Lipolysis and Fatty Acid Metabolism Quiz

Lipid Metabolism: Lipolysis and Fatty Acid β-Oxidation

• Lipolysis is the hydrolytic release of fatty acids and glycerol from TAGs. This process mobilizes stored fat.
• Lipolysis is carried out by three enzymes in adipose tissue: triacylglycerol lipase, hormone-sensitive diacylglycerol lipase (HSL), and monoacylglycerol lipase.
• Lipolysis is regulated by hormones like epinephrine and glucagon, leading to the production of cyclic AMP. This activates a protein kinase, which then phosphorylates and activates lipases.
• Insulin inhibits lipolysis, while hormones like epinephrine, glucagon, and growth hormone stimulate it.
• Regulation of lipolysis is essential for energy production in situations like starvation or insufficient glucose intake.

Steps in Lipolysis

• Lipolysis is a sequential process of hydrolysis of TAGs in adipocytes and lipid droplets.
• Enzymes progressively remove fatty acids from TAGs resulting in free fatty acids and glycerol.

Regulation of Lipolysis

• Cyclic AMP production in adipocytes is critical for lipolysis. Hormones (epinephrine, glucagon) trigger this production.
• The subsequent activation of adenylate cyclase leads to the activation of a protein kinase, which activates lipases, thus promoting the release of fatty acids from TAGs.

Fatty Acid Oxidation (β-Oxidation)

• Lipids, particularly fatty acids, provide significant energy, undergoing oxidation in three main pathways: α, β, and ω.
• Fatty acid oxidation, specifically β-oxidation, involves enzymes in the mitochondria.
• β-oxidation sequentially cleaves two-carbon units from fatty acids, forming acetyl-CoA.
• The acetyl-CoA enters the citric acid cycle for further energy production.
• The β-oxidation process occurs in mitochondria of liver, kidney, heart, and skeletal muscles. It does not occur in erythrocytes or brain tissues due to the absence of mitochondria in erythrocytes or impermeable blood-brain barrier.

Steps in Beta Oxidation

• Fatty acid activation in the cytoplasm by esterification with CoASH.
• Transport of fatty acyl CoA esters across the mitochondrial membrane.
• A sequential series of reactions known as the carbon backbone reaction sequence of: dehydrogenation, hydration, and subsequent dehydrogenation and cleavage (thiolase reaction) happens within the mitochondrial matrix.

Carnitine Shuttle

• Carnitine is essential for transporting long-chain acyl CoA from the cytosol to the mitochondria.
• The carnitine shuttle is a 3-step process mediated by carnitine acyltransferases (I and II) and a translocase.
• The shuttle delivers fatty acyl groups to the appropriate cellular location inside the mitochondria

Carnitine Structure and Sources

• Carnitine is a compound synthesized from amino acids lysine and methionine primarily in the liver and kidneys (but not in skeletal or heart muscle).
• Dietary sources of carnitine include meat products.

Oxidation of Odd-Number Fatty Acids

• Odd-numbered fatty acids undergo the same β-oxidation steps as even-numbered fatty acids until the final three carbons are reached.
• The three-carbon compound (propionyl CoA) requires a specialized three-step pathway for metabolism.
• The final product of this metabolism is succinyl CoA, a valuable intermediate in various metabolic pathways.

Sources and Fate of Succinyl CoA

• Succinyl CoA arises from the oxidation of odd-numbered fatty acids, as well as from the catabolism of various amino acids (e.g., valine, leucine, isoleucine).
• Succinyl CoA plays a critical role in the citric acid cycle, glucose formation, heme synthesis, oxidation in the citric acid cycle, activation of ketone bodies, and detoxification.

α-Oxidation of Fatty Acids

• α-oxidation is a specific pathway that occurs primarily in the brain and nervous tissues.
• It removes one carbon atom at a time from the α-position of branched fatty acids.
• This process does not require coenzyme A and doesn't produce high-energy phosphate compounds.
• A key example of the importance of this pathway is the metabolism of phytanic acid. Phytanic acid is a branched fatty acid and its metabolism via α-oxidation in brain and nervous tissue is critical.

Refsum Disease

• Refsum disease is a genetic disorder affecting the brain and nervous tissues.
• It arises from a defect in a-oxidation, creating phytanic acid accumulation in the body.
• Symptoms include nervous system dysfunction.

Differences between α and β oxidation

• β-oxidation occurs in the liver and requires CoA.
• α-oxidation occurs in brain and does not require CoA.
• β-oxidation removes 2 carbons per cycle and generates energy, while α-oxidation removes 1 carbon per cycle and doesn't generate energy.

Test your knowledge on lipolysis and the metabolism of fatty acids with this comprehensive quiz. Cover essential concepts including hormonal regulation, enzymatic activity, and the role of carnitine in energy production. Perfect for students studying biochemical processes in advanced biology or biochemistry courses.