Biochemistry · MU Pleven

25. Lipid Transport in Serum: Triacylglycerol Transport by Lipoproteins. Metabolism.

Explains why lipids need lipoprotein carriers in blood and what these particles contain, including the core, surface layer and apolipoproteins that stabilise, target and activate them. Goes through chylomicrons, VLDL, LDL and HDL in turn, showing how lipoprotein lipase releases fatty acids from triacylglycerols, how particles are remodelled into remnants, IDL and LDL, and how HDL returns cholesterol to the liver. Closes with a summary of the flow of triacylglycerol transport and a list of the key apolipoproteins and their roles.

Inside: 5 sections · 2 labelled pictures · 10 flashcards · 3 min

Topic 25 of 76 in the MU Pleven syllabus for Biochemistry (Medicine, Year 2). The free AceMed note covers it in 5 sections, about 361 words (3 min of reading), with 10 flashcards.

In this note

  1. Lipid Transport in Serum: Triacylglycerol Transport by Lipoproteins.
  2. Lipid Transport in Serum
  3. Major Lipoproteins Involved in Triacylglycerol (TAG) Transport
  4. Putting It All Together (Flow of TAG Transport)
  5. Key Apolipoproteins

Lipid Transport in Serum

  • Lipids are hydrophobic, so they cannot travel freely in blood.
  • To be transported, they are packaged into lipoproteins (lipid + protein complexes).
  • Each lipoprotein has:
  • A core of triacylglycerols (TAGs) and cholesteryl esters
  • A surface layer of phospholipids + free cholesterol
  • Apolipoproteins (Apos) which:
  • Stabilize the particle
  • Act as address labels (direct to tissues)
  • Activate enzymes

Major Lipoproteins Involved in Triacylglycerol (TAG) Transport

1. Chylomicrons (Dietary TAG Transport)

  • Formed in intestinal cells after a meal.
  • Contain ApoB-48 (their structural protein).
  • Carry dietary TAGs from intestine → bloodstream → tissues.
  • Acquire ApoC-II and ApoE from HDL in the blood.
  • ApoC-II activates Lipoprotein Lipase (LPL) on capillary walls (muscle + adipose).
  • LPL hydrolyzes TAGs → glycerol + fatty acids.
  • Muscles use fatty acids for energy.
  • Adipose stores them as fat.
  • After TAG removal → Chylomicron remnants remain.
  • Remnants are taken up by the liver via ApoE recognition.

2. VLDL (Very Low-Density Lipoproteins) (Endogenous TAG Transport)

  • Synthesized in the liver to transport liver-made TAGs to tissues.
  • Contain ApoB-100.
  • In circulation, also receive ApoC-II & ApoE from HDL.
  • LPL again breaks down TAGs in VLDL for tissue use/storage.
  • As TAGs are removed → VLDL shrinks → becomes IDL, then further → LDL.

3. LDL (Low-Density Lipoproteins) (Cholesterol Delivery)

  • Formed from VLDL metabolism.
  • Rich in cholesterol, not TAGs.
  • Delivers cholesterol to all tissues.
  • Contains ApoB-100, which binds to LDL receptors on cells.
  • Excess LDL → deposits in arteries → atherosclerosis risk.

4. HDL (High-Density Lipoproteins) (Reverse Cholesterol Transport)

  • Synthesized in liver and intestine.
  • Collects excess cholesterol from tissues and blood vessels.
  • Returns cholesterol to liver for:
  • Recycling into bile
  • Or excretion
  • Carries ApoA-I, which activates LCAT → esterifies cholesterol for safe storage.

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