Lipid metabolism is a complex process involving the breakdown, synthesis, and regulation of fats within the body. One of the key processes in lipid metabolism is β-oxidation, a catabolic pathway by which fatty acid molecules are broken down in the mitochondria to generate acetyl-CoA, NADH, and FADH2. This note will provide a detailed overview of β-oxidation, using palmitic acid (a common saturated fatty acid) as an example.
Overview of β-Oxidation
β-Oxidation occurs in the mitochondria and involves a series of four recurring steps that systematically shorten the fatty acid chain by two carbon atoms at a time, producing acetyl-CoA, NADH, and FADH2. The acetyl-CoA enters the citric acid cycle, while NADH and FADH2 donate electrons to the electron transport chain, ultimately producing ATP.
Palmitic Acid: An Example
Palmitic acid (C16:0) is a saturated fatty acid with the chemical formula CH3(CH2)14COOH. It undergoes β-oxidation through the following steps:
1. Activation: Conversion of palmitic acid to palmitoyl-CoA.
2. Transport into the mitochondria: Using the carnitine shuttle.
3. β-Oxidation proper: A cycle of four enzymatic steps repeated until the fatty acid is completely converted to acetyl-CoA.
Step-by-Step Breakdown
1. Activation
Enzyme: Acyl-CoA synthetase (Thiokinase)
Location: Outer mitochondrial membrane
Reaction:
Palmitic acid + CoA + ATP → Palmitoyl-CoA + AMP + PPi
This step involves the formation of a thioester bond between the carboxyl group of palmitic acid and the sulfhydryl group of coenzyme A (CoA), utilizing ATP.
2. Transport into the Mitochondria
Enzyme: Carnitine acyltransferase I and II (CPT-I and CPT-II)
Mechanism:
CPT-I located on the outer mitochondrial membrane transfers the palmitoyl group from CoA to carnitine, forming palmitoyl-carnitine.
Palmitoyl-carnitine is then shuttled across the inner mitochondrial membrane by the carnitine-acylcarnitine translocase.
CPT-II on the inner mitochondrial membrane transfers the palmitoyl group back to CoA, regenerating palmitoyl-CoA and free carnitine.
3. β-Oxidation Cycle
Cycle Steps: Four main reactions in each cycle.
1. Oxidation:
Enzyme: Acyl-CoA dehydrogenase
Reaction:
Palmitoyl-CoA → Trans-Δ2-enoyl-CoA + FADH2
This step introduces a double bond between the C2 and C3 atoms (α and β carbons), producing FADH2.
2. Hydration:
Enzyme: Enoyl-CoA hydratase
Reaction:
Trans-Δ2-enoyl-CoA + H2O → L-β-Hydroxyacyl-CoA
Water is added across the double bond, forming L-β-hydroxyacyl-CoA.
3. Oxidation:
Enzyme: β-Hydroxyacyl-CoA dehydrogenase
Reaction:
L-β-Hydroxyacyl-CoA → β-Ketoacyl-CoA + NADH + H+
The hydroxyl group on the β-carbon is oxidized to a keto group, producing NADH.
4. Thiolysis:
Enzyme: β-Ketothiolase (Thiolase)
Reaction:
β-Ketoacyl-CoA} + CoA} → Acetyl-CoA + Acyl-CoA (14 carbons)
The ketoacyl-CoA is cleaved by CoA, yielding an acetyl-CoA and an acyl-CoA shortened by two carbon atoms.
Repetition: The shortened acyl-CoA re-enters the cycle, undergoing the same series of reactions until the entire fatty acid is converted into acetyl-CoA units.
Energy Yield from Palmitic Acid β-Oxidation
Number of cycles: Palmitic acid (16 carbons) undergoes 7 cycles of β-oxidation.
Products per cycle:
– 1 – FADH2
– 1 NADH
– 1 Acetyl-CoA (except the final cycle, which produces 2 Acetyl-CoA)
Total products:
– 7 FADH2
– 7 NADH
– 8 Acetyl-CoA
ATP Yield Calculation:
– Each of FADH2 yields approximately 1.5 ATP.
– All NADH yields approximately 2.5 ATP.
– Each Acetyl-CoA (via the citric acid cycle) yields approximately 10 ATP.
Total ATP = 7 × 1.5 + 7 × 2.5 + 8 × 10 – 2 (ATP used in activation) = 106 ATP
Summary
β-Oxidation of palmitic acid is a highly efficient process for producing energy. Through a series of well-coordinated enzymatic steps, palmitic acid is broken down into acetyl-CoA units, which are then used in the citric acid cycle and electron transport chain to produce ATP. The energy yield from one molecule of palmitic acid is significant, highlighting the importance of fatty acids as a dense source of energy in biological systems.
