Fatty Acid Catabolism: Mechanisms and Energetics

Fatty Acid Catabolism

Introduction to Fatty Acid Catabolism

Fatty acids are a major class of lipids and serve as one of the primary sources of stored metabolic energy in animals. Their catabolism is essential for energy production, especially during fasting or prolonged exercise. Fatty acid breakdown involves several steps, including mobilization from adipose tissue, transport into mitochondria, and sequential oxidation to generate ATP.

• Fatty acids: Long hydrocarbon chains with a carboxylic acid group; stored mainly as triacylglycerols in adipose tissue.

• Energy yield: Fatty acids provide more energy per gram than carbohydrates or proteins.

• Storage: Adipose tissue stores triacylglycerols, which are mobilized when energy is needed.

Energy Storage and Mobilization

Fatty acids are stored as triacylglycerols in adipose tissue and mobilized in response to hormonal signals such as epinephrine and glucagon. The process involves hydrolysis of triacylglycerols to release free fatty acids and glycerol.

• Hormonal regulation: Epinephrine and glucagon stimulate lipolysis via activation of hormone-sensitive lipase.

• Hydrolysis reaction: Triacylglycerol + 3 H2O → Glycerol + 3 Fatty acids

• Transport: Released fatty acids bind to serum albumin for transport to tissues; glycerol is taken up by the liver.

Stored Metabolic Fuels	Average 70 kg person (g)	Stored Potential Energy (kcal)
Triacylglycerols (adipose tissue)	15,000	135,000
Protein (muscle)	6,000	24,000
Glycogen (liver)	150	600
Glycogen (muscle)	350	1,400
Glucose (body fluids)	20	80

Mobilization of Fatty Acids from Adipose Tissue

Fatty acids are released from adipose tissue by the action of lipases, which hydrolyze triacylglycerols. The process is regulated by hormones and involves the following steps:

• Lipolysis: Hormone-sensitive lipase and monoacylglycerol lipase catalyze the breakdown of triacylglycerols.

• Transport: Free fatty acids are released into the bloodstream and carried by albumin to target tissues.

• Glycerol metabolism: Glycerol is phosphorylated by glycerol kinase in the liver and enters glycolysis or gluconeogenesis.

Digestion and Absorption of Dietary Fatty Acids

Dietary triacylglycerols are digested in the intestine and absorbed as fatty acids and monoacylglycerols. These are re-esterified to triacylglycerols and packaged into chylomicrons for transport via the lymphatic system.

• Chylomicrons: Lipoprotein particles that transport dietary lipids from the intestine to tissues.

• Enzymatic hydrolysis: Lipoprotein lipase releases fatty acids from chylomicrons for uptake by tissues.

Activation and Transport of Fatty Acids into the Mitochondrial Matrix

Fatty acids must be activated and transported into the mitochondria for oxidation. Activation occurs in the cytosol, and transport involves the carnitine shuttle system.

• Activation: Fatty acid + ATP + CoA → acyl-CoA + AMP + PPi

• Carnitine shuttle: Transfers long-chain acyl groups into the mitochondrial matrix.

Equation for activation:

$\text{Fatty acid} + \text{ATP} + \text{CoA} \rightarrow \text{acyl-CoA} + \text{AMP} + \text{PP}_i$

Equation for carnitine transfer:

$\text{Acyl-CoA} + \text{Carnitine} \rightarrow \text{Acyl-carnitine} + \text{CoA}$

Acyl-carnitine is shuttled across the inner mitochondrial membrane, then reconverted to acyl-CoA in the matrix.

β-Oxidation of Fatty Acids

β-Oxidation is the process by which fatty acids are broken down in the mitochondrial matrix to generate acetyl-CoA, NADH, and FADH2. Each cycle shortens the fatty acid by two carbons.

• Steps of β-oxidation:

  1. Dehydrogenation by acyl-CoA dehydrogenase (produces FADH2)

  2. Hydration by enoyl-CoA hydratase

  3. Dehydrogenation by β-hydroxyacyl-CoA dehydrogenase (produces NADH)

  4. Thiolysis by thiolase (releases acetyl-CoA)

• Energy yield: Each round produces 1 FADH2, 1 NADH, and 1 acetyl-CoA.

General equation for one cycle of β-oxidation:

$\text{Acyl-CoA} + \text{FAD} + \text{NAD}^+ + \text{H}_2\text{O} + \text{CoA} \rightarrow \text{Acyl-CoA (shortened by 2C)} + \text{FADH}_2 + \text{NADH} + \text{Acetyl-CoA}$

Complete Oxidation of Palmitate

Palmitate (C16:0) is a common saturated fatty acid. Its complete oxidation involves seven cycles of β-oxidation, followed by entry of acetyl-CoA into the citric acid cycle.

• ATP yield calculation:

  • 7 FADH2 × 1.5 ATP = 10.5 ATP

  • 7 NADH × 2.5 ATP = 17.5 ATP

  • 8 acetyl-CoA × 10 ATP (from citric acid cycle) = 80 ATP

  • Total ATP: 108 ATP (minus 2 ATP for activation = 106 ATP net)

Equation for complete oxidation:

$\text{Palmitoyl-CoA} + 7 \text{FAD} + 7 \text{NAD}^+ + 7 \text{H}_2\text{O} + 7 \text{CoA} \rightarrow 8 \text{Acetyl-CoA} + 7 \text{FADH}_2 + 7 \text{NADH} + 7 \text{H}^+$

Each acetyl-CoA enters the citric acid cycle, producing additional NADH, FADH2, and GTP/ATP.

Summary Table: ATP Yield from Palmitate Oxidation

Step	ATP Produced
β-Oxidation (7 cycles)	7 FADH2 × 1.5 = 10.5 7 NADH × 2.5 = 17.5
Citric Acid Cycle (8 acetyl-CoA)	8 × 10 = 80
Total	108
Activation cost	-2
Net ATP	106

Key Terms and Concepts

• Triacylglycerol: Storage form of fatty acids in adipose tissue.

• Chylomicron: Lipoprotein particle for dietary lipid transport.

• Carnitine shuttle: Mechanism for transporting long-chain fatty acids into mitochondria.

• β-Oxidation: Cyclic process of fatty acid degradation in mitochondria.

• Acyl-CoA: Activated form of fatty acid for metabolism.

Example: Clinical Relevance

Defects in the carnitine shuttle or β-oxidation enzymes can lead to metabolic disorders such as carnitine deficiency or medium-chain acyl-CoA dehydrogenase deficiency (MCAD), resulting in impaired fatty acid oxidation and energy production.

Additional info: The notes above expand on the original content by providing definitions, equations, and clinical context for key steps in fatty acid catabolism, ensuring a comprehensive and self-contained study guide.