Apolipoprotein
Apolipoproteins are proteins that bind lipids (oil-soluble substances such as fat and cholesterol) to form lipoproteins. They transport lipids (and fat soluble vitamins) in blood, cerebrospinal fluid and lymph. 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 a lipoprotein particle that is itself water-soluble, and can thus be carried through water-based circulation (i.e., blood, lymph). In addition to stabilizing lipoprotein structure and solubilizing the lipid component, apolipoproteins interact with lipoprotein receptors and lipid transport  proteins, thereby participating in lipoprotein uptake and clearance. They also serve as enzyme cofactors for specific enzymes involved in the metabolism of lipoproteins. Apolipoproteins are also exploited by hepatitis C virus (HCV) to enable virus entry, assembly, and transmission. They play a role in viral pathogenesis and viral evasion from neutralizing antibodies. Functions. In lipid transport, apolipoproteins function as structural components of lipoprotein particles, ligands for cell-surface receptors and lipid transport proteins, and cofactors for enzymes (e.g. apolipoprotein C-II for lipoprotein lipase and apolipoprotein A-I (apoA1) for lecithin-cholesterol acyltransferase). Different lipoproteins contain different classes of apolipoproteins , which influence their function. Apolipoprotein A-I (apoA1) is the major structural protein component of high-density lipoproteins (HDL), although it is present in other lipoproteins in smaller amounts. Apolipoprotein A-IV (apoA4) is present in chylomicrons, very-low-density lipoproteins (VLDL), and HDL. It is thought to act primarily in reverse cholesterol transport and intestinal lipid absorption via chylomicron assembly and secretion. ApoA-IV synthesized in hypothalamus is suggested to be a satiating factor which regulate the food intake of the rodent. Apolipoprotein B plays a particularly important role in lipoprotein transport being the primary organizing protein of many  lipoproteins. Apolipoprotein C-III (apoC3) plays an important role in lipid metabolism specific in regulating the metabolism of triglyceride-rich lipoproteins (TRLs). Apolipoprotein D (apoD) is a soluble carrier protein of lipophilic molecules in neurons and glial cells within the central and peripheral nervous system and apoD can also modulate the stability and oxidation status of these molecules. Apolipoprotein E (apoE) plays an important role in the transport and uptake of cholesterol by way of its high affinity interaction with lipoprotein receptors, including the low-density lipoprotein (LDL) receptor. ApoE is the major lipoprotein in the central nervous system. Recent  findings with apoA1 and apoE suggest that the tertiary structures of these two members of the human exchangeable apolipoprotein gene family are related. The three-dimensional structure of the LDL receptor-binding domain of apoE indicates that the protein forms an unusually elongated four-helix bundle that may be stabilised by a tightly packed hydrophobic core that includes leucine zipper-type interactions and by numerous salt bridges on the mostly charged surface. Basic amino acids important for LDL receptor binding are clustered into a surface patch on one long helix. Apolipoprotein F (apoF) is one of the minor apolipoprotein in  blood plasma and it is a lipid transfer inhibit protein to inhibit cholesteryl ester transfer protein-mediated transfers of cholesteryl esters and triglycerides. Apolipoprotein M (apoM) participates in the lipid metabolism and exhibit anti‑atherosclerotic functions and it is presented in high-density lipoprotein (HDL), low-density lipoprotein (LDL) and very low-density lipoprotein (VLDL). Classes. There are multiple classes of apolipoproteins and several sub-classes: 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 ancestral gene. Apo-AI 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. Evolution. The cluster of exchangable apoliproteins is well conserved in vertebrates. The family diversified by duplication, with the ancestal gene most similar to ApoC1. Beyond vertebrates, proteins similar to the exchangable ApoA/C/E and the nonexchangable Apo-B are found in a wide range of animals and choanoflagellates. This suggests that the ancestral animal already has both kinds of apolipoproteins. In arthropods in particular, diacylglycerol-carrying apolipoproteins are known as "apolipophorins