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Review of Lipoproteins March 21 2003 Bryant Miles Lipids by definition are insoluble in water In order to transport lipids such as fatty acids triacylglycerols steroids and fat soluble vitamins in the blood plasma a carrier protein is required Fatty acids are carried from the adipose tissue to the muscle heart and liver tissues by serum albumin Vitamin A is carried by the retinol binding protein There are steroid carrier proteins that carry steroids to the target cells The bulk of the body s lipids cholesterol phospholipids and triacylglycerols are transported in the plasma by large complexes called lipoproteins These lipoproteins consist of a core of hydrophobic lipids surrounded by a shell of phosphotidyl glycerols and proteins The protein components of lipoproteins solubilize the hydrophobic lipids and contain the cell targeting signals Lipoproteins are classified according to their density The lowest density lipoproteins are the chylomicrons followed by the chylomicron remnants very low density lipoproteins VLDLs intermediate density lipoproteins IDLs low density lipoproteins LDLs and high density lipoproteins HDLs The densities of these lipoproteins are related to the relative amounts of lipids to proteins in the complex The higher the protein content the higher the density of the lipoprotein Low Density Lipoprotein Shown to the left is a low density lipoprotein The core of the LDL is composed of triacylglycerols and cholesteryl esters This lipid core is surrounded by a layer of amphipathic phospholipids and unesterified cholesterol The lone hydroxyl group of cholesterol molecules is oriented towards the outer surface shown here as black dots The lipoproteins also contain apoproteins in their outer shell The apoproteins have important roles in lipid transport and metabolism They have specific structural domains that are recognized by cell receptors All of the apoproteins have amphipathic helixes with the hydrophobic side chains facing the lipid interior of the

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