PHOSPHOGLYCERIDES: STRUCTURE, FUNCTIONS AND EXAMPLES - BIOLOGY - 2025

The phosphoglycerides or glycerophospholipids are abundant molecules of lipid nature in biological membranes. A phospholipid molecule is made up of four basic components: fatty acids, a skeleton attached to the fatty acid, a phosphate, and an alcohol attached to the latter.

Generally, on carbon 1 of glycerol there is a saturated fatty acid (only single bonds), while on carbon 2 the fatty acid is of the unsaturated type (double or triple bonds between the carbons).

Structure

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Among the most prominent phosphoglycerides in cell membranes we have: sphingomyelin, phosphatidylinositol, phosphatidylserine and phosphatidylethanolamine.

Foods that are rich in these biological molecules are white meats such as fish, egg yolks, some organ meats, shellfish, and nuts, among others.

Structure

Components of a phosphoglyceride

A phosphoglyceride is made up of four basic structural elements. The former is a skeleton to which fatty acids, a phosphate and an alcohol are attached - the latter is attached to phosphate.

The phosphoglyceride backbone can be formed from glycerol or sphingosine. The former is a three-carbon alcohol, while the latter is another alcohol with a more complex structure.

In glycerol, the hydroxyl groups located at carbon 1 and 2 are esterified by two large chain carboxyl groups of fatty acids. The missing carbon, located at position 3, is esterified with a phosphoric acid.

Although glycerol does not have an asymmetric carbon, the alpha carbons are not stereochemically identical. Thus, the esterification of a phosphate at the corresponding carbon confers asymmetry to the molecule.

Characteristics of fatty acids in phosphoglycerides

Fatty acids are molecules composed of hydrocarbon chains of varying length and degree of unsaturation, and terminate in a carboxyl group. These characteristics vary considerably, and determine their properties.

The fatty acid chain is linear if it is of the saturated type or if it is unsaturated in the trans position. In contrast, the presence of a cis-type double bond creates a twist in the chain, so it no longer needs to be represented linearly, as is often the case.

Fatty acids with double or triple bonds significantly influence the state and the physicochemical characteristics of biological membranes.

Hydrophobic and hydrophilic properties

Each of the mentioned elements differs in their hydrophobic properties. Fatty acids, being lipids, are hydrophobic or apolar, which means that they do not mix with water.

In contrast, the rest of the elements of phospholipids allow them to interact in an environment, thanks to their polar or hydrophilic properties.

In this way, phosphoglycerides are classified as amphipathic molecules, which means that one end is polar and the other is apolar.

We can use the analogy of a match or a match. The head of the match represents the polar head made up of the charged phosphate and the substitutions of it on the phosphate group. The prolongation of the match is represented by a nonpolar tail, formed by hydrocarbon chains.

Groups of a polar nature are charged at pH 7, with a negative charge. This is due to the ionization phenomenon of the phosphate group, which has a pk close to 2, and the charges of the esterified groups. The number of charges depends on the type of phosphoglyceride studied.

Features

Structure of biological membranes

Lipids are hydrophobic biomolecules that present solubility in organic solvents - such as chloroform, for example.

These molecules have a wide variety of functions: play a role as fuels by storing concentrated energy; as signaling molecules; and as structural components of biological membranes.

In nature, the most abundant group of lipids that exist are phosphoglycerides. Their main function is structural, since they are part of all cell membranes.

Biological membranes are grouped in the form of a bilayer. Which means that lipids are grouped into two layers, where their hydrophobic tails face the interior of the bilayer and the polar heads face the exterior and interior of the cell.

These structures are crucial. They delimit the cell and are responsible for the exchange of substances with other cells and with the extracellular environment. However, the membranes contain other lipid molecules other than phosphoglycerides, and also molecules of a protein nature that mediate the active and passive transport of substances.

Secondary functions

In addition to being part of biological membranes, phosphoglycerides are associated with other functions within the cellular environment. Some very specific lipids are part of the myelin membranes, the substance that covers the nerves.

Some can act as messages in the capture and transmission of signals to the cellular environment.

Metabolism

Synthesis

The synthesis of phosphoglycerides is carried out starting from intermediate metabolites such as the phosphatidic acid molecule and also triacylglycerols.

An activated nucleotide CTP (cytidine triphosphate) forms an intermediate called CDP-diacylglycerol, where the pyrophosphate reaction favors the reaction on the right.

The portion called phosphatidyl reacts with certain alcohols. The product of this reaction is phosphoglycerides, including phosphatidylserine or phosphatidyl inositol. Phosphatidyl ethanolamine or phosphatidylcholine can be obtained from phosphatidylserine.

However, there are alternative routes to synthesize the last mentioned phosphoglycerides. This pathway involves the activation of choline or ethanolamine by binding to CTP.

Subsequently, a reaction occurs that unites them with phosphatidate, obtaining phosphatidyl ethanolamine or phosphatidylcholine as the final product.

Degradation

The breakdown of phosphoglycerides is handled by enzymes called phospholipases. The reaction involves the release of the fatty acids that make up phosphoglycerides. In all tissues of living organisms, this reaction takes place constantly.

There are several types of phospholipases and they are classified according to the fatty acid they are released. Following this classification system, we distinguish between lipases A1, A2, C and D.

Phospholipases are ubiquitous in nature, and we find them in different biological entities. Intestinal juice, secretions of certain bacteria, and snake venom are examples of substances high in phospholipases.

The end product of these degradation reactions is glycerol-3-phosphate. Thus, these released products plus free fatty acids can be reused to synthesize new phospholipids or be directed to other metabolic pathways.

Examples

Phosphatidate

The compound described above is the simplest phosphoglyceride and is called phosphatidate, or also diacylglycerol 3-phosphate. Although it is not very abundant in the physiological environment, it is a key element for the synthesis of more complex molecules.

Phosphoglycerides derived from phosphatidate

From the simplest phosphoglyceride molecule, the biosynthesis of more complex elements can occur, with very important biological roles.

The phosphate group of phosphatidate is esterified with the hydroxyl group of alcohols - it can be one or more. The most common alcohols of the folfoglycerides are serine, ethanolamine, choline, glycerol, and inositol. These derivatives will be described below:

Phosphatidylethanolamine

In cell membranes that are part of human tissues, phosphatidylethanolamine is a prominent component of these structures.

It is composed of an alcohol esterified by fatty acids in the hydroxyls located in positions 1 and 2, while in position 3 we find a phosphate group, esterified with the amino alcohol ethanolamine.

Phosphatidylserine

Generally, this phosphoglyceride is found in the monolayer that faces the interior - that is, the cytosolic side - of the cells. During the process of programmed cell death, the distribution of phosphatidylserine changes and is found throughout the cell surface.

Phosphatidylinositol

Phosphatidylinositol is a phospholipid that is found in low proportions both in the cell membrane and in the membranes of the subcellular components. It has been found to participate in cellular communication events, causing changes in the internal environment of the cell.

Sphingomyelin

In the group of phospholipids, sphingomyelin is the only phospholipid present in membranes whose structure is not derived from the alcohol glycerol. Instead, the skeleton is made up of sphingosine.

Structurally, this last compound belongs to the group of aminoalcohols and has a long carbon chain with double bonds.

In this molecule, the amino group on the backbone is attached to a fatty acid through an amide-type bond. Together, the primary hydroxyl group of the skeleton is esterified to phosphotylcholine.

Plasmalogens

Plasmallogens are phosphoglycerides with heads formed mainly from ethanolamine, choline and serine. The functions of these molecules have not been fully elucidated and there is little information about them in the literature.

Because the vinyl ether group is easily oxidized, plasmallogens are able to react with oxygen free radicals. These substances are products of average cellular metabolism and have been found to damage cellular components. In addition, they have also been related to aging processes.

Therefore, a possible function of plasmalogens is to trap free radicals that can potentially have negative effects on cell integrity.

References

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