Lipoprotein

Overview

Lipoproteins are complex particles composed of a water-repelling core of mainly lipids as non-polar cholesterol esters (CEs) and triglycerides (TGs) surrounded by an amphipathic phospholipid monolayer that includes unesterified free cholesterol (FC) and proteins known as apolipoproteins or aproteins. Apolipoproteins provide a framework for lipoprotein assembly; determine the metabolic fate of the lipoprotein by activating or inhibiting key enzymes; and finally act as ligands for receptor molecules. Lipoproteins are classified into five classes, defined by their respective density on electrophoresis as Chylomicrons, Very Low Density Lipoproteins (VLDL), Intermediate Density Lipoproteins (IDL), Low Density Lipoprotein (LDL) and High Density Lipoproteins (HDL).

Function

The lipids are often an essential part of the complex, even if they seem to have no catalytic activity themselves. To isolate transmembrane lipoproteins from their associated membranes, detergents are often needed.

All cells use and rely on fats and, for all animal cells, cholesterol as building blocks to create the multiple membranes which cells use to both control internal water content, internal water soluble elements and to organize their internal structure and protein enzymatic systems.

Lipoproteins in the blood, a water medium, carry fats around the body. The protein particles have charged groups aimed outward so as to attract water molecules; this makes them soluble in the salt water based blood pool. Triglyceride-fats and cholesterol are carried internally, shielded by the protein particle from the water.

The interaction of the proteins forming the surface of the particles with (a) enzymes in the blood, (b) with each other and (c) with specific proteins on the surfaces of cells determine whether triglycerides and cholesterol will be added to or removed from the lipoprotein transport particles.

Regarding atheroma development and progression vs. regression, the key issue has always been cholesterol transport patterns, not cholesterol concentration itself.

Classification

Lipoproteins can be classified according to their density, size and constituents. It is also possible to classify lipoproteins as "alpha" and "beta", akin to the classification of proteins in serum protein electrophoresis. This terminology is sometimes used in describing lipid disorders such as Abetalipoproteinemia.

Structure

Lipoprotein	Density	Size	% Protein	% Cholesterol	% Triglyceride	Major Lipid	Apolipoprotein
Chylomicron	0.93
Chylomicron remnant
Very low density lipoprotein (VLDL)	0.93-1.006
Intermediate density lipoprotein (IDL)	1.006-1.019
Low density lipoprotein (LDL)	1.019-1.063
High density lipoprotein (HDL)	1.063-1.210
Lipoprotein(a)

Function

Lipoprotein	Function
Chylomicron	Carries triacylglycerol (fat) from the intestines to the liver and to adipose tissue
Chylomicron remnant
Very low density lipoprotein (VLDL)	Carries newly synthesised triacylglycerol from the liver to adipose tissue
Intermediate density lipoprotein (IDL)	Intermediate between VLDL and LDL, not usually detectable in the blood
Low density lipoprotein (LDL)	Carries cholesterol from the liver to cells of the body, sometimes referred to as the "bad cholesterol" lipoprotein
High density lipoprotein (HDL)	Collects cholesterol from the body's tissues and brings it back to the liver, sometimes referred to as the "good cholesterol" lipoprotein
Lipoprotein(a)

Class	Density (gm/mL)	Electrophoretic Mobility	Diameter (nm)	Molecular Weight (Da)
Chylomicrons	0.93	Remains at origin	75-1200	50-1000 x10^6
VLDL	0.93-1.006	Pre-Beta	30-8	10-80 x10^6
IDL	1.006-1.019	Slow Pre-Beta	25-35	5-10 x10^6
LDL	1.019-1.063	Beta	18-25	2-3 x10^6
HDL	1.063-1.210	alpha	5-12	65-386 x10^6

See also

• Apolipoprotein
• Lipid anchored protein
• Lipid anchor

External links

• Database of bacterial lipoproteins at mrc-lmb.cam.ac.uk
• Overview and diagram at washington.edu
• Lipoprotein assembly at wisc.edu
• Lipoprotein circulation at purdue.edu
• Lipoproteins at the US National Library of Medicine Medical Subject Headings (MeSH)
• Proteolipids at the US National Library of Medicine Medical Subject Headings (MeSH)

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