Oxidation of fatty acids, energy yield, carnitine system.

Introduction

Fatty acids are a major source of energy, especially during fasting, prolonged exercise, or starvation. The oxidation of fatty acids occurs primarily in the mitochondria of liver and muscle cells. Long-chain fatty acids, the most abundant dietary type, require carnitine-dependent transport into the mitochondrial matrix before undergoing β-oxidation. The process releases large amounts of ATP, particularly from long-chain saturated fatty acids.

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Activation of fatty acids

Before oxidation, free fatty acids must be activated in the cytosol to form acyl-CoA derivatives. This reaction is catalyzed by acyl-CoA synthetase (thiokinase):

Fatty acid + CoA + ATP → Acyl-CoA + AMP + PPi​

This step consumes two high-energy phosphate bonds (equivalent to 2 ATP).

Source: Nelson DL, Cox MM. Lehninger Principles of Biochemistry. 8th ed. 2021.

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Carnitine transport system

Long-chain acyl-CoA cannot directly cross the inner mitochondrial membrane. Instead, it uses the carnitine shuttle, consisting of three major components:

1. Carnitine palmitoyltransferase I (CPT I)

Located on the outer mitochondrial membrane, CPT I transfers the acyl group from CoA to carnitine, forming acylcarnitine:

Acyl-CoA + Carnitine → Acylcarnitine + CoA

CPT I is inhibited by malonyl-CoA, a key intermediate in fatty acid synthesis, preventing simultaneous synthesis and degradation of fatty acids.

2. Carnitine-acylcarnitine translocase

This inner membrane transporter exchanges acylcarnitine into the mitochondrial matrix while exporting free carnitine.

3. Carnitine palmitoyltransferase II (CPT II)

Located on the inner surface of the inner mitochondrial membrane, CPT II transfers the acyl group back from carnitine to CoA, regenerating mitochondrial acyl-CoA:

Acylcarnitine + CoA →Acyl-CoA + Carnitine

Source: Murray RK et al. Harper’s Illustrated Biochemistry. 31st ed. 2018.

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β-oxidation of fatty acids

Occurs in the mitochondrial matrix and involves repeated removal of two-carbon units as acetyl-CoA from the carboxyl end of acyl-CoA. Each cycle of β-oxidation consists of four main reactions:

1. Oxidation

Acyl-CoA dehydrogenase introduces a trans double bond between C2 and C3, generating FADH₂:

Acyl-CoA → trans-Δ²-enoyl-CoA + FADH2​

2. Hydration

Enoyl-CoA hydratase adds water across the double bond:

trans-Δ²-enoyl-CoA → L-3-hydroxyacyl-CoA

3. Oxidation

L-3-hydroxyacyl-CoA dehydrogenase oxidizes the hydroxyl group to a keto group, forming NADH:

L-3-hydroxyacyl-CoA → 3-ketoacyl-CoA + NADH

4. Thiolysis

β-ketothiolase cleaves 3-ketoacyl-CoA to produce acetyl-CoA and a shortened acyl-CoA (by two carbons):

3-ketoacyl-CoA + CoA → Acetyl-CoA + Acyl-CoA (n-2)

Source: Marks DB et al. Marks’ Basic Medical Biochemistry: A Clinical Approach. 5th ed. 2017.

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Energy yield

Complete oxidation of palmitic acid (C16:0) yields:

• 8 acetyl-CoA → enters TCA cycle
• 7 FADH₂ → 1.5 ATP each = 10.5 ATP
• 7 NADH → 2.5 ATP each = 17.5 ATP
• Total from β-oxidation: 28 ATP
• Acetyl-CoA via TCA cycle and oxidative phosphorylation → ~80 ATP
• Minus 2 ATP for activation → Net yield: ~106 ATP

Source: Devlin TM. Textbook of Biochemistry with Clinical Correlations. 7th ed. 2010.

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Regulation

Allosteric regulation

• Malonyl-CoA inhibits CPT I → prevents β-oxidation during fatty acid synthesis
• High NADH/FADH₂ inhibits β-oxidation enzymes via product feedback
• High AMP → activates AMPK → inhibits acetyl-CoA carboxylase → ↓ malonyl-CoA → ↑ β-oxidation

Hormonal regulation

• Insulin inhibits lipolysis and β-oxidation
• Glucagon and epinephrine stimulate lipolysis in adipose tissue → increase FFA availability → promote β-oxidation

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Clinical significance

Carnitine deficiency

Can be primary (genetic) or secondary (e.g., due to malnutrition, liver disease, or hemodialysis). Leads to impaired transport of long-chain fatty acids into mitochondria.

Symptoms:

• Hypoketotic hypoglycemia
• Muscle weakness
• Cardiomyopathy

Treatment: Oral carnitine supplementation

Medium-chain acyl-CoA dehydrogenase deficiency (MCAD)

Autosomal recessive disorder affecting the second step of β-oxidation. Most common inherited fatty acid oxidation disorder.

Symptoms:

• Hypoketotic hypoglycemia during fasting
• Vomiting
• Seizures
• Sudden infant death

Diagnosis: Acylcarnitine profile, urinary dicarboxylic acids

Treatment: Avoid fasting, high-carb diet

Refsum disease

Defect in α-oxidation of phytanic acid (branched-chain fatty acid). Not a β-oxidation defect per se, but related.

Symptoms:

• Retinitis pigmentosa
• Cerebellar ataxia
• Peripheral neuropathy

Treatment: Avoid phytanic acid in diet (found in dairy and meat of ruminants)

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Peroxisomal β-oxidation

Very long-chain fatty acids (≥ C22) are first shortened in peroxisomes before being transferred to mitochondria.

• Similar steps to mitochondrial β-oxidation
• Generates H₂O₂ (requires catalase for detoxification)
• Does not produce ATP directly

Source: Zempleni J, Suttie JW, Gregory JF III, Stover PJ. Present Knowledge in Nutrition. 10th ed. 2012.

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Summary

Fatty acid β-oxidation is a crucial mitochondrial process for energy production, especially in fasting states. Long-chain fatty acids must be transported into mitochondria by the carnitine shuttle, then degraded in a cyclic four-step β-oxidation pathway. This process is tightly regulated and interconnected with carbohydrate metabolism. Inherited defects in the transport or oxidation systems can result in severe metabolic disorders, often presenting with hypoglycemia and muscle-related symptoms.