Structure of lipoprotein and transport

• Lipoproteins


  Lipid-protein complexes that allow the movement of apolar lipids through aqueous environments like blood and lymph

  
• Classification of lipoproteins


  • Chylomicrons
  • Very low density lipoproteins (VLDL)
  • Intermediate density lipoproteins (IDL)
  • Low density lipoproteins (LDL)
  • High density lipoproteins (HDL)

  
• Density of lipoproteins


  • Inversely proportional to triacylglycerol content
  • As the density increases both the size and diameter of the particle decreases

  
• Lipoproteins consist of a lipid core containing non-protein non polar, triacylglycerol and cholesterol ester surrounded by a single layer of phospholipids and free cholesterol and apoprotein

  
• Functions (Importance) of Lipoproteins

  • Chylomicrons synthesized in the intestine and transports dietary lipids from intestine to peripheral tissues
  • VLDL synthesized in the liver and transports endogenous lipids to peripheral tissues
  • LDL synthesized from plasma VLDL and transports cholesterol from the liver to peripheral tissues (BAD CHOLESTEROL)
  • HDL synthesized in the liver and small intestine and scavenging cholesterol from peripheral tissues (GOOD CHOLESTEROL)

  
• Exogenous pathway (Chylomicron clear dietary lipid), Endogenous pathway (VLDL transport and distribute endogenously synthesized lipid)

  
• Exogenous pathway


  1. Dietary lipids incorporated into chylomicrons

  2. Chylomicrons acquire Apo C-II and Apo E from HDL

  3. Chylomicrons acted upon by lipoprotein lipase

  4. Chylomicron remnants taken up by liver

  
• Endogenous pathway


  1. VLDL carries triacylglycerol and cholesterol from liver

  2. VLDL acquires Apo C-II and Apo E from HDL

  3. Lipoprotein lipase hydrolyzes triacylglycerol in VLDL

  4. VLDL converted to IDL then LDL

  
• LDL


  Contains only cholesterol and cholesterol ester, considered bad cholesterol as it cannot be further degraded and will be absorbed by liver and other cells

  
• Reverse cholesterol transport by HDL


  • Movement of cholesterol from peripheral tissues back to the liver via the plasma compartment
  • Cholesterol from non-hepatic peripheral tissues transferred to HDL by ABCA1
  • LCAT converts free cholesterol to cholesterol ester which can be taken up by HDL
  • Cholesteryl esters can be transferred to other lipoproteins like LDL and VLDL

  
• Dyslipidemia/hyperlipidemia


  High level of lipids (cholesterol, triglycerides, or both) or a low high-density lipoprotein (HDL) cholesterol level

  
• Primary hyperlipoproteinemia (Fredrickson's classification)


  • Type I (Deficiency of lipoprotein lipase)
  • Type IIA (Defective LDL receptors)
  • Type IIB (Defective Apo B 100)
  • Type III (Abnormality in Apo E)
  • Type IV (Overproduction of VLDL)
  • Type V (Defective Apo C-II gene)
• Secondary hyperlipoproteinemia


  Lipid disorders arising as a consequence of a disease, drug treatment, or defective nutrition
• Hypocholesterolaemia is an abnormally low concentration of cholesterol in the blood, mainly due to genetic defects or non-inherited forms arising from malignancy, malnutrition, intestinal malabsorption, and liver disease