Metabolism — Triglyceride Metabolism

Triglyceride

• 3x long fatty acid molecules joined by glycerol
• eg. stearic/oleic/palmitic acid

Minimal Breakdown in the Upper GIT.

After exiting the stomach:

• Pancreatic lipase
  • Triglycerides to FFAs and 2-monoglycerides
• Pancreatic esterase
  • Cholesterol esters to cholesterol
• Actions accelerated by emulsification from bile salts
  • Facilitate intestinal digestion of lipids
    • aids pancreatic lipase activity
  • Enhances intestinal absorption of lipids by
    • emulsifying lipids
    • forming “micelles” (as it is amphipathic)
  • Induces intestinal motility
  • Choleretic action
    • bile salts → ↑ hepatocyte bile production

Transfer into Enterocytes by Passive diffusion as FFAs

In Enterocytes:

1. Short/Medium Chain FFAs
   • Diffusion into portal circulation
   • Bound to albumin and trasported to liver
2. Triglyceride formation
   • Cholesterol
     • Esterified in enterocytes
   • Long Chain FFAs
     • Encapsulated in phospholipids
   • Cholesterol and LC FFAs combine to form TGs
     • Packaged in Chylomicrons
     • Exocytosed to lymphatics for transport to liver

Transport to Liver

• TAGs absorbed as glycerol, FAs, MAG, DAG
• Transported to the liver:
  • Short-chain FAs (< 12 C)
    • transferred directly tvia portal vein without re-esterification
  • Long-chain FAs
    • re-esterified to form TAGs
    • packaged with cholesterol esters within Chylomicrons
    • transported via lymphatics to adipocytes
      • Lipids deposited in liver
      • Chylomicron remnants with remaining lipids are then taken up by liver

Fate

• Glycerol can be converted to Glucose by the liver
• Fatty acids transported from ECF to ICF by fatty acid transporter
  • FACoA synthase in cytosol makes FACoA
  • Converteted to Acyl carnitine by CPT1 and transported into cell
  • Converted back to FACoA by CPT2
  • Undergo β-oxidation
    • Each cycle generates:
      • 1 molecule of Acetyl CoA (by cleaving the 2 carbon-CoA from the whole molecule) which is used in TCA cycle
      • 1 molecule of both FADH2 and NADH to be used in the Electron Transport Chain.
    • These together produce up to 17 molecules of ATP
• Transported as chylomicrons to Adipocytes
  • Degraded by Lipoprotein lipase to FFAs and Glycerol
  • FFAs and Glycerol transported into Adipocyte
  • Converted back to Triglycerides for storage.
• In Starvation:
  • β-oxidation → ↑ acetyl-CoA → converted to HMG-CoA in liver → ketone bodies (AcAc, β-OH-butyric acid, acetone)
  • used as alternate fuel source in liver, heart and brain

Gladwin 2016

Answer by Gladwin