Lipoproteins are lipid-protein complexes that transport lipids through the blood

Question

Lipoproteins are lipid-protein complexes that transport lipids through the bloodstream from dietary and biosynthesized sources to peripheral tissues and the liver. They included LDLs (the so-called “bad cholesterol”), HDLs (the “good cholesterol”), and others. They are essentially fat droplets, composed of triacylglycerols and cholesterol, enclosed by a monolayer of phospholipids and protein. What is the functional significance of the monolayer of amphipathic lipids? How are the lipoprotein components spatially arranged to allow for travel through the bloodstream?

Explanation / Answer

The liver takes up IDLs after they have interacted with the LDL receptor to form a complex, which is endocytosed by the cell. For LDL receptors in the liver to recognize IDLs requires the presence of apoB-100 and is enhanced in the presence of apoE. The LDL receptor is also sometimes referred to as the apoB-100/apoE receptor. The importance of apoE in cholesterol uptake by LDL receptors has been demonstrated in transgenic mice lacking functional apoE genes. These mice develop severe atherosclerotic lesions at 10 weeks of age even when fed a low-fat diet. Of significance to overall serum total cholesterol levels is the fact that one of the events that results in the conversion of IDL to LDL is the loss of apoE which is returned to HDL. Although LDL particles still possess apoB-100 which is required for LDL receptor binding, the loss of apoE amkes their affinity for the receptor reduced.

** HDLs represent a heterogeneous population of lipoproteins in that they exist as functionally distinct particles possessing different sizes, protein content, and lipid composition. One of the major functions of HDLs is to acquire cholesterol from peripheral tissues and transport this cholesterol back to the liver where it can ultimately be excreted following conversion to bile acids. This function is referred to as reverse cholesterol transport (RCT). The role of HDLs in RCT represents the major atheroprotective (prevention of the development of atherosclerotic lesions in the vasculature) function of this class of lipoprotein. In addition to RCT, HDLs exert anti-inflammatory, antioxidant, and vasodilatory effects that together represent additional atheroprotective functions of HDLs. Evidence has also been generated that demonstrates that HDLs possess anti-apoptotic, anti-thrombotic, and anti-infectious properties. With respect to these various atheroprotective functions of HDLs, it is the small dense particles (referred to as HDL3) that are the most beneficial.

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