Lipoproteins: General Structure and Functions
Lipids are insoluble in the body's aqueous environments; therefore, specialized supramolecular complexes—lipoproteins—ensure their transport in lymph and blood. Despite their functional diversity, all lipoproteins share a similar structure.
- Hydrophobic core — contains triacylglycerols, cholesterol esters, and fat-soluble vitamins.
- Surface monolayer — formed by phospholipids oriented with polar heads outward and hydrophobic "tails" inward toward the core; free cholesterol is embedded within it.
- Apolipoproteins (apoproteins) — the protein component that confers functional properties to the particle.
Apolipoproteins: Integral and Peripheral
Apoproteins are divided into two groups based on the nature of their binding to the particle:
- Integral — embedded into the lipoprotein during its synthesis, remain part of the particle, and serve as its structural marker. These include apoB-48 (chylomicrons) and apoB-100 (VLDL, IDL, LDL).
- Peripheral — can be transferred from one lipoprotein to another. Key representatives: apoC-II, apoE, apoA-I.
Formation of the Immature Chylomicron in the Intestine
Chylomicron — the largest and least dense lipoprotein, specialized in the transport of exogenous (dietary) triacylglycerols and cholesterol. The chylomicron is assembled in the enterocytes of the small intestinal mucosa. At this stage, the particle contains resynthesized triacylglycerols, cholesterol esters, phospholipids, and a single integral apoprotein — apoB-48. Such a particle is called an immature chylomicron. It is secreted into lymphatic capillaries (lacteals) and enters the venous bloodstream via the thoracic duct, bypassing the liver.
Chylomicron Maturation in the Bloodstream
In the blood, the immature chylomicron interacts with high-density lipoproteins (HDL), which act as donors of peripheral apoproteins. From HDL, the chylomicron acquires:
- apoC-II — an activator of lipoprotein lipase;
- apoE — a ligand for the hepatic receptor.
After this, the particle is referred to as a mature chylomicron and acquires the ability to deliver fats to tissues.
Action of Lipoprotein Lipase and Fat Delivery to Tissues
The mature chylomicron circulates until it contacts lipoprotein lipase (LPL), an enzyme anchored to the capillary endothelium of muscle and adipose tissue. ApoC-II on the chylomicron surface activates LPL, after which the enzyme hydrolyzes the triacylglycerols of the core into free fatty acids and glycerol.
- Fatty acids enter myocytes, where they are oxidized to produce energy, or adipocytes, where they are re-esterified and stored as triacylglycerols.
- Glycerol is carried by the bloodstream to the liver, where it enters glycolysis or gluconeogenesis.
Remnant Chylomicrons and Their Hepatic Uptake
As triacylglycerols are released, the particle shrinks and becomes enriched with cholesterol, forming a remnant chylomicron (remnant particle). The remnant is captured by hepatocytes via the apoE receptor. In hepatocyte lysosomes, the particle components are broken down by lysosomal hydrolases into end products: fatty acids, cholesterol, and amino acids, which are subsequently utilized by the cell.
Comparison with Other Lipoprotein Classes
In addition to chylomicrons, four other classes of lipoproteins circulate in the blood, differing in density, composition, and function:
- VLDL (very low-density lipoproteins) — transport endogenous triacylglycerols synthesized in the liver to peripheral tissues; the integral apoprotein is apoB-100.
- IDL (intermediate-density lipoproteins) — a transitional form between VLDL and LDL.
- LDL (low-density lipoproteins) — the primary transport of cholesterol to peripheral tissues; contain apoB-100.
- HDL (high-density lipoproteins) — perform reverse cholesterol transport from tissues to the liver, removing excess cholesterol; serve as a depot and donor of peripheral apoproteins (apoC-II, apoE) for other lipoproteins.
Key points
- Chylomicrons — lipoproteins that transport exogenous fats from enterocytes to tissues.
- The structural marker of chylomicrons is the integral apoprotein apoB-48.
- The immature chylomicron matures in the blood by receiving apoC-II and apoE from HDL.
- ApoC-II activates lipoprotein lipase on the capillary endothelium.
- LPL hydrolyzes triacylglycerols into fatty acids (muscle, adipose tissue) and glycerol (liver).
- Remnant chylomicrons are captured by the liver via the apoE receptor.
- VLDL, IDL, and LDL share the integral apoprotein apoB-100.
- HDL provide reverse cholesterol transport and act as a donor of peripheral apoproteins.