Lipoproteins Lecture Notes

Introduction to Lipoproteins

• Definition: Proteins synthesized in the small intestine and liver.
• Function: Transport hydrophobic lipids (cholesterol, triglycerides, phospholipids) throughout the body.
• Types:
  • Chylomicron
  • Chylomicron remnants
  • VLDL (Very Low-Density Lipoprotein)
  • IDL (Intermediate Density Lipoprotein)
  • LDL (Low-Density Lipoprotein)
  • HDL (High-Density Lipoprotein)
• Density: Chylomicrons are least dense; HDL are most dense.

Lipids in Lipoproteins

• Types of Lipids:
  • Triglycerides
  • Phospholipids
  • Free cholesterol
  • Cholesterol ester (structure)
• Structure:
  • Core: Hydrophobic lipids (cholesterol esters, triglycerides)
  • Shell: Hydrophilic components (free cholesterol, phospholipids, apoproteins)

Basics of Lipid Transport

• Nature of Lipids: Nonpolar
• Medium: Blood is polar, making direct lipid transport impossible.
• Solution: Lipoproteins make lipids polar by combining them with proteins, enabling blood transport.

Types of Lipoproteins

Chylomicrons

• Function: Carry lipids and cholesterol from small intestine to adipocytes, cardiac and skeletal muscle.
• Composition: Mostly triglycerides, some cholesterol.
• Secreted by: Intestinal epithelial cells into lymphatic system.
• Apoproteins: Apo E, Apo C2 (activates capillary lipoprotein lipase), Apo B48.

Very Low-Density Lipoprotein (VLDL)

• Function: Transports triglycerides from liver to peripheral tissues.
• Conversion: Becomes LDL after hydrolysis by capillary lipoprotein lipase.
• Apoproteins: Apo E, Apo C2, Apo B100.

Intermediate Density Lipoprotein (IDL)

• Composition: Cholesterol and triglycerides.
• Function: Transports triglycerides and cholesterol to liver; formed from VLDL degradation.
• Apoproteins: Apo E, Apo B100.

Low-Density Lipoprotein (LDL)

• Function: Transports cholesterol from liver to peripheral tissues; often called "bad cholesterol."
• Formation: From IDL via hepatic lipase.
• Apoproteins: Apo B100.

High-Density Lipoprotein (HDL)

• Function: Transports cholesterol from peripheral tissues to liver (reverse cholesterol transport); "good cholesterol."
• Secreted by: Intestinal epithelium and liver.
• Apoproteins: Apo A1 (activates LCAT), Apo C2, Apo E.

Apolipoproteins (Apo)

• Location of Synthesis: Rough endoplasmic reticulum.
• Types:
  • Apo A1
  • Apo A2
  • Apo B48
  • Apo B100
  • Apo C1
  • Apo C2
  • Apo C3
  • Apo E
• Functions:
  • Apo A1: Activates LCAT, major structural protein for HDL.
  • Apo C2: Activates capillary lipoprotein lipase, releases fatty acids and glycerol.
  • Apo B48: Component of chylomicrons.
  • Apo B100: Contains LDL receptor recognition domain.
  • Apo E: Mediates uptake of chylomicron remnants and IDL by the liver.

Density of Lipoproteins

• Relation to Proteins and TG:
  • Density ∝ Protein percentage
  • Density ∝ 1/TG content and size
• Order of Density and Size:
  • Chylomicron (least dense, largest size)
  • HDL (most dense, smallest size)

Electrophoretic Separation

• Order (from cathode to anode):
  • Chylomicron
  • LDL (β-lipoprotein)
  • VLDL (pre-β-lipoprotein)
  • IDL (broad-β-lipoprotein)
  • HDL (α-lipoprotein)