Apolipoprotein
Encyclopedia
Apolipoproteins are protein
s that bind lipids (oil-soluble substances such as fat and cholesterol
) to form lipoproteins and transport the lipids through the lymphatic
and circulatory
systems.
The lipid components of lipoproteins are not soluble 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 ligand
s, and lipid transfer carriers that regulate the metabolism of lipoproteins and their uptake in tissues.
Hundreds of genetic polymorphisms
of the apolipoproteins have been described, and many of them alter their structure and function.
Apolipoprotein synthesis in the liver is controlled by a host of factors, including dietary composition, hormones (insulin
, glucagon
, thyroxin, estrogens, androgens), alcohol
intake, and various drugs (statins, niacin
,and fibric acids). Apo B is an integral apoprotein whereas the others are peripheral apoproteins.
Protein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...
s that bind lipids (oil-soluble substances such as fat and cholesterol
Cholesterol
Cholesterol is a complex isoprenoid. Specifically, it is a waxy steroid of fat that is produced in the liver or intestines. It is used to produce hormones and cell membranes and is transported in the blood plasma of all mammals. It is an essential structural component of mammalian cell membranes...
) to form lipoproteins and transport the lipids through the lymphatic
Lymphatic system
The lymphoid system is the part of the immune system comprising a network of conduits called lymphatic vessels that carry a clear fluid called lymph unidirectionally toward the heart. Lymphoid tissue is found in many organs, particularly the lymph nodes, and in the lymphoid follicles associated...
and circulatory
Circulatory system
The circulatory system is an organ system that passes nutrients , gases, hormones, blood cells, etc...
systems.
The lipid components of lipoproteins are not soluble 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
Blood
Blood is a specialized bodily fluid in animals that delivers necessary substances such as nutrients and oxygen to the cells and transports metabolic waste products away from those same cells....
, lymph
Lymph
Lymph is considered a part of the interstitial fluid, the fluid which lies in the interstices of all body tissues. Interstitial fluid becomes lymph when it enters a lymph capillary...
).
Apolipoproteins also serve as enzyme
Enzyme
Enzymes are proteins that catalyze chemical reactions. In enzymatic reactions, the molecules at the beginning of the process, called substrates, are converted into different molecules, called products. Almost all chemical reactions in a biological cell need enzymes in order to occur at rates...
cofactors
Cofactor (biochemistry)
A cofactor is a non-protein chemical compound that is bound to a protein and is required for the protein's biological activity. These proteins are commonly enzymes, and cofactors can be considered "helper molecules" that assist in biochemical transformations....
, receptor ligand
Ligand
In coordination chemistry, a ligand is an ion or molecule that binds to a central metal atom to form a coordination complex. The bonding between metal and ligand generally involves formal donation of one or more of the ligand's electron pairs. The nature of metal-ligand bonding can range from...
s, and lipid transfer carriers that regulate the metabolism of lipoproteins and their uptake in tissues.
Functions
- They are enzyme coenzymes (C-II for lipoprotein lipase and A-I for lecithin-cholesterol acyltransferase)
- Lipid transport proteins
- Ligands for interaction with lipoprotein receptors in tissues ( apoB100 and apoE for LDL-receptors, apoA-I for HDL receptors)
Classes
There are six major classes of apolipoproteins and several sub-classes:- A (apo A-IApolipoprotein A1Apolipoprotein A-I is a protein that in humans is encoded by the APOA1 gene. It has a specific role in lipid metabolism.Apolipoprotein A-I is the major protein component of high density lipoprotein in plasma. Chylomicrons secreted from the intestinal enterocyte also contain ApoA1 but it is quickly...
, apo A-IIAPOA2-Further reading:...
, apo A-IVAPOA4Apolipoprotein A-IV is plasma protein that is the product of the human gene APOA4.- Gene :APOA4 resides on chromosome 11 in close linkage to APOA1 and APOC3. APOA4 contains 3 exons separated by two introns, and is polymorphic, although most of the reported sequence polymorphisms occur in exon 3...
, and apo A-VAPOA5Apolipoprotein A-V is a protein that in humans is encoded by the APOA5 gene.The protein encoded by this gene is an apolipoprotein and an important determinant of plasma triglyceride levels, a major risk factor for coronary artery disease. It is a component of several lipoprotein fractions including...
) - B (apo B48 and apo B100Apolipoprotein BApolipoprotein B is the primary apolipoprotein of low-density lipoproteins , which is responsible for carrying cholesterol to tissues. While it is unclear exactly what functional role APOB plays in LDL, it is the primary apolipoprotein component and is absolutely required for its formation...
) - CApolipoprotein CIn the field of molecular biology, apolipoprotein C is a family of four low molecular weight apolipoproteins, designated as C-I, C-II, C-III, and C-IV that are surface components of chylomicrons, VLDL, and HDL. In the fasting state, the C apolipoproteins are mainly associated with HDL...
(apo C-IApolipoprotein C1Apolipoprotein C-I is a protein component of lipoproteins that in humans is encoded by the APOC1 gene.- Function :The protein encoded by this gene is a member of the apolipoprotein C family. This gene is expressed primarily in the liver, and it is activated when monocytes differentiate into...
, apo C-II, apo C-IIIApolipoprotein C3Apolipoprotein C-III is a protein component of very low density lipoprotein . APOC3 inhibits lipoprotein lipase and hepatic lipase; it is thought to inhibit hepatic uptake of triglyceride-rich particles. The APOA1, APOC3 and APOA4 genes are closely linked in both rat and human genomes...
, and apo C-IVApolipoprotein C4Apolipoprotein C-IV, also known as apolipoprotein C4, is a protein that in humans is encoded by the APOC4 gene.- Function :Apolipoprotein C4 gene is a member of the apolipoprotein C gene family. It is expressed in the liver and has a predicted protein structure characteristic of the other genes in...
) - DApolipoprotein DApolipoprotein D is a protein that in humans is encoded by the APOD gene.- Function :Apolipoprotein D is a component of high density lipoprotein that has no marked similarity to other apolipoprotein sequences...
- EApolipoprotein EApolipoprotein E is a class of apolipoprotein found in the chylomicron and IDLs that binds to a specific receptor on liver cells and peripheral cells. It is essential for the normal catabolism of triglyceride-rich lipoprotein constituents.-Function:...
- HApolipoprotein HApolipoprotein H , previously known as , is a multifunctional apolipoprotein. One of its functions is to bind cardiolipin. When bound the structure of cardiolipin and Apo-H both undergo large changes in structure...
Hundreds of genetic polymorphisms
Polymorphism (biology)
Polymorphism in biology occurs when two or more clearly different phenotypes exist in the same population of a species — in other words, the occurrence of more than one form or morph...
of the apolipoproteins have been described, and many of them alter their structure and function.
Synthesis and regulation
Apolipoprotein synthesis in the intestine is regulated principally by the fat content of the diet.Apolipoprotein synthesis in the liver is controlled by a host of factors, including dietary composition, hormones (insulin
Insulin
Insulin is a hormone central to regulating carbohydrate and fat metabolism in the body. Insulin causes cells in the liver, muscle, and fat tissue to take up glucose from the blood, storing it as glycogen in the liver and muscle....
, glucagon
Glucagon
Glucagon, a hormone secreted by the pancreas, raises blood glucose levels. Its effect is opposite that of insulin, which lowers blood glucose levels. The pancreas releases glucagon when blood sugar levels fall too low. Glucagon causes the liver to convert stored glycogen into glucose, which is...
, thyroxin, estrogens, androgens), alcohol
Alcohol
In chemistry, an alcohol is an organic compound in which the hydroxy functional group is bound to a carbon atom. In particular, this carbon center should be saturated, having single bonds to three other atoms....
intake, and various drugs (statins, niacin
Niacin
"Niacin" redirects here. For the neo-fusion band, see Niacin .Niacin is an organic compound with the formula and, depending on the definition used, one of the forty to eighty essential human nutrients.Niacin is one of five vitamins associated with a pandemic deficiency disease: niacin deficiency...
,and fibric acids). Apo B is an integral apoprotein whereas the others are peripheral apoproteins.