PHOSPHOLIPIDS: CHARACTERISTICS, STRUCTURE, FUNCTIONS, TYPES - BIOLOGY - 2024

The term phospholipid is used to refer to biomolecules of a lipid nature that have in their structures, specifically in their polar heads, a phosphate group, and that may have a glycerol 3-phosphate or a sphingosine molecule as their main skeleton.

Many authors, however, when they mention phospholipids, usually refer to glycerophospholipids or phosphoglycerides, which are lipids derived from glycerol 3-phosphate to which they are esterified, at the carbons of positions 1 and 2, two chains of fatty acids of varying lengths and degrees of saturation.

Scheme of the structure of a phospholipid (Source: OpenStax via Wikimedia Commons)

Phosphoglycerides represent the most important group of membrane lipids and are distinguished mainly by the identity of the substituent groups attached to the phosphate group at the C3 position of glycerol.

Phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine, and phosphatidylinositol are among the most prominent phospholipids, both for their abundance and for the importance of the biological functions they exert in cells.

characteristics

Like any other lipid, phospholipids are also amphipathic molecules, that is, they have a hydrophilic polar end, often known as the "polar head" and an apolar end called the "apolar tail," which has hydrophobic characteristics.

Depending on the nature of the head groups or polar groups and the aliphatic chains, each phospholipid has different chemical, physical and functional characteristics. Polar substituents can be anionic (with a net negative charge), zwitterionic, or cationic (with a net positive charge).

Phospholipids are distributed "asymmetrically" in cell membranes, since these can be more or less enriched of one type or another, which is also true for each monolayer that makes up the lipid bilayer, since a phospholipid can be preferentially located towards the exterior or interior cell.

The distribution of these complex molecules generally depends on the enzymes responsible for their synthesis, which are modulated, at the same time, by the intrinsic needs of each cell.

Structure

Most phospholipids, as discussed above, are lipids that are assembled on a glycerol 3-phosphate backbone; and that is why they are also known as glycerophospholipids or phosphoglycerides.

Its polar head is made up of the phosphate group attached to the carbon at the C3 position of the glycerol to which the substituent groups or "head groups" are attached by means of a phosphodiester bond. It is these groups that give each phospholipid its identity.

The apolar region is represented in the apolar tails, which are composed of the fatty acid chains attached to the carbons at positions C1 and C2 of the glycerol 3-phosphate molecule by means of ester or ether bonds (ether-phospholipids).

Schematic of a phospholipid in a membrane (Source: Tvanbr via Wikimedia Commons)

Other phospholipids are based on a molecule of dihydroxyacetone phosphate to which the fatty acids also bind through ether bonds.

In many biologically important phospholipids, the fatty acid at the C1 position is a saturated fatty acid of between 16 and 18 carbon atoms, while that at the C2 position is often unsaturated and longer (18 to 20 carbon atoms). carbon).

Normally, in phospholipids, no branched chain fatty acids are found.

The simplest phospholipid is phosphatidic acid, which consists of a glycerol 3-phosphate molecule attached to two chains of fatty acids (1,2-diacyl glycerol 3-phosphate). This is the key intermediate for the formation of the other glycerophospholipids.

Features

Structural

Phospholipids, together with cholesterol and sphingolipids, are the main structural elements for the formation of biological membranes.

Biological membranes make possible the existence of the cells that make up all living organisms, as well as that of the organelles inside these cells (cellular compartmentalization).

Phospholipids are an essential part of the lipid bilayer that makes up biological membranes (Source: Bekerr, via Wikimedia Commons)

The physicochemical properties of phospholipids determine the elastic characteristics, fluidity, and the ability to associate with integral and peripheral proteins of cell membranes.

In this sense, the proteins associated with the membranes interact mainly with the polar groups of the phospholipids and it is these groups, in turn, which give special surface characteristics to the lipid bilayers of which they are part.

Certain phospholipids also contribute to the stabilization of many transporter proteins and others help to increase or enhance their activity.

Cellular communication

In terms of cell communication, there are some phospholipids that fulfill specific functions. For example, phosphoinositols are important sources of second messengers that participate in cell signaling processes in the membranes where they are found.

Phosphatidylserine, an important phospholipid essentially associated with the inner monolayer of the plasma membrane, has been described as a “reporter” or “marker” molecule in apoptotic cells, as it is translocated to the outer monolayer during programmed cell death processes.

Energy and metabolism

Like the rest of the membrane lipids, phospholipids are an important source of caloric energy, as well as precursors for membrane biogenesis.

The aliphatic chains (the fatty acids) that make up their apolar tails are used through complex metabolic pathways by which large amounts of energy are extracted in the form of ATP, energy that is necessary to carry out most of the cellular processes vital.

Other functions

Certain phospholipids fulfill other functions as part of special materials in some tissues. Dipalmitoyl-phosphatidylcholine, for example, is one of the main components of lung surfactant, which is a complex mixture of proteins and lipids whose function is to decrease the surface tension in the lungs during expiration.

Types

The fatty acids attached to the glycerol 3-phosphate backbone can be very varied, therefore, the same type of phospholipid can consist of a large number of molecular species, some of which are specific for certain organisms, for certain tissues and even for certain cells within the same organism.

-Glycerophospholipids

Glycerophospholipids or phosphoglycerides are the most abundant class of lipids in nature. So much so that they are the model commonly used to describe all phospholipids. They are mainly found as structural elements of cell membranes, but they can also be distributed in other parts of the cell, although in much lower concentration.

As has been commented throughout this text, its structure is formed by a molecule of 1,2-diacyl glycerol 3-phosphate to which another molecule with polar characteristics is attached, through a phosphodiester bond, that gives a specific identity to each glycerolipid group.

These molecules are generally alcohols such as ethanolamine, choline, serine, glycerol, or inositol, forming phosphatidylethanolamines, phosphatidylcholines, phosphatidylserines, phosphatidylglycerols, and phosphatidylinositols.

In addition, there may be differences between phospholipids belonging to the same group related to the length and degree of saturation of the aliphatic chains that make up their apolar tails.

Classification

According to the characteristics of the polar groups, glycerophospholipids are classified as:

- Negatively charged glycerophospholipids, such as phosphatidylinositol 4,5-bisphosphate.

- Neutral glycerophospholipids, such as phosphatidylserine.

- Positively charged glycerophospholipids, such as phosphatidylcholine and phosphatidylethanolamine.

-Ether-phospholipids and plasmallogens

Although their function is not known for sure, it is known that this type of lipid is found in the cell membranes of some animal tissues and in those of some unicellular organisms.

Its structure differs from the more common phospholipids by the type of bond through which the fatty acid chains are attached to glycerol, since it is an ether and not an ester bond. These fatty acids can be saturated or unsaturated.

In the case of plasmallogens, the fatty acid chains are attached to a dihydroxyacetone phosphate backbone by means of a double bond at the C1 or C2 carbons.

Plasmalogens are especially abundant in the cells of the heart tissue of most vertebrates; and many invertebrates, halophytic bacteria and some ciliated protists have membranes enriched with this type of phospholipids.

Among the few known functions of these lipids is the example of the platelet activating factor in vertebrates, which is an alkyl phospholipid.

-Sphingomyelins

Although they could be classified together with sphingolipids, since in their main skeleton they contain a sphingosine molecule instead of a glycerol 3-phosphate molecule, these lipids represent the second most abundant class of membrane phospholipids.

A fatty acid chain is attached to the amino group of sphingosine through an amide bond, thereby forming a ceramide. The primary hydroxyl group of sphingosine is esterified with a phosphorylcholine, giving rise to sphingomyelin.

These phospholipids, as their name indicates, enrich the myelin sheaths that surround nerve cells, which play a major role in the transmission of electrical nerve impulses.

Where are they found?

As their functions indicate, phospholipids are mostly found as a structural part of the lipid bilayers that make up the biological membranes that enclose both cells and their internal organelles in all living organisms.

These lipids are common in all eukaryotic organisms and even in many prokaryotes, where they perform analogous functions.

Example of major phospholipids

As has been repeatedly commented, glycerophospholipids are the most important and abundant phospholipids in the cells of any living organism. Of these, phosphatidylcholine represents more than 50% of the phospholipids in eukaryotic membranes. It has an almost cylindrical shape, so it can be organized into flat lipid bilayers.

Phosphatidylethanolamine, on the other hand, is also extremely abundant, but its structure is "conical", so it does not self-assemble as bilayers and is normally associated with places where there are curvatures in the membrane.

References

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