STEROLS: STRUCTURE, FUNCTIONS AND EXAMPLES - CHEMISTRY - 2025

The sterols are a class of steroids, lipids unsaponifiables, which are characterized by the alcoholic OH functional group. Bottom line: they are alcoholic steroids. They are part of the biochemistry of almost all living beings, multicellular or unicellular, since they are found in animals, plants, fungi, bacteria and algae.

Being steroids, they represent the starting materials for the synthesis of steroid hormones and vitamins that are solubilized in fats. They are derived in three main types: the zoosterols, present in animals and especially in mammals; phytosterols, in plants or vegetables; and mycosterols, found in microorganisms.

Cholesterol, present in sausages, cheeses and red meats, is the best known sterol of all. Source: Pexels.

The name 'sterol' sounds strange to the ears in general; but it is not the same when they hear the word 'cholesterol'. And it is that cholesterol is in fact a sterol. It is the most important sterol in animal cells, so many foods derived from them, such as sausages or black pudding, are rich in cholesterol.

One of the main and most interesting functionalities of sterols is to make the lipid bilayer of cell membranes more flexible. In this way they become more dynamic, behaving as if it were a liquid that flows in an orderly fashion (crest or wave type).

Structure of sterols

General structure of sterols and their derivatives. Source: Vaccinationist

In the image above we have the basic structure for sterols. They have four rings, three of which are hexagonal and one pentagonal, which together receive the specific name of cyclopentaneperhydrophenanthrene; the structural skeleton of steroids and this family of lipids.

In C-3 we have the hydroxyl group, OH, which gives the molecule the alcoholic character. At the other end, at C-17 there is a side chain R, which will differentiate different types of sterols from each other, as well as the presence of substituents on other carbons.

Sterols are said to be amphipathic molecules because they have fully localized polar and apolar regions in their structure. The OH group becomes the hydrophilic polar head or region; while the rest of the carbon body is the tail or apolar region, hydrophobic.

The sterol structure may initially give the false impression of being completely flat; but in reality it resembles a leaf with slight folds.

Unless there are double bonds, the rings are not completely flat, as their carbons are sp ³ hybridized. However, the sterols are laminar enough to "sneak" into the tight spaces of cell membranes.

Cholesterol

Structure of cholesterol. Source: BorisTM via Wikipedia.

Although the first structure shown is the most general of all, that of cholesterol is practically the basis for comparing the structures of other important sterols; that is, they are very similar, but with extra double bonds, or with other additional alkyl substituents such as methyl or ethyl groups.

Features

Synthesis of fat soluble vitamins

Sterols are starting materials for four essential vitamins for the body: A, D, E and K. Therefore, indirectly sterols help to improve vision, ensure healthy skin, strengthen bones and the immune system, and provide antioxidants to the body.

Steroid hormone synthesis

As stated before, cholesterol is the main sterol in animals. Participates in the synthesis of bile acids (bile) in the liver, which help in the assimilation of nutrients and dissolve fats. It represents the organic material for the synthesis of vitamin D in our body, and for the construction of cell membranes.

Likewise, our body needs cholesterol for the synthesis of steroid hormones; such as aldosterone, cortisol, testosterone, and estrogen. Consequently, sterols play an important role in cell signaling, so that distant organs communicate by exchanging molecules.

Stability of cell membranes

The lipid bilayer of cell membranes is not rigid, but moves or partially undulates, thus having fluidity, which is essential for cells to respond effectively to external stimuli or signals. The membrane adopts a movement similar to that of a wave, as if it were a liquid flowing in an orderly manner.

Sterols regulate the dynamics of the membranes so that they are not too stiff, nor do they flow out of order. They thus impose an order. They do this by joining between the apolar tails of the lipid bilayer, interacting with them through dispersive forces and forcing them to move or stop.

These membranes not only correspond to those that separate the cytoplasm from the extracellular environment, but also to those that line the mitochondria and endoplasmic reticulum.

There are studies that actually investigate the effect of sterol substituents on the fluidity of membranes; if there is an increase or increase in their dynamics in the event that the substituents are alkyl and branched, or polar groups such as OH and NH ₂.

Examples of sterols

In animals

Cholesterol is by far the most important animal sterol. Among other zoosterols we can also mention cholestenol, coprastenol and demosterol.

In plants

Until now, no mention has been made of any phytosterol or plant sterol, which are equally important for our consumption, since they are marketed as nutritional supplements to reduce high levels of cholesterol avoiding their absorption.

Among the phytosterols we have: campesterol, sitosterol, stigmasterol, avenasterol and brassicaesterol, the first three being the main phytosterols found in different species of plants and vegetable oils (corn, sunflower, soybean, palm, etc.).

Cholesterol is also part of the sterols present in plants. Again, it is acceptable to think that part of the sterols are derived in some way from cholesterol, since their structures are very similar, except that they have additional methyl or ethyl groups.

In mushrooms

The main sterol in fungi is called ergosterol, usually abbreviated as ERG:

Structure of ergosterol. Source: Mysid via Wikipedia.

Ergosterol differs a little more from cholesterol by having two extra double bonds, one of them being located in the R side chain on the far right. This sterol helps protect fungi from the action of certain antibiotics.

In bacteria

And finally we have the hopanoids, which are considered primitive sterols and allow bacteria to withstand hostile conditions of temperature, pressure, acidity or highly saline environments. Hopanoids are not even structurally based on the four condensed rings, but rather consist of five rings.

References

1. Morrison, RT and Boyd, R, N. (1987). Organic Chemistry. 5th Edition. Editorial Addison-Wesley Interamericana.
2. Carey F. (2008). Organic Chemistry. (Sixth edition). Mc Graw Hill.
3. Graham Solomons TW, Craig B. Fryhle. (2011). Organic Chemistry. Amines. (10th edition.). Wiley Plus.
4. Wikipedia. (2019). Sterol. Recovered from: en.wikipedia.org
5. Elsevier BV (2019). Sterols. ScienceDirect. Recovered from: sciencedirect.com
6. Dufourc EJ (2008). Sterols and membrane dynamics. Journal of chemical biology, 1 (1-4), 63–77. doi: 10.1007 / s12154-008-0010-6
7. William W. Christie. (2019). Sterols 3. Sterols and their Conjugates from Plants and Lower Organisms. The lipid web. Recovered from: lipidhome.co.uk