6 lipoprotein

• Lipoproteins are molecular complexes that consist of lipids and proteins
• Lipoproteins


  Function as transport vehicles for lipids in blood plasma
• A lipoprotein consists of a neutral lipid core surrounded by a coat shell of phospholipids, apoproteins, and cholesterol
• Classification of lipoproteins


  Based on their separation by electrophoresis
• Major classes of lipoproteins

  • Chylomicrons
  • Very low density lipoproteins (VLDL)
  • Low density lipoproteins (LDL)
  • High density lipoproteins (HDL)
  • Free fatty acids-albumin
• Chylomicrons


  Synthesized in the intestine and transport exogenous triacylglycerol
• Chylomicrons consist of highest quantity of lipid and lowest concentration of protein
• Very low density lipoproteins (VLDL)

  Produced in liver and intestine, responsible for transport of endogenously synthesized triacylglycerols
• Low density lipoproteins (LDL)


  Transport cholesterol from liver to other tissues
• High density lipoproteins (HDL)

  Synthesized in liver, transport cholesterol from peripheral tissues to liver
• Free fatty acids in circulation are in a bound form to albumin
• Each molecule of albumin can hold about 20-30 molecules of free fatty acids
• Apolipoproteins


  Protein components of lipoproteins
• Functions of apolipoproteins


  • Act as structural components of lipoproteins
  • Recognize cell membrane surface receptors
  • Activate enzymes involved in lipoprotein metabolism
• The comparative characteristic features of different lipoproteins are given in a table
• Chylomicrons are synthesized in the small intestine during fat absorption
• Chylomicrons contain apoprotein B48 and mostly triacylglycerols
• Role of lipoprotein lipase


  1. Hydrolyses triacylglycerols in chylomicrons and VLDL

  2. Liberates free fatty acids and glycerol
• Lipoprotein lipase is activated by apo CII
• Uptake of chylomicron remnants by liver

  Chylomicron remnants are taken up by receptors on hepatocytes
• Conversion of VLDL to LDL

  1. Intermediate density lipoprotein (IDL) is formed

  2. IDL loses apo E and gets converted to LDL
• LDL contains high cholesterol and less triacylglycerol
• CETP is synthesized in the liver
• CETP


  Facilitates the exchange of components between different lipoproteins
• LDL binds to specific receptor pits on the cell membrane
• Apo B100 is responsible for the recognition of LDL receptor sites
• A defect in LDL receptors results in the elevation of plasma LDL
• Deficiency of LDL receptors is observed in type IIa hyperbetalipoproteinemia
• This disorder is associated with a very high risk of atherosclerosis
• High density lipoproteins are synthesized in the liver as discoidal particles
• Nascent HDL contains free cholesterol and phospholipids
• Role of LCAT in HDL metabolism

  1. Catalyses the esterification of free cholesterol

  2. Transfers to HDL
• Apoprotein A promotes the activity of LCAT
• Uptake of HDL particles


  1. Enter hepatocytes by receptor-mediated endocytosis

  2. Cholesteryl esters are degraded to cholesterol
• Functions of HDL


  • Transport of cholesterol from peripheral tissue to liver
  • Reservoir of apoproteins
  • Activator of lipoprotein lipase