Structure and Classification of Lipoproteins

 

Lipoproteins are macromolecular structures (supramolecular complexes) formed by the association of proteins and lipids, whose main function is to facilitate the transport of lipids (apolar compounds) in the aqueous environment (polar) that is blood.

 

We can distinguish in a lipoprotein an envelope and the core. The envelope is formed mainly by amphipatic lipids of the kind of phospholipids, like lecithins,  cephalins and others. These phospholipids have their polar part oriented to the aqueous surface, while the apolar part interacts with the hydrophobic core of the lipoprotein, formed mainly by cholesterol esters and triacylglycerols.  

 

 Free Cholesterol also appears forming part of the lipoprotein envelope, with the Carbon 3 hydroxyl oriented to the polar environment, and the apolar rest of the molecule embedded in the hydrophobic portion of the monolayer formed by the phospholipids.

 

The proteins that form these structures are called apoproteins (like all proteinic part of conjugated proteins).  The hydrophilic amino acids of these apoproteins are oriented to the outside, while the hydrophobic amino acids interact with the apolar part of the lipoprotein structure.

 

The low density of lipids and in particular, the low density of neutral fats (triacylglycerols) in relation to water is very well known.  The proportion of lipids in the lipoprotein structure influence  this physical property, and it is the basis for a simple way of classifying lipoproteins:

 

Very Low Density Lipoproteins (VLDL)

Intermediate Density Lipoproteins (IDL)

Low Density Lipoproteins (LDH)

High Density Liporprotein (HDL). 

 

Chylomicrons, lipoproteins formed in the intestines for facilitating the transportation in blood of the lipids absorbed from the diet, have a density even lower that the VLDL.

 

The following table shows, for each lipoprotein, the  Density, lipid and protein  proportion, and the predominant kind of lipid.

 

Lipoprotein	Density	Protein (% total weight)	Lipids % total weight)	Predominant Lipid (% of total weight)
Chylomicrons	<1.006	2	98	Triacylglycerol (85 %)
VLDL	0.950-1.006	10	90	Triacylglycerol (50%)
IDL	1.006-1.019	12	88	About 40 %  Cholesterol (30 % esterified and 10 % free)
LDL	1.019-1.063	25	75	About 50 %  Cholesterol (40 % esterified + 10% free)
HDL	1.063-1.210	55	45	Phospholipids (35 %)

 

More information about general features of Lipoproteins can be found here:

 

http://themedicalbiochemistrypage.org/lipoproteins.html#composition

 

Related Posts:

 

Metabolism of VLDL and formation of IDL

 

Metabolism of Chylomicrons

 

Apolipoproteins