LIPOPROTEINS: STRUCTURE, COMPOSITION, FUNCTIONS, TYPES, EXAMPLES - BIOLOGY - 2025

The lipoproteins are complex particles that function in the transport and absorption of lipids through the blood from and to different tissues. They are made up mainly of nonpolar lipids such as cholesterol and triglycerides, in addition to some phospholipids and proteins.

These are molecular aggregates that are synthesized mainly in the intestines and the liver, which, when entering circulation, are in a state of constant flux, which implies that they change their composition and physical structure regularly as they are "captured. "And metabolized by peripheral body tissues that are" targeted. "

Structure of a lipoprotein (Source: AntiSense via Wikimedia Commons)

Lipid components that are not absorbed through lipoproteins in a given tissue usually return as "carry-overs" to the liver, where they are further processed.

Most lipoproteins are studied due to their relationship with human health, according to which four types with clinical relevance have been defined, each of which with different physiological functions: chylomicrons, very low lipoproteins. density, low-density lipoproteins, and high-density lipoproteins.

In this sense, plasma lipoproteins are closely related to pathological processes of great importance for humans, such as arteriosclerosis and coronary artery disease.

Structure

Plasma lipoproteins are particles with an almost spherical structural morphology, since they are, in reality, complex micelles of lipids and proteins, in which the hydrophobic or apolar regions of the lipids face each other in the center, while the hydrophilic or poles are exposed towards the surface, in contact with the aqueous medium.

The hydrophilic “shell” or “coat” of these particles consists mainly of unesterified cholesterol molecules, phospholipids with their polar “heads” “facing” outward, and proteins called apolipoproteins; while the central portion or the "core" consists of cholesterol esters and triglycerides.

Composition of lipoproteins

As explained, lipoproteins are particles that basically consist of a mixture of lipids and proteins that perform transport functions.

- Lipid portion

Depending on the type of lipoprotein considered, the lipid composition can vary, especially in relation to the amounts of phospholipids and free or esterified cholesterol molecules.

In addition to the composition, the mass or the proportion of lipids in lipoproteins is also highly variable. In chylomicrons, for example, lipids represent more than 98% of the lipoprotein mass, while for high-density lipoproteins these can be less than 50%.

Lipoproteins are generally associated with the transport of triglycerides for storage (adipose tissue) or their use in metabolism (cells or muscle fibers).

These triglycerides can be of exogenous origin (absorbed in the intestines from food) or of endogenous origin (synthesized and secreted by the liver and intestinal cells).

Chylomicrons and very low-density lipoproteins are the two types of lipoproteins that have the highest abundance of triglycerides and cholesterol, so their density is considerably lower than that of other lipoproteins.

In contrast, the lipid component of low-density and high-density lipoproteins consists mainly of cholesterol and phospholipids. Among the most abundant phospholipids found in lipoproteins are sphingomyelins and phosphatidylcholines, whose molar ratios vary from lipoprotein to lipoprotein.

It is important to note that many physical properties of lipoproteins are related to their lipid content and composition, including flotation properties, surface charge, and migration tendency in electric fields.

- Protein portion

The proteins that are associated with all plasma lipoproteins are known as apolipoproteins or apoproteins and, as with lipids, the amount of these molecules present in the different classes of lipoproteins is highly variable.

Chylomicrons, of the least dense lipoproteins, have around 1% protein and the maximum content has been reported in some high-density lipoproteins (of the smallest lipoproteins) with values ​​close to 50%.

More or less 10 different types of apoproteins have been isolated and described in humans, which are named, according to the "ABC" nomenclature, as: Apo AI, Apo A-II, Apo A-IV, Apo B- 100, Apo B-48, Apo CI, Apo C-II, Apo C-III, Apo D and Apo E.

The main protein of high-density lipoproteins is known as apolipoprotein A (Apo AI and Apo A-II), that of low-density lipoproteins is apoprotein B (which is also found in chylomicrons and very low-density particles) and the apolipoprotein of chylomicrons is Apo B-48, smaller than Apo B of low-density lipoproteins.

The apoproteins CI, C-II and C-III are low molecular weight proteins that are found in plasma as part of high and very low density particles.

Some of the proteins associated with lipoproteins are glycoproteins, such is the case of Apo E, which has been isolated from very low and high density lipoproteins.

Principal functions

In general, apolipoproteins are in charge of functions such as:

- Be part of the main structure of lipoproteins.

- To function as enzymatic cofactors for some proteins with enzymatic activity that participate in their metabolism.

- Specific ligands for lipoprotein receptors on the surface of "target" or "target" tissue cells for the transport of triglycerides and cholesterol.

Features

Lipoproteins actively participate in the transport and intestinal absorption of fatty acids that are obtained from food, in addition, these particles also contribute in the transport of lipids from the liver to peripheral tissues and in reverse transport, that is,, from the peripheral tissues to the liver and intestine.

Lipoprotein metabolism (Source: Npatchett via Wikimedia Commons)

These molecular aggregates, then, make the hydrophobic lipid substances "compatible" with the aqueous medium that constitutes most of the animal body fluids, allowing their "transport and delivery" to the tissues where they are needed.

A secondary function that has been attributed to lipoproteins consists, additionally, in the transport of foreign toxic compounds with hydrophobic and / or amphipathic characteristics (one extreme hydrophobic and the other hydrophilic), as is the case of some bacterial toxins, etc.

They can also transport fat-soluble vitamins and antioxidant molecules throughout the body.

Types (classification)

Lipoproteins are classified according to their density, a characteristic directly related to the relationship between the proportion of lipids and proteins that compose them, and which is very useful when they are separated by ultracentrifugation processes.

Thus, these particles have been classified into four different groups, each of which fulfills a specific function and has different properties. These groups are: chylomicrons, very low-density lipoproteins, low-density lipoproteins, and high-density lipoproteins.

Chylomicrons

Chylomicrons (CMs) are formed from fatty acids and lipids that enter the body with food, which, once they are absorbed by the cells of the intestinal epithelium, mix and recombine with each other and with some proteins.

Structure of a chylomicron (Source: Posible2006 via Wikimedia Commons)

The formation of chylomicrons precedes their release or secretion into the lymphatic system and later into the circulation.

Once they reach certain extrahepatic tissues, these particles are initially metabolized by an enzyme known as lipoprotein lipase, capable of hydrolyzing triglycerides and releasing fatty acids that can be incorporated into the tissues or oxidized as fuel.

Very low-density lipoproteins

Very low density lipoproteins or VLDL (Very Low Density Lipoprotein), also known as “pre-β lipoproteins” are produced in the liver and fulfill the function of exporting triglycerides, which represent one of its main components.

These are one of the lipoproteins found in the plasma of fasting animals and their concentration increases with advancing age.

Low-density lipoproteins

These lipoproteins, known as LDL (Low Density Lipoprotein) or as β-lipoproteins, represent the final steps in the catabolism of very low-density lipoproteins and are rich in cholesterol molecules.

Low-density lipoproteins are the most abundant, representing about 50% of the total mass of plasma lipoproteins and responsible for the transport of more than 70% of cholesterol in the blood. Like very low-density lipoproteins, the plasma concentration of these lipoproteins increases as the body ages.

High-density lipoproteins

High-density lipoproteins (HDL) or α-lipoproteins, are lipoproteins involved in the metabolism of very low-density lipoproteins and chylomicrons, but they also participate in cholesterol transport. These particles are rich in phospholipids.

Some authors also suggest that there are other types of lipoproteins such as the so-called Intermediate Density Lipoprotein (IDL) and various subdivisions of high-density lipoproteins (HDL1, HDL2, HDL3 and so on).

Examples of lipoproteins

High-density lipoproteins are associated with some important diseases in humans. These particles participate in the transfer of excess cholesterol from peripheral tissues to the liver, and such cholesterol molecules are known as “good cholesterol”.

Cholesterol metabolism or "cycle" (Source: Hisashi Shinkai via Wikimedia Commons)

However, in recent decades, cholesterol associated with high-density lipoproteins has been associated with greater risks of suffering from "events" or cardiovascular diseases, which is why it has been one of the most studied risk factors for such pathological conditions.

Another good example of lipoproteins is that of chylomicrons, which are the particles that are formed from fats consumed with food and are transported in the first instance by the circulatory stream, once they are formed by the epithelial cells of the mucosa intestinal.

References

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