Beta Oxidation Quiz: Test Your Knowledge on Fatty Acids

Study Notes

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.

Description

Test your knowledge on the beta oxidation process of fatty acids, including the hydrolysis of triacylglycerol to fatty acids in adipose tissue, the role of hormone sensitive lipase, and the conversion of TAG to glycerol and dihydroxyacetone phosphate. Explore how fatty acids are carried by albumin to the liver or muscle.