Lipoproteins: Structure, Types, Functions and The Lipid Profile

They are a core full of fat and cholesterol and a lipid membrane containing proteins called apolipoproteins.

Lipoproteins are compounds with links of a biochemical nature between simple soluble proteins and non-soluble lipids, bound to facilitate the displacement of non-soluble fats through the blood and the lymphatic system to the various cells throughout the body.

According to their composition and density, they are classified into five types. In a lipoprotein, the polar ends of all the phospholipid molecules facing outward interact with the water.

This allows lipoprotein to be transported in the blood instead of going up to the top, like cream in milk.

The non-polar fat is accumulated within the phospholipid layer, in the center of the lipoprotein. It is transported to where it must be stored or metabolized, through the bloodstream, despite being insoluble in the blood.

Lipoproteins can be compared with cargo trucks at the molecular level that transport fats wherever they are stored or required.

Structure

Lipoproteins are macromolecular structures, the nucleus and a shell can be differentiated into a lipoprotein.

 

The envelope is formed mainly by amphipathic lipids of the phospholipid type, such as cephalins, lecithins, and others.

These phospholipids have their polar part oriented outwards to contact the aqueous surface. In contrast, the apolar position interacts with the hydrophobic nucleus of the lipoprotein, formed mainly by triacylglycerols and cholesterol esters.

It also appears as part of the envelope of lipoproteins, the free cholesterol with the carbon hydroxyl oriented to the polar environment and the apolar rest of the molecule embedded in the monolayer’s hydrophobic portion formed by the monolayer phospholipids.

The hydrophilic amino acids of these apoproteins are oriented towards the outside, while the hydrophobic amino acids interact with the apolar part of the lipoprotein structure.

Types

According to the density, there are five types of lipoproteins. The three main ones are LDL, HDL, and VLDL.

The lipid profile or lipid panel measures the values ​​of each of these lipoproteins in the blood plasma by a blood test.

The most common form in which lipoproteins are classified is according to the density of the apolipoproteins and the weight ratio per unit volume.

  1. Chylomicrons:

These are the lipoproteins of greater size and low density and have the highest content of triglycerides.

It is a component of protein synthesized in the liver, which involves cholesterol and fats derived from food.

It travels from the intestinal lymphatic vessels to the large veins. It attaches to the inner surface of the small capillary blood vessels found within the muscles and fat storage cells distributed in various body parts.

There the fat is digested, while the cholesterol remains, it travels to the liver, where the cholesterol is metabolized.

Thus, chylomicrons release cholesterol and fats to the intestines’ muscles, liver, and fat cells.

  1. VLDL

It is a lipoprotein composed of proteins, fats, and cholesterol synthesized in the liver and very low density.

It is associated with five apoproteins: B – 100, CI, C – II, C – III, and E.

It is converted into IDL and LDL by eliminating apoproteins, except for one called apoprotein B100, together with the esterification of cholesterol.

They are the second, after the chylomicrons.

  1. IDL:

The IDL is a lipoprotein of intermediate density created by VLDL metabolism.

  1. LDL

This low-density protein is the last remnant of VLDL and contains mainly cholesterol. All forms transport fats and cholesterol produced in the liver to tissues, the only apoprotein associated with it is apoB-100.

It is known as bad cholesterol. It is the one that we should avoid in our diet.

  1. HDL

It is the densest and has the highest protein-lipid ratio. It has apoprotein A-1. Also called “good cholesterol” or high density, because it reduces harmful cholesterol levels in the blood, it carries cholesterol from the tissues to the liver.

High HDL levels are associated with a lower risk of cardiovascular disease.

The size of the lipoprotein particles varies from 10 to 1000 nanometers.

Generally, the higher the density of lipoproteins, the smaller the lipoprotein particle size.

Functions

Both triglycerides and cholesterol are not soluble in blood plasma.

But they must move in the bloodstream throughout the body so that the body can use its benefits and functions, which is why soluble proteins help transport them by joining them and helping them to move freely through the bloodstream in a state miscible in blood.

In general, lipoproteins are essential and responsible for transporting triglycerides, cholesterol, and phospholipids through the bloodstream to the various tissue cells.

The lipid profile

The lipid profile is an important test to evaluate the variable patterns related to the risks in our medical history and indicate the need for treatment.

This test allows us to measure the total cholesterol in plasma, the levels of LDL, VLDL, and HDL, as well as the level of triglycerides.

Together with other risk factors, these results will originate from atherosclerosis to a fatal cardiovascular event.

Children should have a lipid profile between 9-11 years and repeat it between 17 and 21 years.

Adults without other risk factors should have a blood lipid profile at least once every five years.