SAPONIFIABLE LIPIDS: CHARACTERISTICS, STRUCTURE, FUNCTIONS, EXAMPLES - BIOLOGY - 2024

The saponifiable lipids are those lipids having a functional ester group can be hydrolyzed under alkaline conditions. Saponifiable lipids are waxes, phospholipids, triglycerides and sphingolipids.

The chemical process of alkaline hydrolysis of esters in the presence of a base in aqueous solution (NaOH or KOH) is known as saponification. This reaction consists of breaking the carbon-oxygen bond that "holds" the acidic portion and the alcoholic portion of the ester.

Image by jacqueline macou from www.pixabay.com

Saponification is the process through which carboxylated salts are obtained, which are the raw material for the manufacture of soaps that we use daily for personal or home hygiene.

The saponification of lipids results in the release of glycerol molecules and the salts of their fatty acids.

Saponification or hydrolysis of a lipid (Source: SVG version: WhiteTimberwolfPNG version: Bryan Derksen, H Padleckas / Public domain, via Wikimedia Commons)

In view of the fact that the lipids that make up the tissues of both animals and plants are, for the most part, saponifiable lipids, throughout history man has used various natural sources to obtain soapy substances with different domestic and industrial utilities.

Traditionally, bovine tallow (beef fat) and bleach (ash, impure KOH source) were used, however, nowadays animal and vegetable fats of different types are used and the alkali is usually sodium carbonate.

Characteristics and structure of saponifiable lipids

The saponifiable lipids, as already mentioned, are waxes, phospholipids, triglycerides and sphingolipids. Like all lipids known in nature, these are amphipathic molecules, that is, they are molecules with a polar (hydrophilic) end and an apolar (hydrophobic) end.

Structurally speaking, the apolar portion of saponifiable lipids consists of one or two fatty acid chains of different lengths and varying degrees of saturation, which may or may not be branched.

Structural representation of a fatty acid (Image from WikimediaImages at www.pixabay.com)

A fatty acid also has amphipathic characteristics, as it is a carboxylic acid made up of an apolar aliphatic (hydrocarbon) chain. These compounds are not free in the biological context, but are always chemically associated with other molecules.

Thus, the fundamental characteristic of all saponifiable lipids is that they are molecules composed of fatty acids esterified to different types of "skeletons" or "frameworks".

Phospholipids

Phospholipids are esterified to a glycerol molecule, which also has a phosphate group attached to one of its carbon atoms capable of interacting with different groups to form, through a phosphodiester bond, the different types of phospholipids that are known as phosphatidylcholine and phosphatidylethanolamine, for example.

General structure of a phospholipid (Source: Rupertsciamenna. Spanish version by Alejandro Porto. / CC BY-SA (https://creativecommons.org/licenses/by-sa/3.0) via Wikimedia Commons)

Triglycerides

Triglycerides, similar to phospholipids, are lipid molecules assembled on a glycerol skeleton, but they differ from the previous ones in that instead of a phosphate group they are esterified to a third fatty acid.

Formation of a triacylglyceride (Source: Iacopo Leardini. Spanish version by Alejandro Porto. / CC0, via Wikimedia Commons)

Sphingolipids

Sphingolipids are made up of a molecule of sphingosine (an amino alcohol of 18 carbon atoms) that is linked to a fatty acid through an amide bond.

Phosphoesphingolipids or sphingomyelins

There are phosphoesphingolipids or sphingomyelins, which are those that have a phosphate group attached to one of the OH groups of sphingosine and to which choline or ethanolamine molecules can be esterified, constituting the polar "head" of the molecule.

Structure of a sphingolipid (Source: Javier Velasco / CC BY-SA (https://creativecommons.org/licenses/by-sa/4.0) via Wikimedia Commons)

Glucosphingolipids

There are also the glycosphingolipids, which instead of a phosphate group have a carbohydrate (monosaccharide or oligosaccharide) linked through a glycosidic bond to one of the OH groups of sphingosine.

Waxes

Finally, waxes are also very long chain fatty acid esters whose "backbone" is a high molecular weight alcohol (with chains of up to 30 carbon atoms).

Features

Biologically speaking, saponifiable lipids are of paramount importance for the functioning of all living beings, since most of them, especially phospholipids and sphingolipids, fulfill structural, metabolic and even intracellular signaling functions.

The cell membranes of eukaryotic and prokaryotic organisms are composed of lipid bilayers.

These bilayers are mainly made up of phospholipids, which are organized in such a way that their apolar ends are "protected" from the aqueous medium inside them, while their polar "heads" are in permanent interaction with the surrounding environment.

From the above it is understood the importance of these molecules for the existence of cells as we know them today.

Sphingolipids also enrich the membranes of many types of cells and, in addition to this structural function, they are widely studied for their participation in cell signaling phenomena, as they have been implicated in processes such as apoptosis, mitosis and cell proliferation, among others.

These molecules are particularly important for the cells of the nervous system of many animals, since they comprise, for example, more than 5% of the gray matter of the human brain.

Economic and industrial importance

Saponifiable lipids have been industrially exploited by man for dozens of years for the production of soaps through saponification.

The use of animal fats and more recently vegetable fats, such as palm oil and coconut oil, for example, has been of great relevance for the development of soaps with different properties and characteristics.

The ability to remove grease and the "cleaning power" of detergents or soaps that are currently used for personal, domestic and industrial hygiene is related to the structure of the ions present in the salts of fatty acids that are produced. by the saponification of lipids.

This is due to the ability of these ions to participate in the formation of micelles, which are spherical structures formed by these amphipathic molecules, in which the fatty acids face each other in the center and the ions face the hydrophilic surface.

Examples of saponifiable lipids

Due to their abundance, the most recognized examples of saponifiable lipids are phospholipids. Phosphatidylcholine, phosphatidylserine, phosphatidylethanolamine, and phosphatidylinositol are phospholipids, for example.

Beeswax and palm wax are good examples of saponifiable wax-like lipids, meanwhile body fat from animals, as well as many of the vegetable fats are good examples of saponifiable triglyceride-type lipids.

References

1. Clayden, J., Greeves, N., Warren, S., & Wothers, P. (2001). Organic chemistry.
2. Law, SQ, Halim, R., Scales, PJ, & Martin, GJ (2018). Conversion and recovery of saponifiable lipids from microalgae using a nonpolar solvent via lipase-assisted extraction. Bioresource technology, 260, 338-347.
3. Nelson, DL, Lehninger, AL, & Cox, MM (2008). Lehninger principles of biochemistry. Macmillan.
4. Stoker, HS (2012). General, organic, and biological chemistry. Nelson Education.
5. Vance, DE, & Vance, JE (Eds.). (nineteen ninety six). Biochemistry of lipids, lipoproteins and membranes. Elsevier.