β-Oxidation of saturated fatty acid (Palmitic acid)

• β-Oxidation is the primary pathway for breaking down fatty acids to generate energy.
• In this process, fatty acids are converted into acetyl-CoA, which enters the citric acid cycle (TCA cycle) to produce ATP.
• Here’s a detailed explanation of the beta-oxidation of palmitic acid, a saturated fatty acid with 16 carbons (C16:0).

1. Activation and Transport into Mitochondria

1. Activation:

   • Process: Before beta-oxidation can occur, palmitic acid must be activated by attaching a coenzyme A (CoA) molecule.
   • Enzyme: Acyl-CoA synthetase (also called fatty acid thiokinase).
   • Product: Palmitoyl-CoA.
   • Energy: This reaction occurs in the cytosol and consumes one ATP molecule.

2. Transport:

   • Palmitoyl-CoA needs to be transported into the mitochondrial matrix for β-oxidation.

     • Step 1:

       • Carnitine palmitoyl transferase I (CPT I), located on the outer mitochondrial membrane, transfers the fatty acyl group from CoA to carnitine, forming acyl-carnitine.

     • Step 2:

       • Acyl-carnitine is transported across the inner mitochondrial membrane by the carnitine-acylcarnitine translocase

     • Step 3:

       • Inside the mitochondrial matrix, carnitine palmitoyl transferase II (CPT II) transfers the fatty acyl group from carnitine back to CoA, regenerating acyl-CoA and releasing free carnitine.

2. Beta-Oxidation Steps

• Beta-oxidation consists of a series of four enzymatic reactions that are repeated until the entire fatty acid chain is broken down into acetyl-CoA units.

β-Oxidation of saturated fatty acid (Palmitic acid)

1. Dehydrogenation:

   • Enzyme: Acyl-CoA dehydrogenase.
   • Process: The first step introduces a double bond between the α (alpha) and β (beta) carbons of the fatty acid chain.
   • Products: Formation of trans-enoyl-CoA and reduction of flavin adenine dinucleotide (FAD) to FADH2.

2. Hydration:

   • Enzyme: Enoyl-CoA hydratase.
   • Process: A water molecule is added across the double bond formed in the previous step.
   • Product: L-3-hydroxyacyl-CoA.

3. Dehydrogenation:

   • Enzyme: 3-hydroxyacyl-CoA dehydrogenase.
   • Process: The hydroxyl group on the β-carbon is oxidized to a carbonyl group (ketone), forming β-ketoacyl-CoA.
   • Product: Reduction of NAD+ to NADH.

4. Thiolysis:

   • Enzyme: β-ketothiolase.
   • Process: The bond between the α and β carbons is cleaved.
   • Products: One molecule of acetyl-CoA and a fatty acyl-CoA that is two carbons shorter.

3. Repetition:

• For palmitic acid (C16:0), these four reactions are repeated seven times, producing a total of eight acetyl-CoA molecules.

  • Energy Production

    • TCA Cycle Entry: The acetyl-CoA molecules generated through beta-oxidation enter the TCA cycle.
    • Additional Products: The TCA cycle generates more NADH, FADH2, and ATP.
    • Electron Transport Chain: NADH and FADH2 produced during beta-oxidation and the TCA cycle are used in the electron transport chain to generate ATP via oxidative phosphorylation.

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