Fat Metabolism: Fatty Acid Synthesis and Breakdown 

Fat metabolism involves the breakdown and synthesis of fatty acids, which are essential for energy storage, membrane structure, and cellular signaling. This process is tightly regulated and occurs in different cellular compartments, mainly in the cytoplasm and mitochondria.

Fatty Acid Breakdown (Beta-Oxidation)

Fatty acids stored in adipose tissue or derived from dietary fats are released into the bloodstream as free fatty acids (FFAs). These fatty acids are transported to various tissues where they are utilized for energy production, primarily through beta-oxidation, which occurs in the mitochondria.

Step-by-Step Process of Beta-Oxidation:

• Fatty Acid Activation:
  • Fatty acids must first be activated to fatty acyl-CoA before they can undergo beta-oxidation. This activation occurs in the cytoplasm.
  • Enzyme involved: Acyl-CoA synthetase.
  • Reaction: Fatty acid + CoA + ATP → Fatty acyl-CoA + AMP + PPi.

• Transport into Mitochondria:
  • Fatty acyl-CoA is transported into the mitochondria via the carnitine shuttle.
  • The fatty acyl-CoA is first converted into fatty acylcarnitine by carnitine palmitoyltransferase I (CPT-I) in the outer mitochondrial membrane.
  • The fatty acylcarnitine is transported into the mitochondrial matrix by the carnitine-acylcarnitine translocase.
  • Inside the matrix, carnitine palmitoyltransferase II (CPT-II) reconverts fatty acylcarnitine back to fatty acyl-CoA.

• Beta-Oxidation in Mitochondria: Once inside the mitochondria, fatty acyl-CoA undergoes beta-oxidation, which occurs in a cyclic process consisting of four reactions, each of which shortens the fatty acid chain by two carbon atoms at a time.

Four Steps of Beta-Oxidation:

1. Oxidation (Dehydrogenation): Fatty acyl-CoA undergoes oxidation by the enzyme acyl-CoA dehydrogenase to form a trans-enoyl-CoA and FADH2 (which enters the electron transport chain for ATP production).
2. Hydration: The trans-enoyl-CoA is hydrated by enoyl-CoA hydratase, adding water across the double bond to form L-3-hydroxyacyl-CoA.
3. Oxidation (Dehydrogenation): L-3-hydroxyacyl-CoA is oxidized by L-3-hydroxyacyl-CoA dehydrogenase to form 3-ketoacyl-CoA and NADH.
4. Thiolysis (Cleavage): Finally, 3-ketoacyl-CoA undergoes cleavage by thiolase, where the bond between the α- and β-carbon is broken, resulting in a shortened fatty acyl-CoA (two carbons shorter) and acetyl-CoA. The remaining fatty acyl-CoA continues to undergo the same cycle until the fatty acid is completely converted into acetyl-CoA units.

• ATP Production: Each round of beta-oxidation yields 1 FADH2, 1 NADH, and 1 acetyl-CoA. Acetyl-CoA enters the citric acid cycle (Krebs cycle), generating more ATP through oxidative phosphorylation. Example: For a 16-carbon fatty acid (palmitic acid), the process of beta-oxidation will yield 8 acetyl-CoA molecules, 7 NADH, 7 FADH2, and a large amount of ATP after entering the citric acid cycle and oxidative phosphorylation.

Fatty Acid Synthesis (Lipogenesis)

Fatty acid synthesis is the process by which the body produces fatty acids from acetyl-CoA, primarily occurring in the cytoplasm. It is the reverse of beta-oxidation but involves different enzymes and cofactors. This process is mainly active in liver cells and adipocytes (fat cells), especially when there is an excess of carbohydrates.

Step-by-Step Process of Fatty Acid Synthesis:

• a) Acetyl-CoA Transport to the Cytoplasm:
  • Acetyl-CoA is generated in the mitochondria through glycolysis and pyruvate oxidation. Since acetyl-CoA cannot cross the mitochondrial membrane, it is transported in the form of citrate.
  • In the mitochondria, acetyl-CoA combines with oxaloacetate to form citrate. Citrate is then transported into the cytoplasm, where it is converted back into acetyl-CoA and oxaloacetate by the enzyme ATP-citrate lyase.

• b) Carboxylation of Acetyl-CoA:
  • In the cytoplasm, acetyl-CoA undergoes carboxylation to form malonyl-CoA in a reaction catalyzed by acetyl-CoA carboxylase (ACC).
  • ATP and biotin are required as cofactors for this reaction:
    • Acetyl-CoA + CO₂ + ATP → Malonyl-CoA + ADP + Pi.
  • Malonyl-CoA is the building block for fatty acid elongation.

• c) Fatty Acid Chain Elongation:
  • Fatty acid synthase (FAS) is a multi-enzyme complex that catalyzes the addition of two-carbon units (from malonyl-CoA) to the growing fatty acid chain.

The elongation cycle involves the following steps:

1. Loading: The acetyl-CoA (2 carbons) is loaded onto the fatty acid synthase complex, and malonyl-CoA (also 2 carbons) is used to extend the chain.
2. Condensation: The acetyl group is condensed with the malonyl group to form a 4-carbon intermediate.
3. Reduction: The intermediate undergoes reduction by NADPH, converting it to a 4-carbon alcohol group.
4. Dehydration: Water is removed to form a double bond between carbons 2 and 3.
5. Reduction (Second Reduction): The double bond is reduced by NADPH, forming a saturated fatty acid. This cycle repeats, with the growing fatty acid chain elongating by 2 carbon units with each cycle, until the chain reaches 16 carbon atoms (palmitate).

• d) Termination: Once the fatty acid reaches 16 carbons (palmitate), it is released from the fatty acid synthase complex and can be further modified (e.g., desaturation or elongation).
• e) Fatty Acid Modifications: After synthesis, fatty acids may be desaturated (addition of double bonds) by fatty acyl desaturases or elongated by elongase enzymes to form longer fatty acids like stearic acid (C18) or arachidonic acid (C20).

Regulation of Fat Metabolism

Both fatty acid breakdown and synthesis are tightly regulated to maintain balance in the body, particularly in response to energy needs.

• Hormonal Regulation:
  • Insulin promotes fatty acid synthesis by activating acetyl-CoA carboxylase and fatty acid synthase, and it inhibits lipolysis (fat breakdown).
  • Glucagon and epinephrine promote fatty acid breakdown by activating hormone-sensitive lipase (HSL) in adipose tissue and stimulating beta-oxidation.
• Allosteric Regulation:
  • Citrate activates acetyl-CoA carboxylase in the synthesis of fatty acids.
  • Malonyl-CoA inhibits the transport of fatty acyl-CoA into mitochondria (prevents simultaneous synthesis and breakdown).
• Energy Status: When energy is abundant (high blood glucose), the body synthesizes fats for storage. When energy is low (fasting, exercise), fatty acids are mobilized and broken down for ATP production.

Summary

• Fatty Acid Breakdown (Beta-Oxidation) occurs in the mitochondria and involves the sequential removal of two-carbon units from fatty acids, producing acetyl-CoA, which enters the citric acid cycle and generates ATP.
• Fatty Acid Synthesis occurs in the cytoplasm, where acetyl-CoA is converted into malonyl-CoA, and then fatty acids are built up in a cyclic process by fatty acid synthase, primarily producing palmitic acid (C16).