Lipoproteins &
Lipid Transport
Explore how hydrophobic lipids travel through the aqueous bloodstream using specialized lipoprotein particles: from chylomicrons to \( \text{HDL} \).
Biochemistry
Why Do Lipids Need Transport Vehicles?
Lipids such as triglycerides, cholesterol, and cholesterol esters are hydrophobic. Blood plasma is an aqueous environment. Without a transport system, lipids would aggregate into insoluble droplets, blocking blood vessels and never reaching target tissues.
Anatomy of a Lipoprotein Particle
Core
Triglycerides and cholesterol esters: the cargo
Shell
Phospholipid monolayer + free cholesterol: the interface
Proteins
Apolipoproteins: targeting, enzyme activation, structural roles
The Five Lipoprotein Classes
The Exogenous Pathway: Dietary Lipids
The Endogenous Pathway: Liver to Tissues
Reverse Cholesterol Transport: \( \text{HDL} \)
Key Apolipoproteins and Their Roles
| Apolipoprotein | Found On | Function |
|---|---|---|
| \( \text{ApoB-48} \) | Chylomicrons | Structural (intestinal) |
| \( \text{ApoB-100} \) | \( \text{VLDL, IDL, LDL} \) | \( \text{LDL} \) receptor ligand |
| \( \text{ApoA-I} \) | \( \text{HDL} \) | Activates \( \text{LCAT} \) |
| \( \text{ApoC-II} \) | Chylomicrons, \( \text{VLDL} \) | Activates \( \text{LPL} \) |
| \( \text{ApoE} \) | Chylomicron remnants, \( \text{IDL} \) | Hepatic receptor ligand |
Critical Enzymes in Lipid Transport
\( \text{LPL} \)
Lipoprotein Lipase: on capillary endothelium, hydrolyzes \( \text{TG} \) in chylomicrons and \( \text{VLDL} \). Activated by \( \text{ApoC-II} \).
Hepatic Lipase
On liver sinusoids: converts \( \text{IDL} \to \text{LDL} \) by removing remaining triglycerides. Also remodels \( \text{HDL} \).
\( \text{LCAT} \)
Lecithin-Cholesterol Acyltransferase: esterifies free cholesterol on \( \text{HDL} \) surface. Activated by \( \text{ApoA-I} \).
\( \text{CETP} \)
Cholesteryl Ester Transfer Protein: exchanges cholesterol esters from \( \text{HDL} \) for triglycerides from \( \text{VLDL}/\text{LDL} \).
\( \text{LDL} \) vs \( \text{HDL} \): Good vs Bad?
| Feature | \( \text{LDL} \) ("Bad") | \( \text{HDL} \) ("Good") |
|---|---|---|
| Direction | Liver → Tissues | Tissues → Liver |
| Key Apolipoprotein | \( \text{ApoB-100} \) | \( \text{ApoA-I} \) |
| Main Cargo | Cholesterol esters | Cholesterol esters |
| Atherosclerosis | Promotes (deposits in walls) | Protects (removes from walls) |
| Target | \( \text{LDL} \) receptor (\( \text{ApoB-100} \)) | \( \text{SR-BI} \) receptor (\( \text{ApoA-I} \)) |
How to Trace a Dietary Lipid
1
Absorption: Dietary triglycerides are digested by pancreatic lipase, absorbed as monoglycerides and fatty acids by enterocytes.
2
Packaging: Re-esterified into \( \text{TG} \), packaged into chylomicrons with \( \text{ApoB-48} \) in the enterocyte.
3
Lymph to blood: Chylomicrons enter lacteals → thoracic duct → bloodstream.
4
\( \text{LPL} \) action: In capillaries, \( \text{LPL} \) (activated by \( \text{ApoC-II} \)) hydrolyzes \( \text{TG} \to \) free fatty acids taken up by muscle/adipose.
5
Remnant clearance: Depleted chylomicron remnant returns to liver via \( \text{ApoE} \) receptor-mediated endocytosis.
Example: Familial Hypercholesterolemia
A patient has a mutation in the \( \text{LDL} \) receptor gene. Predict the biochemical consequences for lipoprotein metabolism.
1
Defective \( \text{LDL} \) receptors on hepatocytes cannot bind and internalize \( \text{LDL} \) particles normally.
2
Plasma \( \text{LDL} \) accumulates because it is not cleared. Serum cholesterol rises dramatically.
3
Liver senses low intracellular cholesterol → upregulates \( \text{HMG-CoA} \) reductase → synthesizes even more cholesterol → more \( \text{VLDL} \) secretion → even more \( \text{LDL} \).
4
Clinical result: Severe atherosclerosis, xanthomas, corneal arcus, early myocardial infarction.
Example: \( \text{LPL} \) Deficiency
A child presents with eruptive xanthomas and milky (lipemic) serum. Lab tests show massively elevated triglycerides (\( > 1000 \text{ mg/dL} \)). Which lipoprotein is most elevated?
Cause
\( \text{LPL} \) or \( \text{ApoC-II} \) deficiency: \( \text{TG} \) hydrolysis blocked
Risk
Pancreatitis from extreme hypertriglyceridemia
Treatment
Strict dietary fat restriction (medium-chain \( \text{TG} \) bypass chylomicron pathway)
Example: \( \text{LCAT} \) Deficiency
A patient has very low \( \text{HDL} \) levels despite normal diet and exercise. Genetic testing reveals an \( \text{LCAT} \) mutation. Explain the mechanism.
1
\( \text{LCAT} \) normally esterifies free cholesterol on \( \text{HDL} \) surface, pulling it into the hydrophobic core.
2
Without \( \text{LCAT} \), free cholesterol accumulates on the surface. \( \text{HDL} \) cannot mature from discoidal to spherical form.
3
Immature \( \text{HDL} \) is rapidly cleared, leading to very low circulating \( \text{HDL} \) levels.
4
Excess free cholesterol deposits in corneas (corneal opacities) and kidneys (renal failure in severe cases).
Common Mistakes
✕ Confusing \( \text{ApoB-48} \) and \( \text{ApoB-100} \)
\( \text{ApoB-48} \) is intestinal (chylomicrons only). \( \text{ApoB-100} \) is hepatic (\( \text{VLDL, IDL, LDL} \)). They are NOT interchangeable.
✕ Thinking \( \text{LDL} \) causes pancreatitis
Elevated triglycerides (chylomicrons/\( \text{VLDL} \)) cause pancreatitis. Elevated \( \text{LDL} \) cholesterol causes atherosclerosis. Different lipids, different diseases.
✕ Forgetting \( \text{CETP} \)'s role
\( \text{CETP} \) transfers cholesterol esters from \( \text{HDL} \) to \( \text{VLDL}/\text{LDL} \) and triglycerides the other way. This is the indirect pathway of reverse cholesterol transport.
Master Summary: All Lipoproteins
| Class | Source | Main Cargo | Key Apo | Fate |
|---|---|---|---|---|
| Chylomicron | Intestine | Dietary \( \text{TG} \) | \( \text{B-48, C-II, E} \) | Remnant → Liver |
| \( \text{VLDL} \) | Liver | Endogenous \( \text{TG} \) | \( \text{B-100, C-II, E} \) | → \( \text{IDL} \) → \( \text{LDL} \) |
| \( \text{IDL} \) | \( \text{VLDL} \) catabolism | \( \text{TG} \) + \( \text{CE} \) | \( \text{B-100, E} \) | \( 50\% \) liver, \( 50\% \to \text{LDL} \) |
| \( \text{LDL} \) | \( \text{IDL} \) catabolism | Cholesterol esters | \( \text{B-100} \) | \( \text{LDL} \) receptor |
| \( \text{HDL} \) | Liver, intestine | Cholesterol esters | \( \text{A-I, A-II} \) | \( \text{SR-BI} \) / \( \text{CETP} \) |
Question 1 of 5
Which apolipoprotein is the activator of lipoprotein lipase (\( \text{LPL} \))?
\( \text{ApoC-II} \) is the obligate activator of \( \text{LPL} \). Without \( \text{ApoC-II} \), \( \text{LPL} \) cannot hydrolyze triglycerides in chylomicrons or \( \text{VLDL} \). \( \text{ApoA-I} \) activates \( \text{LCAT} \), \( \text{ApoB-100} \) is an \( \text{LDL} \) receptor ligand, and \( \text{ApoE} \) is a hepatic receptor ligand.
Question 2 of 5
A patient with familial hypercholesterolemia has a defective \( \text{LDL} \) receptor. Which lipoprotein will be most elevated in their blood?
\( \text{LDL} \) is cleared from plasma primarily by \( \text{LDL} \) receptor-mediated endocytosis. A defective \( \text{LDL} \) receptor means \( \text{LDL} \) particles cannot be taken up by the liver or peripheral cells, causing massive \( \text{LDL} \) accumulation. Chylomicrons use \( \text{ApoE} \) receptors; \( \text{VLDL}/\text{IDL} \) clearance may be partly affected but \( \text{LDL} \) accumulation is the hallmark.
Question 3 of 5
Which enzyme is responsible for converting free cholesterol to cholesterol esters on the surface of \( \text{HDL} \), enabling reverse cholesterol transport?
\( \text{LCAT} \) esterifies free cholesterol on \( \text{HDL} \) using a fatty acid from lecithin (phosphatidylcholine). This converts discoidal nascent \( \text{HDL} \) into mature spherical \( \text{HDL} \). \( \text{LPL} \) hydrolyzes triglycerides, hepatic lipase converts \( \text{IDL} \) to \( \text{LDL} \), and \( \text{HMG-CoA} \) reductase is the rate-limiting enzyme of cholesterol synthesis.
Question 4 of 5
Chylomicrons enter the bloodstream via the thoracic duct rather than the hepatic portal vein. This means they first bypass which organ?
Chylomicrons are too large to cross intestinal capillary fenestrations, so they enter lacteals (lymphatic vessels) instead. The lymph drains into the thoracic duct → left subclavian vein, bypassing the liver on first pass. This allows dietary fats to reach peripheral tissues (muscle, adipose) before the liver processes the remnants.
Question 5 of 5
\( \text{CETP} \) (Cholesteryl Ester Transfer Protein) transfers cholesterol esters from \( \text{HDL} \) to \( \text{VLDL}/\text{LDL} \). What does \( \text{HDL} \) receive in exchange?
\( \text{CETP} \) mediates a heteroexchange: cholesterol esters leave \( \text{HDL} \) and enter \( \text{VLDL}/\text{LDL} \), while triglycerides move from \( \text{VLDL}/\text{LDL} \) into \( \text{HDL} \). This is the indirect pathway of reverse cholesterol transport. The \( \text{TG} \)-enriched \( \text{HDL} \) is then remodeled by hepatic lipase.