Lipoproteins: Structure, Types, Functions and The Lipid Profile

They are basically a core full of fat and cholesterol, along with a lipid membrane that contains 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, with the polar ends of all the phospholipid molecules facing outward, to 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 and is transported to the place where it must be stored or metabolized, through the bloodstream, despite being insoluble in the blood.

It can be compared, lipoproteins 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 be in contact with the aqueous surface, while the apolar part 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 hydrophobic portion of the monolayer formed by the 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, 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, and attaches to the inner surface of the small capillary blood vessels found within the muscles and fat storage cells distributed in various parts of the body.

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 from the intestines to the muscles, liver and fat cells.

    2. VLDL

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

It is associated with 5 different 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 esterification of cholesterol.

They are the second, after the chylomicrons.

    3. IDL:

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

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

    5. 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 the levels of bad cholesterol 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 make use of its benefits and functions, which is why soluble proteins help to transport them by joining them and helping them to move freely through the bloodstream in a state miscible in blood.

In general, lipoproteins are important and responsible for the transport of triglycerides cholesterol and phospholipids through the bloodstream to the various cells of tissues throughout the body.

The lipid profile

The lipid profile is an important test to evaluate the variable patterns that are 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.

These results, together with other risk factors, 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 5 years.