UNSAPONIFIABLE LIPIDS: FUNCTIONS AND CLASSIFICATION - BIOLOGY - 2025

The unsaponifiable lipids are lipids contain fatty acids as essential structural components. On the contrary, saponifiable or complex lipids do have fatty acids, which can be released by alkaline hydrolysis, producing salts of fatty acids (soaps), in a process called saponification.

Numerically, unsaponifiable lipids are lower than complex ones, but among them are molecules with very intense and specialized biological activity. Examples of them are some vitamins, hormones, coenzymes, carotenoids, among others.

Unsaponifiable lipids, terpenes. Taken and edited from: Alejandro Porto.

Lipids

Lipids are organic biomolecules that do not present solubility in water, but are soluble in nonpolar solvents, such as benzene, ether or chloroform. Its chemical constitution is mainly carbon, hydrogen and oxygen. They can also have, to a lesser degree, other components, such as phosphorus, nitrogen and sulfur.

Lipids often bind with other biomolecules, through weak bonds or covalent bonds, forming hybrid molecules, among which are glycolipids and lipoproteins.

Lipids have been classified in different ways, however, the most stable classification is based on the presence (saponifiable lipids) or absence (unsaponifiable lipids) of fatty acids in their structure.

Function of unsaponifiable lipids

Unsaponifiable lipids fulfill various complex and specific functions within living beings, including:

-Vitamins

Vitamins are organic compounds that in very small amounts are essential for the function of all cells, and must be contained in the diet of some species, since they are unable to synthesize them themselves. Fat-soluble vitamins all belong to the group of non-saponifiable lipids.

Vitamin A

Vitamin A is essential for vision, since in the form of aldehyde it is a constituent part of rhodopsin, a visual pigment. Deficiency of this vitamin causes night blindness in adults and xerophthalmia or dry eyes in infants and children, which can lead to permanent blindness.

The role of vitamin A in other biological activities is still unknown, its deficiency in the diet causes, in addition to visual problems, retarded growth, incomplete development of bones and nervous system, thickening and dryness of the skin, sterility and degeneration of kidneys and other organs.

Vitamin D

Its function is related to the adequate calcification of the bones, and its deficiency causes rickets. There are several compounds that have vitamin D function; in mammals, the most important are vitamin D ₂ (ergocalciferol) and D ₃ (cholecalciferol).

The presence of this vitamin in the diet is very scarce, or nonexistent, except in the liver of fish. Vitamin D can be synthesized by the body itself from a compound called 7-dehydrocholesterol, present in the skin, which requires exposure to sunlight.

Vitamin E

Also known as tocopherol, it has an antioxidant function by preventing the autoxidation of highly unsaturated fatty acids in the presence of molecular oxygen. Its deficiency produces sterility (at least in guinea pigs), necrosis of the liver, degeneration of the kidneys and skeletal muscles, among others.

Vitamin K

Compound that is synthesized by bacteria that are part of the intestinal flora. It is necessary for proper blood clotting, possibly because it acts as a substrate in the liver for the production of an enzyme (proconvertin) that participates in the coagulation cascade.

-Fotopigments

Some unsaponifiable lipids act as photosynthetic pigments, or are part of them; for example, phytol, a diterpene that is part of chlorophyll. Carotenoids are polyisoprenoids that have conjugated double bonds and that can also act as receptors for light energy.

There are two main types of carotenoids, carotenes and xanthophylls; the fundamental difference between the two is the absence (carotenes) or presence (xanthophylls) of oxygen in their molecular constitution.

-Hormones

Among the unsaponifiable lipids there are components with hormonal function, among them:

Androgens

They are the male sex hormones, made up of tetosterone and dihydrotetosterone. These hormones regulate the growth and development of sexual structures such as the penis, sperm duct, and accessory glands.

They also allow the appearance of secondary sexual characteristics (such as beard and tone of voice) and act on reproductive behavior.

Estrogens

There are three types of estrogens: estradiol, estrone, and estriol. Its function, in the female, is similar to androgens in males, by allowing the development of sexual structures, regulating the appearance of secondary sexual characteristics and intervening on sexual desire and reproductive behavior.

Progesterone

The pregnancy hormone stimulates changes in the walls of the uterus for the implantation of the fetus during reproduction and intervenes in the development of the mammary gland, among other activities.

Prostaglandins

All prostaglandins have hormonal activity.

-Other functions

Additionally the unsaponifiable lipids can have other functions; including bile salts, which act by saponifying saponifiable lipids during the digestion process.

Others have functions of coenzymes or pseudo coenzymes, such as coenzyme Q, which has the function of transporting hydrogen in mitochondrial respiration. While the phosphoric esters of dolichol and bactoprenol participate in the biosynthesis of lipopolysaccharides.

Classification

There are three classes of unsaponifiable lipids: terpenes, steroids, and prostaglandins. The first two are very similar from the structural point of view, since they are derived from hydrocarbon units of five carbon atoms.

Prostaglandins, for their part, come from the cyclization of unsaturated fatty acids composed of 20 carbon atoms.

-Terpenes

They are molecules made up of many units of isoprene, a hydrocarbon with five carbon atoms. They are also called terpenoids or isoprenoids. These molecules can be linear, cyclic, or contain both types of structures.

The union between the different units that make up a terpene generally follows an order called "head-tail", although sometimes it can be "tail-tail". Most of the double bonds present in terpenes are of the trans type, however, cis bonds can also be present.

Terpenes can be subdivided according to the number of isoprene units that make them up:

Monoterpenes

Formed by two isoprene units. Many are constituents of essential oils present in plants, such as menthol, the main component of peppermint oil, or camphor, a fundamental constituent of the oil of the same name.

Sesquiterpenes

They contain three isoprene units. Farnesol, an acyclic hydrocarbon present in many plants and which is used in perfumery to enhance the odors of some perfumes, is a sesquiterpene.

Diterpenes

They are made up of four isoprene units. An example of diterpenes is phytol, a fundamental component of chlorophyll, a photosynthetic pigment in plants.

Triterpenes

They are made up of six isoprene units. Such is the case of squalene, a precursor of cholesterol, a sterol that is part of the plasma membrane and body tissues of all animals.

Tetraterpenes

They contain eight units of isoprene. Among these we have carotenoids, organic pigments present in plants and other organisms that carry out photosynthesis, such as algae, protists and bacteria.

Polyterpenes

Composed of more than eight isoprene units, such as natural rubber and wadding. An important group of polyterpenes are polyprenools, which, in addition to having numerous linearly linked isoprene units, possess a terminal primary alcohol.

Examples of polyterpenes are bactoprenol, or undecaprenyl alcohol, present in bacteria, and dolichol, present in animals. These, in their phosphoric ester form, possess pseudo-coenzymatic functions.

-Steroid

They are organic compounds that originate from a linear triterpene called squalene. This squalene has the ability to cycle very easily. There are many steroids in nature, each of them with particular functions or activities.

Steroids will differ from each other by the amount of double bonds, by their location within the molecule, and by the type, amount, and position of their substituent groups.

They also differ in the configuration of the bonds between these substituent functional groups (alpha or beta configuration) and the nucleus; and the configuration of the rings between them.

Lanosterol

Steroid isolated for the first time from the wax coating of wool. It is the first product to be obtained from the cyclization of squalene. In animal tissues it is a precursor of cholesterol, but it is also found in plant membranes.

It is a steroid alcohol characterized by having a branched chain of at least 8 carbon atoms at carbon 17 (c17), as well as a hydroxyl group at carbon 3 of ring A.

Cholesterol

Another steroid alcohol, derived from lanosterol, present in the plasma membranes of a large number of animal cells, as well as in the lipoproteins of blood plasma. Cholesterol is the precursor to many other steroids such as bile acids, estrogens, androgens, progesterone, and adrenocortical hormones.

Structure of cholesterol. Taken and edited from BorisTM.

Other steroids

Phytosterols are a group of steroids found in higher plants, among which are stigmasterol and sitosterol. Fungi and yeasts, on the other hand, present mycosterols, such as ergosterol, a precursor of vitamin D.

-Eicosanoids

C20 molecules derived from 20 carbon essential fatty acids, such as linoleic, linolenic, and arachidonic acids. They are fundamental constituents of the immune system and also serve important functions in the central nervous system.

Prostaglandins

Family of derivatives of fatty acids with important hormonal or regulatory activity. They were isolated for the first time from seminal plasma, prostate, and seminal vesicle. There are many types of prostaglandins with different functions, but all of them lower blood pressure; they also cause smooth muscle contraction.

Thromboxanes

They are compounds derived from arachidonic acid, with both autocrine (affects the emitting cell) and paracrine (affects neighboring cells) effect. Its main function is related to coagulation and the accumulation of platelets.

Leukotrienes

Other arachidonic acid derivatives, isolated for the first time from leukocytes and characterized by having four conjugated double bonds in their structure. They have smooth muscle constricting activity and participate in inflammatory processes.

References

1. A. Lehninger (1978). Biochemistry. Ediciones Omega, SA
2. L. Stryer (1995). Biochemestry. WH Freeman and Company, New York.
3. Lipid. On Wikipedia. Recovered from en.wikipedia.org.
4. Unsaponifiable lipids. On Wikipedia. Recovered from es.wikipedia.org.
5. Terpene. On Wikipedia. Recovered from es.wikipedia.org.
6. Steroid. On Wikipedia. Recovered from es.wikipedia.org.