Lipoproteins: Definition, Structure, Functions, Classification & Composition

Lipoproteins are simple proteins and fats bonded together to facilitate transport of the non-soluble fats through blood. They are classified into five types according to their composition and density.

The lipoprotein structure consists of a core of lipids surrounded by a covering of proteins.

The functions of the lipoproteins can only be described as extremely crucial.

Too much of lipoproteins in the blood is to be avoided because then, they can cause complications that can be described as extremely dangerous.

Definition

A lipoprotein is a bond of biochemical nature between simple soluble proteins and non-soluble fats (cholesterol and triglycerides) whose main purpose is to transport the lipids through the blood and the lymphatic system to the various cells throughout the body.

Lipoproteins or plasma lipoproteins as they are also called, have a core made of lipid and covered by soluble proteins (apolipoprotein).

Structure of lipoproteins

A lipoprotein consists of a non-polar core of hydrophobic (water non-soluble) lipids surrounded by a covering of relatively more polar proteins.

The core of the lipoprotein particle consists of triacylglycerol and/or cholesteryl esters, fatty acids and fat-soluble vitamins like Vitamin E.

The surface single layer is made up of phospholipids, unesterified cholesterol and specific type of water soluble apolipoproteins.

Each type of lipoprotein has a specific type of Apolipoprotein. It is the type of apolipoprotein that determines the function of the lipoprotein.

  • LDL particles contain apolipoprotein B-100.
  • The HDL particle contains apolipoprotein A1.
  • Chylomicrons and VLDL contain Apo C-II.
  • Apo E is important in Chylomicrons, VLDL and IDL in facilitating the binding of these lipoproteins to the hepatocytes.

Functions and importance

Both cholesterol and triglycerides are not soluble in the blood plasma. But, they have to move in the blood stream throughout the body so that the body can avail of their benefits and functions.

Soluble proteins help transport them by binding with them and helping them move freely through the bloodstream in a blood miscible state.

This combination of these fats and lipids with the proteins is called lipoproteins and apolipoproteins are the lipid-free protein portion of a lipoprotein.

High levels of lipoproteins, especially LDL and VLDL, the bad cholesterols are associated with atherosclerosis. HDL, the good cholesterol is associated with lowering the elevated LDL levels and reversing the atherosclerotic changes. Healthy HDL levels, therefore, protect you from the dangers of high cholesterol and your heart from coronary artery disease.

Hence, high levels of lipoproteins, LDL and VLDL are not desirable, while HDL levels on the higher side are preferred.

Lipoproteins are important and responsible for transporting cholesterol, triglycerides and the phospholipids via the bloodstream to the various tissue cells throughout the body.

Classification, density, and composition of lipoproteins

There are five types of lipoproteins. Among these, the three major ones are LDL, HDL, and VLDL. The values of each of these lipoproteins in the blood plasma are measured by a blood test known as the lipid profile or lipid panel.

The most common way lipoproteins are classified is according to the density of the apolipoproteins or less commonly by weight per unit volume.

The four lipoprotein fractions, in order of increasing density, are

(1) Chylomicrons

(2) Very-low-density lipoprotein (VLDL)

(3) Low-density lipoprotein (LDL), and

(4) High-density lipoprotein (HDL)

LDL and HDL contain the least amount of triglycerides and relatively more amounts of cholesterol and protein.

Lipoprotein particles vary in size from 10 to 1000 nanometers. Even the density of the lipoproteins differs. Generally, more the density of the lipoproteins smaller is the size of the lipoprotein particle. Again, as a general rule, lipoproteins with a higher fat to protein ratio are larger and less dense.

The sizes and the type of lipoprotein are distinguished by the processes of lipoprotein electrophoresis and ultracentrifugation. This test determines the abnormal distribution and concentration of lipoproteins in the serum, which is a potential risk factor in the development of coronary artery disease (CAD)

Chylomicrons

Chylomicrons are the largest and the least dense of all the lipoproteins. Their lesser known name is Ultra Low-Density Lipoproteins (ULDL).

They originate in the intestinal mucosa and their function is to transport dietary triglycerides and cholesterol absorbed by the small intestine epithelial cells from the foods that we eat to the liver and other tissues.

They are high in triglycerides content. Chylomicrons contain about 1-2% protein, 85-88% triglycerides, 6~8% phospholipids, and 1-3% cholesterol. Due to the high triglyceride content, their density is less than 0.95.

Very Low-Density Lipoproteins (VLDL)

VLDL is synthesized in the liver from triglycerides, cholesterol, and apolipoproteins. It is referred to as the bad fat along with LDL cholesterol.

According to Medscape, triglycerides form the major component and VLDL serves as the means of transporting triglycerides through the blood.

The blood takes them to the tissues throughout the body for storage in the adipose tissue so that the triglycerides can be used by the body for energy when required.

There is no direct method to measure the VLDL levels in the blood. Generally, its value is taken as 20% of the triglyceride levels.

High levels of VLDL along with high LDL numbers are a high-risk factor for coronary heart disease and stroke. Although LDL is lipoprotein most commonly linked to atherosclerosis, VLDL may also be atherogenic.

VLDL is a precursor of LDL. According to the National Institute of Health, 90% of it is ultimately converted to LDL cholesterol in the liver.

These lipoproteins are composed of 5-12% protein, 50-55% triglycerides, 18-20% phospholipids, 12-15% cholesteryl esters and 8-10% cholesterol.

VLDL particles are approximately 25-90 nanometers in size and have a density of ~0.98.

Low-Density Lipoproteins (LDL)

Low-density lipoproteins contain the highest percentage of cholesterol. It is a combination of 10-15% triglycerides, 45% cholesterol, 22% phospholipids and 25% proteins. They measure approximately 26 nanometers in size and have a density of ~1.04.

Among other functions, the main functions of LDL are the formation of the cell wall, Vitamin D synthesis and helping in the production of certain hormones.

High LDL cholesterol levels can lead to the formation of plaques on the inner wall surfaces of the arteries. This significantly increases the chances of the individual to cardiovascular events such as heart attack and stroke. That is why LDL is referred to as the “bad cholesterol.”

High-Density Lipoproteins (HDL)

HDL Cholesterol has the smallest particle size amongst the lipoprotein particles measuring  6-12.5 nanometers in size and a density of ~1.12. They carry about 25 to 33% of the all the cholesterol in the blood.

Their composition consists of approximately 55% protein, 3-15% triglycerides, 26-46% phospholipids, 15-30% cholesteryl esters, and 2-10% cholesterol

Among other functions, its main benefits accrue from the fact that HDL carries the excess LDL particles from the cells back to the liver to be processed and got rid of through the bile into the intestines.  This process is known as reverse cholesterol transport.

It also clears the atherosclerotic plaques and carries them to the liver to be got rid of. That is why though high levels of LDL are not desirable, the reverse applies to HDL cholesterol.

The desirable levels of HDL stand at 40 mg/dL. But, to have levels at 60 mg/dl is very good and protects you from cardiovascular complications.

Intermediate Density Lipoproteins (IDL)

Intermediate Density Lipoprotein (IDL) is another lipoprotein that enables fats and cholesterol to move freely through the water-based bloodstream.

IDLs are smaller than VLDL in size approximately measuring 40 nanometers and have a density of ~1.0.

They are composed of 10-12% protein, 24-30% triglycerides, 25-27% phospholipids, 32-35% cholesteryl esters and 8-10% cholesterol.

Their size is 25 to 35 nm in diameter, and they are primarily made of a range of triacylglycerols and cholesterol esters.

IDL transports endogenous triglyceride fats and cholesterol through the blood from the liver to the tissues and its high levels can promote the formation of atherosclerosis.

IDL is formed by the degeneration of VLDL and goes through two fates: it is either taken up by the liver cells or IDL goes on losing its triglyceride content and is converted to LDL in the liver.

Intermediate density lipoproteins are not an intermediate between the LDL and HDL, but in fact, the “intermediate-density” refers to the density between that of low-density (LDL) and very-low-density lipoproteins (VLDL).

According to the National Institute of Health, moderately increased levels of IDL cholesterol are closely associated with a high frequency of coronary artery disease (CAD) as is also the case with increased levels of the cholesterol-rich LDL and the triglyceride-rich VLDL.

 

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