Lecture Notes on Lipoproteins

Overview

• Lipoproteins are molecules made of fat (lipo) and proteins.
• There are four main types:
  1. Chylomicrons
  2. Very Low-Density Lipoproteins (VLDL)
  3. Low-Density Lipoproteins (LDL)
  4. High-Density Lipoproteins (HDL)
• Protein content in lipoproteins increases from chylomicrons to HDL.

Types of Lipoproteins

1. Chylomicrons

• Composition: High lipid, low protein.
• Function: Transport dietary fats from the intestine to body tissues.
• Pathway:
  • Fats from diet (lipid droplets) are digested and absorbed in the small intestine.
  • Absorbed lipids are packaged with proteins to form chylomicrons.
  • Chylomicrons enter lymphatic system → blood via subclavian vein → transport triglycerides to tissues.
  • Remnants are cleared by the liver.

2. Very Low-Density Lipoproteins (VLDL)

• Composition: High lipids, especially triglycerides.
• Function: Transport triglycerides from the liver to body tissues.
• Pathway:
  • Synthesized in the liver (via Acetyl-CoA and HMG-CoA reductase).
  • VLDL releases triglycerides, transforming into Intermediate Density Lipoprotein (IDL).

3. Low-Density Lipoproteins (LDL)

• Composition: High cholesterol.
• Function: Deliver cholesterol to cells for membrane integrity and hormone synthesis.
• Pathway:
  • Formed from VLDL → IDL → LDL.
  • LDL binds to cell receptors, delivers cholesterol, and returns to liver.
  • Excess cholesterol is excreted via bile.
  • Note: High LDL ('bad cholesterol') is associated with heart disease risk.

4. High-Density Lipoproteins (HDL)

• Composition: Higher protein relative to lipids.
• Function: Collects excess cholesterol from tissues and returns it to the liver.
• Pathway:
  • Synthesized as "empty HDL" in the liver.
  • Collects cholesterol from tissues and brings it back to the liver.
  • Full HDL binds to scavenger receptors on liver hepatocytes for recycling or excretion.
  • Note: Known as 'good cholesterol' due to its role in reducing excess cholesterol.

Enzyme Inhibition

• HMG-CoA reductase: Key enzyme in cholesterol synthesis.
• Statins: Drugs that inhibit HMG-CoA reductase, lowering cholesterol synthesis.

Summary

• Understanding the function and pathway of each lipoprotein is crucial.
• Balance between LDL and HDL levels is important for cardiovascular health.