirror those seen in genetic deficiencies of carnitine palmitoyl transferase I or II, leading to metabolic disturbances during periods of high energy demand.

Steps of the β-Oxidation Cycle

1. Dehydrogenation:

   • The initial step involves the oxidation of acyl-CoA by FAD, resulting in the formation of a trans-alkene, specifically an unsaturated fatty acid.

2. Hydration:

   • In this step, water is added across the double bond on the β-carbon atom, yielding the corresponding β-hydroxyacid.

3. Dehydrogenation:

   • A second round of oxidation occurs with NAD+, yielding a β-oxoacid as the product.

4. Cleavage:

   • The β-oxoacid undergoes thiolytic cleavage by CoA, forming acetyl-CoA and a newly shortened acyl-CoA that is now two carbon atoms shorter than the original fatty acid.

• Enzymatic Reactions:

  • Subsequent reactions involve a cascade of enzymes, including acyl-CoA dehydrogenase, enoyl-CoA hydratase, and β-ketoacyl-CoA thiolase, facilitating the effective conversion of substrate molecules:

  • $ext{acyl-CoA} \ <br>ightarrow ext{acetyl-CoA}$

  • The pathway begins with acyl-CoA:

    • $ext{acyl-CoA} \ <br>ightarrow ext{trans-} riangle 2 ext{-enoyl-CoA} \ <br>ightarrow ext{L-β-hydroxyacyl-CoA} \ <br>ightarrow ext{β-ketoacyl-CoA}$

  • Notably, the coenzymes involved in these processes include FAD, which gets converted to FADH2, and NAD+, which is reduced to NADH + H+ during the dehydrogenation steps, thereby contributing to the energy yield from fatty acid oxidation.