Principles of Biochemistry (2+1)

6.2.1.2.3 Lipoproteins

A lipoprotein is a conjugated protein that contains lipids in addition to proteins. Triglycerides, phospholipids, cholesterol cholesteryl esters free fatty acids associate with specific proteins to form lipoproteins. In blood plasma, there are four major classes of plasma lipoproteins, which may contain from 50 to 90 percent lipid. These are chylomicrons, very low density (VLDL), low density (LDL), and high density (HDL) lipoproteins. In these conjugated proteins, no covalent linkages exist between the tightly bound lipid molecules and the polypeptides.

Chylomicrons are droplets of nearly pure triglycerides, coated by a very thin layer of proteinChylomicrons carry triglycerides from the small intestine, where they are absorbed during digestion, to the fat depots. Plasma lipoproteins contain both polar lipids and triglycerides, cholesterols and its esters. The nonpolar triglyceride and cholesterol are hidden inside an outer coat of hydrophilic polar heads. This structure gives these lipid-rich structures water solubility, well adapted for the transport of lipids via the blood from the small intestine to liver.

The various types of lipoproteins have different functions. VLDLs are the principal carriers of triglycerides in the blood. Both LDLs and HDLs are involved in the cholesterol transport.

• LDLs carry about 80% of cholesterol while the remaining is carried by HDLs.
• VLDL and LDLs transport cholesterol from liver to tissue for their use where as HDLs carry excess cholesterol away from the cells to the liver for processing and excretion from the body.
• The levels of LDL correlate directly with heart disease, where as HDL levels correlates inversely with heart disease risk.
• Thus HDL is some times referred to as “good” cholesterol and LDL as “bad” cholesterol.

 

Composition of Blood plasma Lipoprotein

Type	Density    g/ml	Protein %	Triglycerides %	Phospholipids  %	Cholesterols     %
Chylomicrons	0.92-0.96	1.7	96	0.8	1.7
Very low Density VLDL	0.95-                1.00	10.0	60	18	15
Low density LDL	1.00-   1.06	25.0	10	22	45
High density HDL	1.06-1.21	50.0	3	30	18