Triacylglycerol and Fatty Acid Catabolism Quiz

Triacylglycerol and Fatty Acid Catabolism

• Triacylglycerols (TAGs) and fatty acids (FAs) are broken down to provide energy during fasting or increased energy demand. This involves lipolysis and β-oxidation.

Lipolysis

• Definition: The breakdown of TAGs into glycerol and free fatty acids (FFAs).
• Location: Adipose tissue.
• Hormonal Regulation:
  • Stimulators: Glucagon, epinephrine, norepinephrine.
  • Inhibitor: Insulin (inhibits lipolysis during fed state).
  • Hormones activate hormone-sensitive lipase (HSL) via cAMP-dependent protein kinase (PKA).
• Enzymatic Action:
  • Adipose triglyceride lipase (ATGL): Converts TAG to diacylglycerol (DAG).
  • Hormone-sensitive lipase (HSL): Converts DAG to monoacylglycerol (MAG).
  • Monoacylglycerol lipase (MAGL): Converts MAG to glycerol and a free fatty acid (FFA).
• Products:
  • Glycerol: Transported to the liver for gluconeogenesis or glycolysis.
  • FFAs: Bind to albumin in the blood and are transported to energy-demanding tissues.

β-Oxidation of Fatty Acids

• Definition: Stepwise removal of two-carbon units (as acetyl-CoA) from fatty acids for energy.

• Location: Mitochondrial matrix. Long-chain fatty acids are first activated in the cytoplasm.

• Activation:

  • Fatty acid + CoA + ATP → Acyl-CoA + AMP + PPi.
  • Enzyme: Acyl-CoA synthetase (thiokinase).

• Transport into Mitochondria:

  • Carnitine shuttle system moves acyl-CoA into the mitochondrial matrix.
    • CPT I (carnitine palmitoyltransferase I): Converts acyl-CoA to acyl-carnitine (rate-limiting step).
    • Acyl-carnitine transported into the matrix by a translocase.
    • CPT II regenerates acyl-CoA in the matrix.

• β-Oxidation Proper (Spiral Pathway):

  • 1st Step: Oxidation by acyl-CoA dehydrogenase → forms a double bond; FAD → FADH₂.
  • 2nd Step: Hydration by enoyl-CoA hydratase → Adds water.
  • 3rd Step: Oxidation by β-hydroxyacyl-CoA dehydrogenase → forms NADH.
  • 4th Step: Cleavage by thiolase → produces acetyl-CoA and a shortened acyl-CoA.

• Energy Yield (per cycle of β-oxidation):

  • 1 FADH₂ → 1.5 ATP
  • 1 NADH → 2.5 ATP
  • Per Acetyl-CoA: Enters the TCA cycle for further ATP generation (1 Acetyl-CoA → 10 ATP).

• Example (Palmitic Acid, C16):

  • 7 cycles of β-oxidation → 7 FADH₂, 7 NADH, 8 acetyl-CoA.
  • Total ATP: ~106 ATP (after accounting for activation energy).

Regulation

• Lipolysis:
  • Stimulated by: Glucagon, epinephrine.
  • Inhibited by: Insulin.
• β-Oxidation:
  • Inhibited by: Malonyl-CoA (inhibits CPT I activity during FA synthesis).
  • Stimulated by: High FA availability, fasting state.

Test your knowledge on the catabolism of triacylglycerols and fatty acids, including the processes of lipolysis and β-oxidation. This quiz covers key enzymes, hormonal regulations, and the biochemical pathways involved in energy production during fasting. Perfect for students studying biochemistry or metabolism.