Lipids are transported in blood as large macromolecules; these are complexes with proteins. Free fatty acids are the exception, mainly binding to albumin. Hydrophobic lipids, triglycerides and phospholipids are within the lipoprotein core, with the polar portions of phospholipids and the water-soluble alcohol portion of free cholesterol projecting into the aqueous environment, causing solubilization of the lipoprotein.
Types of lipoproteins are chylomicrons, very-low-density lipoprotein (VLDL), intermediate-density lipoprotein (IDL), Low-density lipoprotein (LDL) and High-density lipoprotein (HDL). Lipoprotein classes can be separated physicochemically, either by electrophoresis wich uses surface charge or by ultracentrifugation which uses relative density.
Chylomicrons are the largest, lightest lipoproteins, they carry dietary triglycerides to be hydrolysed by peripheral tissues lipoprotein lipase (LPL). Fatty acids either provide energy or are stored as triglycerides. Whereas VLDL carry triglycerides synthesized in the liver also to the periphery. LDL are the main cholesterol carriers, delivering cholesterol to peripheral tissues, or back to the liver, through LDL receptors. IDL normally undergo rapid conversion to LDL or are removed by the liver.
As triglycerides are removed, chylomicrons and VLDL shrink in size, and are further catabolized as remnants. Chylomicron remnants are removed by a liver chylomicron remnant receptor, known as LRP (LDL receptor-related protein) or the α2-macroglobulin receptor, VLDL remnants, intermediate-density lipoproteins (IDL), are acted on by hepatic lipase, removed by the liver or converted to low-density lipoproteins (LDL).
Apolipoproteins
The apoliporoteins are major components of lipoproteins. Apolipoproteins have structural and functional roles; stabilization of the molecule; receptor-recognition peptides for cell lipoprotein uptake; or coenzimes for lipid metabolism. Types of apolipoproreins are apoprotein B (apoB48 and apoB100), apoprotein C (apoC-I, apoC-II and apoC-III; the last has three isoforms: C-III0, C-III1 and C-III2), apoprotein E, apoprotein A (apoA-I, apoAII) and apoprotein (a).
Lipoprotein Structure
Generalized structure of a plasma lipoprotein is shown on the figure below.
The similarities with the structure of the plasma membrane are to be noted. A small amount of cholesterol ester are triacylglycerol are to be found in the surface layer and little free cholesterol in the core.
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