Low Density Lipoprotein (LDL)
Lipids are nonpolar molecules and are relatively insoluble in aqueous solutions . At low concentrations, cholesterol and cholesterol esters , as well as other lipids, may form microscopic droplets called chylomicrons (lipid-protein complexes) that are somewhat stable in solution. At high concentrations, the lipids would form larger droplets and clog blood vessels, so they must be transported as complexes of lipid and protein called lipoproteins. Lipoproteins are complexes of lipid and precursor protein molecules called apolipoproteins.
Some portions of the apolipoprotein molecules are nonpolar ( hydrophobic ), and these are usually oriented toward the inside (near the lipid portion) of the complex. Polar amino acid side chains in the protein portions are oriented toward the outside of the complex, where they associate with the aqueous environment, rendering the complex soluble in blood plasma. This type of structure resembles that of micelles.
Lipoprotein complexes usually have a lipid core surrounded by one or more apolipoprotein molecules. These complexes can be separated into classes according to density. They range from very low density lipoproteins (VLDL), having densities of less than 1.006 g/mL, to low density lipoproteins (LDL), having densities of between 1.019 and 1.063 g/mL, to high density lipoproteins (HDL), having densities of between 1.063 and 1.210
g/mL. In general, the density of the lipoprotein increases as the proportion of apolipoprotein increases.
Small amounts of cholesterol may be transported as part of chylomicrons, but cholesterol is usually carried within lipoproteins, including low density lipoprotein (LDL), which carries cholesterol from the liver to muscle and other tissues, and high density lipoprotein (HDL), which carries cholesterol to the liver for conversion to bile acids. Physicians are especially concerned when patients have high levels of LDL (the so-called bad cholesterol) in blood; moderate exercise and low-cholesterol diets help to increase HDL (the so-called good cholesterol). Either high fat intake or problems with transport of cholesterol can lead to atherosclerosis, which in turn can contribute to heart attack (myocardial infarction) or stroke.
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