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Apolipoproteins are proteins that bind lipids (oil-soluble substances such as fat and cholesterol) to form lipoproteins. They transport
the lipids through the lymphatic and circulatory systems.
The lipid components of lipoproteins are insoluble in water. However, because of their detergent-like (amphipathic) properties,
apolipoproteins and other amphipathic molecules (such as phospholipids) can surround the lipids, creating the lipoprotein particle that
is itself water-soluble, and can thus be carried through water-based circulation (i.e., blood, lymph).
Apolipoproteins also serve as enzyme cofactors, receptor ligands, and lipid transfer carriers that regulate the metabolism of
lipoproteins and their uptake in tissues.
There are two major types of apolipoproteins. Apolipoproteins B form low-density lipoprotein ("bad cholesterol") particles. These proteins
have mostly beta-sheet structure and associate with lipid droplets irreversibly. Most of the other apolipoproteins form high-density
lipoprotein ("good cholesterol") particles. These proteins consist of alpha-helices and associate with lipid droplets reversibly. During
binding to the lipid particles these proteins change their three-dimensional structure. There are also intermediate-density
lipoproteins formed by Apolipoprotein E.
There are six classes of apolipoproteins and several sub-classes:
•A (apo A-I, apo A-II, apo A-IV, and apo A-V)
•B (apo B48 and apo B100)
•C (apo C-I, apo C-II, apo C-III, and apo C-IV)
Exchangeable apolipoproteins (apoA, apoC and apoE) have the same genomic structure and are members of a multi-gene family that
probably evolved from a common ancestralgene. ApoA1 and ApoA4 are part of the APOA1/C3/A4/A5 gene cluster on chromosome 11.
Hundreds of genetic polymorphisms of the apolipoproteins have been described, and many of them alter their structure and function.
Lipoprotein lipase (LPL) (EC 188.8.131.52) is a member of
the lipase gene family, which includes pancreatic
lipase, hepatic lipase, and endothelial lipase. It is a water-
soluble enzyme that hydrolyzes triglycerides in lipoproteins,
such as those found in chylomicrons and very low-density
lipoproteins (VLDL), into two free fatty acids and
one monoacylglycerol molecule. It is also involved in
promoting the cellular uptake of chylomicron remnants,
cholesterol-rich lipoproteins, and free fatty acids. LPL requires ApoC-II as a cofactor
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