Lipid catabolism

• What is the process called that hydrolyzes triacylglycerol stored in adipocytes?

  Fat mobilization
• What are the products of lipolysis in adipocytes?

  Free fatty acids (FFA) and glycerol
• How do free fatty acids enter the bloodstream after being released from adipocytes?

  They diffuse from the adipocyte into the blood and bind to serum albumin
• What activates hormone-sensitive TG-lipase?

  Phosphorylation by a cAMP dependent protein kinase
• Which hormone is the most effective in enhancing lipolysis?

  Epinephrine
• What effect does insulin have on lipolysis?

  It decreases cAMP levels and inactivates lipase
• How does caffeine promote lipolysis?

  By increasing cAMP levels through inhibition of phosphodiesterase activity
• What is the relationship between cAMP levels and fatty acid synthesis?

  Increased cAMP levels decrease fatty acid synthesis
• What are the lipolytic and antilipolytic hormones?

  Lipolytic Hormones:

  • Epinephrine
  • Norepinephrine
  • Adrenocorticotropic hormone (ACTH)
  • Thyroid stimulating hormone (TSH)
  • Glucagon

  
  

  Antilipolytic Hormone:

  • Insulin
• Why can't glycerol produced in lipolysis be phosphorylated in adipose tissue?

  Because adipose tissue lacks the enzyme glycerol kinase
• Where is glycerol transported after lipolysis?

  To the liver
• What can glycerol 3-phosphate be used for in the liver?


  For the synthesis of triacylglycerols and phospholipids
• How do free fatty acids enter various tissues after being released from adipocytes?

  They are transported in a bound form to albumin
• What percentage of energy obtained from fat comes from the oxidation of fatty acids?

  About 95%
• What is the primary method of fatty acid oxidation in the body?

  β-oxidation
• What is β-oxidation defined as?

  The oxidation of fatty acids on the β-carbon atom
• What are the three stages of fatty acid catabolism?

  1. Activation of fatty acids occurring in the cytosol
  2. Transport of fatty acids into mitochondria
  3. β-Oxidation proper in the mitochondrial matrix
• What enzyme catalyzes the activation of long chain fatty acids?

  Acyl-CoA Synthetase (Thiokinase)
• What is required for the activation of fatty acids to acyl CoA?

  ATP, coenzyme A, and Mg$^{2+}$
• What happens to ATP during the activation of fatty acids?

  Two high energy phosphates are utilized, converting ATP to pyrophosphate (PPi)
• What makes the activation of fatty acids irreversible?

  The immediate elimination of PPi by inorganic pyrophosphatase
• What is the rate-limiting step in the transport of acyl CoA into the mitochondria?

  • The inner mitochondrial membrane is impermeable to fatty acids.
  • A specialized carnitine carrier system (carnitine shuttle) is used.
• What is the role of carnitine in fatty acid transport?

  Carnitine acts as a carrier to transport activated fatty acids into the mitochondria
• What happens to acyl-carnitine once it enters the mitochondria?

  It is converted back to acyl CoA by carnitine acyl transferase II
• What are the four steps in one round of β-oxidation?

  Dehydrogenate, Hydration, Dehydrogenate, Thiolytic cleavage
• What is the net ATP production from one cycle of β-oxidation?

  4 ATP (2 from FADH$_{2}$ and 3 from NADH)
• What is the total energy yield from one molecule of palmitic acid?

  129 ATP