BICH 410 Lipid Metabolism

Lipid Metabolism Notes

I. Digestion

• Triacylglycerols (TAGs) are the primary form of dietary lipids (~90%).
  • Other components include cholesterol, cholesteryl esters, phospholipids, and free fatty acids.
• Key Enzymes in Lipid Digestion:
  • Pancreatic Lipase: Breaks down TAGs into fatty acids and monoglycerides.
  • Colipase: Activates pancreatic lipase and helps it bind to lipid surfaces.
  • Cholesterol Esterase: Hydrolyzes cholesterol esters.
• Micelle Formation: Bile salts emulsify fats, leading to micelle formation for better absorption.

II. Absorption

• Process of Lipid Absorption:
  1. Digested lipids form micelles with the aid of bile salts.
  2. Micelles facilitate the diffusion of free fatty acids and monoglycerides into mucosal cells of the small intestines.
  3. Inside the cells, fatty acids are reesterified into TAGs.
  4. TAGs, along with cholesterol esters and apoproteins, are packaged into chylomicrons.
  5. Chylomicrons enter the lymphatic system for transport to the bloodstream.

III. Transport

• Lipoprotein Types:
  • Chylomicrons: Transport dietary TAGs and cholesterol from the intestines.
  • Very Low-Density Lipoprotein (VLDL): Transports TAGs from the liver to tissues; derived from chylomicron remnants.
  • Low-Density Lipoprotein (LDL): Major carrier of cholesterol in the blood; delivers cholesterol to tissues.
  • Intermediate Density Lipoprotein (IDL): Formed from VLDL remnants; acts as a transitional particle between TAG and cholesterol transport.
  • High-Density Lipoprotein (HDL): Engages in cholesterol recovery; cleans excess cholesterol from blood for excretion.
• Apolipoproteins: Integral to lipoprotein function.
  • Apolipoprotein A-I (ApoA-I): Involved in HDL formation, assisting in cholesterol clearance.
  • Apolipoprotein B-100 (ApoB-100): Key in LDL receptor-mediated endocytosis.
  • Apolipoprotein E (ApoE): Enhances binding of lipoproteins to receptors, involved in lipid metabolism and transport, related to neurodegenerative diseases

Enzymatic and Cellular Mechanisms

• Emulsification:
  • Bile salts break down fat globules into smaller micelles to facilitate digestion.
  • Involves both bile salts and phospholipids to enhance lipid solubility.
• Lipid Digestion Environment:
  • Begins minimally in the mouth and stomach; significant digestion occurs in the small intestine with enzymatic action.
• Interfacial Activation of Lipase:
  • Pancreatic lipase activity increases when at the lipid-water interface, aided by colipase.

Dietary Lipid Processing Steps

1. Bile salts emulsify fats into micelles.
2. Lipases degrade TAGs into fatty acids and glycerol.
3. Fatty acids are absorbed and converted back into TAGs.
4. TAGs combined with cholesterol form chylomicrons.
5. Chylomicrons are released into lymph, then bloodstream.
6. Lipoprotein lipase hydrolyzes TAGs, releasing fatty acids and glycerol.
7. Cells uptake fatty acids for energy or storage.
8. Adipocytes store fats as TAGs after reesterification.

Summary

• Lipid metabolism involves complex processes of digestion, absorption, and transport, with critical roles played by enzymes and lipoproteins.
• Understanding these steps is essential for grasping lipid biochemistry and its implications for health.