STEROIDS CHARACTERISTICS, STRUCTURE, FUNCTIONS, CLASSIFICATION - BIOLOGY - 2025

The steroids are lipids (fats) and are classified as such because they are hydrophobic compounds and thus are insoluble in water. Unlike the other known lipids, steroids are composed of a 17-carbon nucleus made up of four fused or interlocking rings, named by the letters A, B, C, and D, respectively.

The term "steroid" was introduced by Callow in 1936 to refer to a group of compounds that included sterols, saponins, bile acids, sex hormones, and cardiotoxics.

Basic structure of a steroid (Source: Hati at German Wikipedia / Public domain, via Wikimedia Commons)

Hundreds of steroids are found in plants, animals, and fungi. They all derive from the same molecule called cyclopentaneperhydrophenanthrene, steran or "gonane." This molecule is the one that contributes the four rings, three of which are made up of 6 carbons, each one called cyclohexane (A, B and C) and the last one for five, cyclopentane (D).

Cholesterol is the steroid of greatest biological importance. It is the precursor of vitamin D, progesterone, testosterone, estrogens, cortisol, aldosterone, and bile salts. It is part of the structure of animal cell membranes and participates in cell signaling systems.

Natural steroids are substances synthesized by living organisms that fulfill various functions.

In humans, some steroids are part of cell membranes where they perform functions related to the degree of fluidity of the membrane; others act as hormones and others participate in the emulsion of fats in the digestive processes.

In plants, steroids are part of the membranes, they participate in the growth of lateral roots, in the growth and development of shoots and in flowering.

Although steroids have not been well studied in mushrooms, they are part of the structure of their membranes, their sex hormones and growth factors. In insects, birds, and amphibians, steroids are part of hormones, membranes, and some of the poisons.

Steroid characteristics

When it comes to steroids, many people associate them with ergogenic aids, cheating, or unfair advantages. This association has to do with the use of a type of steroid to increase muscle mass widely used by athletes. These types of steroids are called "androgen anabolic steroids."

While these natural and some synthetic steroids serve the function of promoting the growth and development of skeletal muscle mass, steroids serve many other functions and are widely used in the field of pharmacology.

Steroids are used as anti-inflammatories in processes that affect the joints, in anticancer treatment as premedication together with chemotherapy, it is part of some drugs used for bronchial asthma.

Oral contraceptives are steroid hormones. They are used locally to treat eczema of the skin, etc.

The use of steroids is also important in the field of agribusiness since they are used as rooters, to promote the growth of roots and shoots. They are used to control flowering in crops, etc.

Structure

Steroids are derivatives of cyclopentaneperhydrophenanthrene, to which are added some functional groups and a side chain at carbon 17. The functional groups are hydroxyl, methyl, carboxyl or carbonyl groups, among others. In some steroids double bonds are added.

The length and structure of the side chain makes the difference between the various steroids. Steroids that possess the hydroxyl functional group (-OH) are classified as alcohols and are called "sterols".

From the basic structure of steran with four fused rings, three cyclohexanes named with the letters A, B and C and one cyclopentane named with the letter D, hundreds of natural and synthetic steroid structures are formed.

In humans, three hormonal groups are synthesized from cholesterol: mineralocorticoids such as aldosterone, glucocorticoids such as cortisol, sex hormones such as testosterone and estrogens and progestogens such as progesterone.

Cholesterol is also used for the synthesis of vitamin D and bile salts.

Features

Steroids serve very important and diverse functions. Cholesterol, for example, stabilizes cell membranes and participates in cell signaling mechanisms. Other steroids fulfill hormonal functions and participate in digestive processes through bile salts.

In humans, steroid hormones are released into the circulation, where they are transported until they reach the "target" organs. In these organs, such molecules bind to nuclear receptors, eliciting physiological responses by regulating the expression of specific genes.

This is how steroids participate in the regulation of carbohydrate and protein metabolism, in the control of blood electrolyte levels and plasma osmolarity.

They have anti-inflammatory properties, some are released in response to stress, increasing blood pressure and blood glucose.

Other steroids have to do with the female and male reproductive systems. They participate in the development and maintenance of the distinctive sexual characteristics of each sex, and others have specific functions during pregnancy.

Phytosterols are found in the structure of plant membranes, with the same function as cholesterol to stabilize animal membranes, and ergosterol is found in the membranes of filamentous fungi and yeasts.

The most important steroids of plant origin (phytosterols) are: β-sitosterol, stigmasterol and campesterol.

In insects, birds and amphibians, steroids are found both in their membranes and in the structures of many of their hormones and in some poisons. Steroids are also part of the structure of various poisons produced by fungi.

Types of steroids (classification)

The four-ring structure of the cyclopentaneperhydrophenanthrene common to all steroids allows the possibility of hundreds of substitutions at each position, which makes classification very difficult.

There are several classifications for steroids. The simplest of all groups them into two types: natural and synthetic. However, in 1950 a classification based on the number of carbon atoms was devised; This classification includes 5 types:

1. Cholestanes: with 27 carbons, example: cholesterol
2. Colanos: with 24 carbons, example: cholic acid
3. Pregnanos: with 21 carbons, example: progesterone
4. Androstanes: with 19 carbons, example: testosterone
5. Estranos: with 18 carbons, example: estradiol

Subsequently, a new classification was structured that takes into account the number of carbon atoms in the side chain and the functional groups of carbon number 17.

This classification includes 11 types of steroids, among which are: estrane, androstane, pregnane, cholan, cholestane, ergostane, stigmastane, lanostane, cardanolides, bufanolides and spirostanes.

Strange

Steroids with a backbone of estrane have 18 carbon atoms and have, in the side chain attached to carbon 17, an aromatic ring A without a methyl group at carbon 10. Natural steroids of this class are estrogens and an example is estradiol.

Androstane

Androgens are natural steroids that have an androstane skeleton, also with 18 carbon atoms and with a "keto" substituent attached to the side chain of the 17 carbon atom. Examples of androgens are testosterone and androstenedione.

Pregnant

Steroids with a pregnane skeleton have 21 carbon atoms and have two carbon atoms in the 17-carbon side chain. Progesterone and adrenal steroids, cortisol, and aldosterone belong to this group.

Colano

Bile salts have steroids with colane skeletons, composed of 24 carbon atoms and that have 5 carbon atoms in the 17 carbon side chain. Examples of these are cholic acid.

Cholestane

Sterols are steroids with cholestane skeletons. They have 27 carbon atoms and 8 in the 17 carbon side chain. Cholesterol is, without a doubt, the most exemplary sterol.

Ergostan

Other sterols such as ergosterol are good examples for steroids in this group, which have an ergostane backbone, with 28 carbon atoms and 9 atoms of the same element on the 17 carbon side chain.

Stigmastane

Stigmasterol, another sterol of plant origin, has a skeleton made up of 29 carbon atoms known as stigmastane, which has 10 carbon atoms on the side chain of the carbon at position 17.

Lanostan

Lanosterol, which is the first cyclization product of squalene, the precursor of all steroids, is composed of a skeleton of 27, 30-32 carbon atoms, which has two methyl groups on carbon 4 and 8 carbon atoms on the carbon 17 side chain.

This steroid belongs to a group known as the trimeryl sterols.

Cardanolides

Cardiac glycosides are steroids composed of cardanolide skeletons, formed by 23 carbon atoms and a lactone ring as a substituent at carbon 17. An example of these compounds is digitoxigenin.

Buphanolides

Toad venom is rich in bufotoxin, a compound made up of bufanolide-type steroids, characterized by a 24-carbon structure and a lactone ring with a double bond on the 17-carbon side chain.

Spirostans

Dioscin and diosgenin, a steroidal saponin produced by some plants, are sapogenins with steroidal skeletons of the spirostane type. These have 27 carbon atoms and a spirochetal ring at carbon 22.

Examples of steroids

To cholesterol, due to the action of sunlight on the skin, one of the cyclohexane B bonds is “broken” and forms a double bond that joins a CH2, which transforms it into cholecalciferol or vitamin D3.

This vitamin is necessary for the correct metabolism of calcium and phosphate in the formation and maintenance of bone structure and teeth in animals.

Example of some steroids (Source: Alejandro Porto / CC BY-SA (https://creativecommons.org/licenses/by-sa/3.0) via Wikimedia Commons)

Steroids and hormones

The cholesterol-derived steroid hormones are progestogens, corticosteroids (mineralocorticoids and glucocorticoids), and sex hormones.

The first hormone to be made from cholesterol is pregnenolone. This is formed by losing 6 carbons of the side chain attached to the 17th carbon of cholesterol.

Pregnenolone is then transformed into progesterone, a hormone whose function is to prepare the uterine wall for the implantation of the fertilized egg.

Other steroid hormones are synthesized from pregnenolone and progesterone. Corticosterone and aldosterone are synthesized from successive hydroxylations of progesterone, a process catalyzed by enzyme complexes of cytochrome P450.

Then, hydroxylation and subsequent oxidation of the C-18 methyl group of corticosterone converts it to aldehyde and forms aldosterone.

Corticosterone and aldosterone are mineralocorticoids that regulate blood levels of sodium and potassium and regulate the renal reabsorption of sodium, chloride, and water; these participate in the regulation of plasma osmolarity.

Sex hormones are also derived from progesterone. Androgens are synthesized in the testes and in the cortex of the adrenal glands. To do this, by enzymatic action, progesterone is converted to 17α-hydroxyprogesterone, then to androstenedione, and finally, testosterone is formed.

Testosterone, through multiple enzymatic steps, ends up forming β-estradiol, which is a female sex hormone.

Beta-estradiol is the most important estrogen. Its synthesis occurs mainly in the ovaries and to a lesser extent in the adrenal cortex. These hormones can also be produced in the testicles.

References

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