Lipoprotein Functions and Classification

Lipoprotein Structure and Functions

• Apoproteins solubilize hydrophobic lipids and facilitate receptor recognition for receptor-mediated uptake of plasma lipoproteins by certain tissues.
• Apoproteins act as activators or coenzymes for enzymes involved in lipoprotein metabolism (e.g., apo C activates lipoprotein lipase, which degrades TG into unesterified fatty acids and glycerol).

Classification of Plasma Lipoproteins

• Lipoproteins are classified according to their densities, which are determined by the amount of TG (making them less dense) and apoproteins (making them more dense).

Types of Lipoproteins

• Chylomicron:
  • Synthesized in the intestine after a fat-rich meal
  • Composition: TG 90%, C&CE 5%, PL 3%, Protein 2%
  • Apoproteins: apo B48, apo C-II, apo E
  • Function: Transport of dietary TG from the intestine to extrahepatic tissues
• Very Low Density Lipoprotein (VLDL):
  • Synthesized in the liver
  • Composition: TG 60%, C&CE 20%, PL 5%, Protein 15%
  • Apoproteins: apo B100, apo C-II, apo E
  • Function: Transport of TG synthesized in the liver to extrahepatic tissues
• Low Density Lipoprotein (LDL):
  • Derived from VLDL by losing TG
  • Composition: TG 8%, C&CE 50%, PL 25%, Protein 20%
  • Apoproteins: apo B100
  • Function: Transport of C & CE from the liver to extrahepatic tissues (bad cholesterol)

Lipoprotein Metabolism

• The major lipids in blood are: Triacylglycerol (TG), Cholesterol & Cholesterol esters, Phospholipids, and Free fatty acids.
• Lipids are insoluble in water, so they are transported through the blood as lipoproteins, which are water-lipid-protein complexes.
• Non-polar lipids (triacylglycerol and cholesterol esters) are associated with amphipathic lipids (phospholipids and cholesterol) and proteins (apolipoproteins) to form lipoproteins.
• Free fatty acids bind to albumin.
• Amphipathic lipids have both hydrophilic (water-loving, polar) and lipophilic (fat-loving) properties.

Structure of Lipoproteins

• Inner core of non-polar lipids (triacylglycerol and cholesterol esters)
• Monolayer of amphipathic lipids (phospholipids and cholesterol) with polar groups oriented outward to the aqueous medium and non-polar groups oriented to the lipid core
• Protein moiety (apoproteins):
  • Peripheral apoproteins: Can be transferred to other lipoproteins (e.g., apo C & apo E from HDL)
  • Integral apoproteins: Cannot be transferred to other lipoproteins (e.g., apo B)

This quiz covers the functions of lipoproteins, including solubilizing hydrophobic lipids, receptor recognition, and enzyme activation. It also explores the classification of plasma lipoproteins.