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

By density

General categories of lipoproteins, listed in order from larger and less dense (more fat than protein) to smaller and more dense (more protein, less fat):

• Chylomicrons - carry triacylglycerol (fat) from the intestines to the liver and to adipose tissue.
• Very low density lipoproteins - carry (newly synthesised) triacylglycerol from the liver to adipose tissue.
• Intermediate density lipoproteins - are intermediate between VLDL and LDL. They are not usually detectable in the blood.
• Low density lipoproteins - carry cholesterol from the liver to cells of the body. Sometimes referred to as the "bad cholesterol" lipoprotein.
• High density lipoproteins - collects cholesterol from the body's tissues, and brings it back to the liver. Sometimes referred to as the "good cholesterol" lipoprotein.

Alpha and beta

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.

Lipoprotein(a)

Lipoprotein(a) - Lp(a), Cardiology diagnostic tests

Normal: <14mg/dL

High risk: >19mg/dL

How to lower: exercise, niacin, aspirin.

source: Beyond Cholesterol, Julius Torelli MD, 2005 ISBN 0-312-34863-0 p.91

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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