Beta Oxidation of Fatty Acids Quiz

Lipolysis of Triglycerides

• Lipolysis is the hydrolysis of triacylglycerols to free glycerol and free fatty acids, with both products leaving the adipocyte.
• Fatty acids are used as a metabolic fuel for energy production in most tissues, except the brain due to the blood-brain barrier.
• Glycerol is taken up by the liver or kidney as a substrate for gluconeogenesis, as it cannot be used by adipose tissue cells (no glycerol kinase).

Steps of Lipolysis

• Three different lipases hydrolyze the three fatty acids of the triacylglycerols.
• The rate-limiting step of lipolysis in adipocytes is the first reaction catalyzed by the hormone-sensitive lipase.
• Diacyl- and mono-acylglycerol lipases are present in excess, ensuring that once the hormone-sensitive lipase is activated, lipolysis of the triacylglycerols goes to completion.

Control of Lipolysis in Adipocytes

• Covalent modification: hormone-sensitive lipase is activated by phosphorylation through cAMP-dependent protein kinase.
• Hormones: lipolytic hormones (e.g., epinephrine, norepinephrine, glucagon, and ACTH) stimulate lipolysis, while antilipolytic hormones (e.g., insulin) inhibit lipolysis.

Products of Lipolysis

• Free glycerol and free fatty acids (FFA) are produced, both leaving the adipocyte.
• FFAs are transported in blood as combined with albumin.
• Fatty acids are taken up by liver, kidney, muscles, mammary glands, heart, and other tissues to be oxidized through beta oxidation.

Gluconeogenesis

• Glycerol is absorbed from the diet or derived from lipolysis of fat and is activated by glycerol kinase into glycerol-3-phosphate.
• Glycerol-3-phosphate is oxidized into dihydroxyacetone phosphate, which crosses into gluconeogenesis to produce glucose in the fasted state.

Fatty Acid Oxidation (β-Oxidation)

• Definition: β-oxidation of fatty acids is the principal pathway for the catabolism of fatty acids.
• β-oxidation involves oxidation of the β-carbon to form a β-keto acid catalyzed by a number of enzymes collectively called fatty acid oxidase.

Transport of Acyl CoA into Mitochondrial Matrix

• Carnitine plays a crucial role in the transport of fatty acids through the inner mitochondrial membrane.
• Long-chain acyl-CoA cannot pass through the inner mitochondrial membrane, but its metabolic product, acylcarnitine, can.
• Carnitine palmitoyl transferase I and II are involved in the transport of long-chain fatty acids into mitochondria.

Steps of β-Oxidation

• Step 1: Activation of fatty acids into acyl-CoA.
• Step 2: Dehydrogenation (oxidation) of acyl-CoA to produce FADH2 and NADH.
• Step 3: Hydration of αβ-enoyl-CoA to produce β-hydroxyacyl-CoA.

Energy Gain from β-Oxidation

• Energy gain from β-oxidation is calculated using the formula: [(N/2 – 1) x 4 ATP] + [N/2 x 10 ATP] – 2 ATP.
• Energy gain from β-oxidation of stearic acid (18 carbon atoms) is 120 ATP.

Fates of Active Acetate

• Oxidation: through the TCA cycle.
• Lipogenesis: formation of fatty acids and their elongation.
• Ketogenesis: formation of ketone bodies.
• Steroids formation: cholesterol, bile acids, vitamin D3, and steroid hormones.
• Acetylation reactions: e.g., acetylcholine formation.