Lipoprotein Metabolism and Transport

Lipoprotein Basics - Tiny Lipid Taxis

• Function: Transport water-insoluble lipids (triglycerides, cholesterol, cholesteryl esters) in plasma.

• Structure:

  • Hydrophobic Core: Triglycerides (TG), Cholesteryl Esters (CE).
  • Amphipathic Shell: Phospholipids, Free Cholesterol, Apolipoproteins.

• Major Classes (listed by ↑density & ↓size):

  • Chylomicrons (largest, least dense)
  • VLDL (Very Low-Density Lipoprotein)
  • IDL (Intermediate-Density Lipoprotein)
  • LDL (Low-Density Lipoprotein)
  • HDL (High-Density Lipoprotein; smallest, most dense)

• Apolipoproteins (Apo): Proteins on the surface; roles in structure, receptor ligand binding, enzyme activation/inhibition (e.g., ApoA-I, ApoB-100, ApoC-II, ApoE).

⭐ ApoB-48 is unique to chylomicrons (intestinal origin), while ApoB-100 is found on VLDL, IDL, and LDL (hepatic origin).

Chylomicron Cycle - Gut to Liver Express

• Origin: Intestinal enterocytes package dietary TGs & cholesterol.
• Key Apolipoproteins:
  • ApoB-48: Structural, unique to chylomicrons.
  • ApoC-II: Acquired from HDL; activates Lipoprotein Lipase (LPL).
  • ApoE: Acquired from HDL; ligand for liver remnant receptor.
• Transport Pathway:
  1. Gut: Nascent chylomicrons (ApoB-48) secreted into lymph.
  2. Blood: Mature by gaining ApoC-II, ApoE from HDL.
  3. Periphery: LPL (capillaries) hydrolyzes TGs → FFAs (to tissues) & glycerol (to liver).
  4. Remnant: Chylomicron Remnant (CR) forms, rich in cholesterol esters.
  5. Liver: CR binds ApoE receptor (LRP1) for endocytosis.

⭐ ApoB-48 is synthesized from the same gene as ApoB-100 via mRNA editing (C→U, creating a stop codon) in the intestine.

VLDL/LDL Journey - Liver's Delivery Service

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HDL's Good Deeds - Cholesterol Cleanup Crew

• HDL ("Good cholesterol"): Central to Reverse Cholesterol Transport (RCT); moves cholesterol from periphery to liver.
• Key Players:
  • ApoA-I: Activates LCAT.
  • LCAT: Esterifies free cholesterol (FC) $\rightarrow$ cholesteryl esters (CE) in HDL. $FC + Lecithin \rightarrow CE + Lysolecithin$.
  • CETP: Exchanges HDL's CE for VLDL/LDL's TG.
  • SR-B1: Hepatic receptor for HDL-CE uptake.
  • ABCA1/ABCG1: Efflux FC from cells to HDL.
• RCT: Nascent HDL collects FC (ABCA1/G1) $\rightarrow$ LCAT esterifies $\rightarrow$ Mature HDL delivers CE to liver (SR-B1 or via CETP).

![HDL Reverse Cholesterol Transport Pathway](https://ylbwdadhbcjolwylidja.supabase.co/storage/v1/object/public/notes/L1/Biochemistry_Lipid_Metabolism_Lipoprotein_Metabolism_and_Transport/3ec7d7b2-7ae5-461c-89f0-d22f13050dbf.jpg)

⭐ Low HDL-C (< 40 mg/dL ♂, < 50 mg/dL ♀) is a significant risk for coronary artery disease (CAD).

Lipid Disorders - When Taxis Crash

• Hyperlipidemias (HLP):
  • Type I (LPL/ApoC-II def.): ↑Chylomicrons, ↑↑TG. Eruptive xanthomas.
  • Type IIa (LDL-R def.): ↑LDL, ↑Cholesterol. Tendon xanthomas.
  • Type IIb (FCHL): ↑LDL, ↑VLDL, ↑Cholesterol, ↑TG.
  • Type III (ApoE def.): ↑IDL (remnants), ↑Cholesterol & TG. Palmar xanthomas.
  • Type IV (FHTG): ↑VLDL, ↑TG.
• Hypolipidemias:
  • Abetalipoproteinemia (MTP def.): ↓ApoB; malabsorption, acanthocytes.

⭐ Dysbetalipoproteinemia (Type III HLP) due to ApoE defect causes accumulation of chylomicron and VLDL remnants, presenting with pathognomonic palmar xanthomas.

High‑Yield Points - ⚡ Biggest Takeaways

• Chylomicrons: Transport dietary TGs; unique ApoB-48; largest size.
• VLDL: Carries endogenous TGs from liver; contains ApoB-100.
• LDL: Major cholesterol carrier to periphery ("bad"); ApoB-100 ligand for LDL receptor.
• HDL: "Good cholesterol"; reverse cholesterol transport; primary apolipoprotein is ApoA-I.
• Key Apolipoproteins: ApoA-I activates LCAT; ApoC-II activates LPL; ApoE for remnant uptake.
• Key Enzymes: LPL (TG hydrolysis); LCAT (cholesterol esterification in HDL); CETP (CE/TG exchange).
• Dyslipidemias: Familial Hypercholesterolemia (defective LDL receptors, ↑LDL-C).