UNSATURATED FATTY ACIDS: CHARACTERISTICS, STRUCTURE, FUNCTIONS, EXAMPLES - BIOLOGY - 2024

The unsaturated fatty acids are monocarboxylic acids with hydrocarbon chains in which two or more of their carbon atoms are linked together by a double bond, they have lost their hydrogen atoms by an event dehydrogenation.

These are molecules belonging to the group of lipids, as they have amphipathic characteristics, that is, they have a hydrophilic or polar portion and a hydrophobic or apolar portion. Furthermore, they function as "building blocks" for the construction of more complex lipids and are rarely found free in the cellular environment.

Structural formula of linoleic acid, a polyunsaturated fatty acid (Source: Jü / CC0, via Wikimedia Commons)

Since they form more complex lipids such as phospholipids, sphingolipids, waxes and triglycerides, unsaturated fatty acids participate in various cellular functions such as energy storage, membrane formation, message transmission, formation of protective coatings, etc.

Due to the above, it can be understood that fatty acids are fundamental molecules for living beings and that, in addition, they are extremely diverse: more than 100 different types of fatty acids have been described in isolated lipids from animals, plants and microbes.

Characteristics of unsaturated fatty acids

Fatty acids can be saturated and unsaturated and, in both cases, they are monocarboxylic acids with chains of variable lengths, but always with an even number of carbon atoms and without branches, with some exceptions.

They are not normally found free in the cell or in the extracellular compartments of multicellular organisms, but are always part of lipids or more complex molecules.

They are called "unsaturated fatty acids" because their carbon atoms are not completely saturated with hydrogen atoms, but have lost two or more hydrogens by dehydrogenation and have one or more double or triple bonds between the carbon atoms that make up their structure.

They can be monounsaturated or polyunsaturated, depending on whether they have one or more double bonds, respectively.

Physical properties

The water solubility of fatty acids (whether saturated or unsaturated) is a direct function of the length of their aliphatic chains, that is, the longer the carbon chain, the lower the solubility and vice versa.

The melting point also depends on the length of the chain and also on the degree of unsaturation (the number of double bonds). It is greater the greater the length of the chain (directly proportional) and the lower the more unsaturations the fatty acid has (inversely proportional).

Very long chain saturated fatty acids are usually in the solid state at room temperature, while saturated fatty acids of equal number of carbon atoms remain in the liquid state.

This is explained thanks to the decrease in molecular attractions between the carbon chains of unsaturated fatty acids, which are induced by the presence of unsaturations in cis configuration, as the aliphatic chains "bend", preventing their packing into solid structures.

Structure

Fatty acids are fundamentally apolar molecules, as they are made up of aliphatic chains of hydrogenated carbon atoms and attached, at one end, to a carboxyl group that represents carbon 1 and, at the other, to a terminal methyl group, also known as carbon ω.

The number of carbon atoms can be highly variable: there are very long chain fatty acids, which have between 12 and 26 carbon atoms; medium chain fatty acids, with between 8 and 10 carbon atoms and, finally, short chain fatty acids, which can vary between 4 and 6 carbon atoms.

The presence of double bonds between carbon atoms implies unsaturations. Monounsaturated fatty acids (with only one double bond in the chain) normally have the double bond in the cis configuration.

Polyunsaturated fatty acids that are biochemically relevant in nature can have up to 6 double bonds between their carbon atoms.

Trans-unsaturated fatty acids are produced by fermentation in the rumen of some animals and are obtained from dairy and meat products from these. Furthermore, they are produced industrially by hydrogenation of fish oils, but they are not necessarily natural products and it has been determined that they can be harmful to health.

Denotation or nomenclature

As with many compounds in nature, unsaturated fatty acids can be named by their “vulgar” name or by their IUPAC name, according to the number of carbon atoms in their chain.

To differentiate them from saturated fatty acids with the same number of carbon atoms, chemists have developed a simple system to describe the most important structural characteristics of unsaturated fatty acids.

This system consists of writing two numbers separated by a colon (:) to specify the number of carbon atoms (the first number) and the number of carbon-carbon double bonds they have (the second number).

For example, a saturated fatty acid with 18 carbon atoms can be written as 18: 0, while an unsaturated one with two carbon-carbon double bonds is written as 18: 2.

For the sake of specifying the position of each double bond within the carbon chain, the above denotation can be “expanded” by adding the Greek letter delta (∆) followed by one or more numbers as superscripts to the upper right of the letter.

Thus, a C18 polyunsaturated fatty acid with 3 double bonds can be written as 18: 3 (∆9,12,15), a nomenclature that describes an unsaturated fatty acid with double bonds between carbons 9 and 10, 12 and 13 and 15 and 16.

It is important to note that most of the monounsaturated fatty acids have the double bond in position 9 of their carbon chain and, generally, the additional double bonds of polyunsaturated fatty acids are located in positions 12 and 15 of it, with certain exceptions.

Unsaturated fatty acid families

There are several families of unsaturated fatty acids, the relationships of which are evident when the position of the double bonds is specified with respect to the position of the terminal methyl group (ω) and not of the carboxylic carbon atom.

The position of the double bonds thus determined is then denoted by the Greek letter ω and the number of carbon atoms between the terminal methyl group and the carbon-carbon double bond of the unsaturated fatty acid is indicated.

The most important families of unsaturated fatty acids are the family of omega-3 fatty acids (ω-3) and the family of omega-6 fatty acids (familia-6), however, there are others.

Omega-3 fatty acids are unsaturated fatty acids whose first double bond (in the case of polyunsaturated ones) is 3 carbon atoms from the terminal methyl group, while omega-6 fatty acids have the first double bond on the carbon in position 6 with respect to carbon ω.

Features

Unsaturated fatty acids, as well as saturated fatty acids, have multiple functions in maintaining cell life.

Not only do they serve as energy reserve substances, since their oxidation leads to the production of enormous amounts of energy, but they are also building blocks for the complex lipids that make up the membranes and for others that serve other physiological purposes.

Generally, these fatty acids predominate over saturated fatty acids, particularly in higher plants and in animals that inhabit very low-temperature environments, since they contribute to the fluidity of membranes and tissues.

In the group of unsaturated fatty acids there are some essential fatty acids that cannot be produced by man and that, therefore, must be consumed with daily food. These include linoleic acid and arachidonic acid.

These fatty acids are biosynthetic precursors of many eicosanoids and their derivatives, such as prostaglandins, thromboxanes and leukotrienes, compounds with hormonal characteristics that exert physiological functions of great relevance in humans and other mammals.

On the other hand, unsaturated fatty acids also participate in the absorption of fat-soluble substances such as vitamins and carotenoids that are consumed with food.

Examples of unsaturated fatty acids

Very important examples of mono and polyunsaturated fatty acids are:

- Palmitoleic acid (16: 1, ω-7): it is a common lipid component of human adipose tissue, especially in the liver.

Palmitoleic acid, a monounsaturated fatty acid (Source: Foobar ~ commonswiki, via Wikimedia Commons)

- Oleic acid (18: 1, ω-9): characteristic in vegetable oils such as olive and avocado. It has beneficial actions for blood vessels and is a possible "hypotensive".

Oleic acid, a monounsaturated fatty acid (Source: Andel, via Wikimedia Commons)

- Linoleic acid (18: 3 ∆9,12,15; ω-3): it is also common in oils of vegetable origin, in the meat and milk of ruminants. It seems to be involved in the reduction of cholesterol levels in the blood and the deposition of fat in the body, which is why it is said to work for weight loss.

Linoleic acid, a polyunsaturated fatty acid (Source: Edgar181 / Public domain, via Wikimedia Commons)

- Arachidonic acid (20: 4 ∆5,8,11,14; ω-6): it is found in the phospholipids of practically all cell membranes and functions as a precursor in the synthesis of eicosanoids. It is an essential fatty acid, so it must be consumed with food, especially those of animal origin.

Arachidonic acid, a polyunsaturated fatty acid (Source: Yikrazuulx / Public domain, via Wikimedia Commons)

Health benefits / harms

The health benefits or detriments of the different unsaturated fatty acids are mainly related to their physicochemical characteristics.

It is well known that "trans fats", that is, fats that are rich in lipids that have a high content of trans-unsaturated fatty acids, are harmful to health, as they have effects related to cardiovascular diseases similar to those that are exacerbated by saturated fatty acids.

Cis-unsaturated fatty acids, on the other hand, are the most commonly found in foods and therefore can be processed more easily by the human body, making them essential for the diet of man.

Thus, in addition to some benefits related to the appearance of the skin and hair, for example, the consumption of unsaturated fatty acids has great benefits at an organic level, as they contribute to the proper functioning of cells.

Monounsaturates are found in olive and peanut oil, in avocados or avocados, in most nuts and seeds. Polyunsaturated ones, on the other hand, enrich the tissues of fish such as sardines, tuna, salmon and others; of flax seeds, soybeans, sunflower, chia and some walnuts.

They are also in corn, canola and soybean oil, and many publications related to the fatty acids of the omega-3 and omega-6 families indicate that they can reduce the risk of suffering from some cardiovascular diseases and improve antioxidant capacities of the body.

References

1. Engelking, LR (2015). Chemical composition of living cells. Textbook of veterinary physiological chemistry, 2-6.
2. Ha, CE, & Bhagavan, NV (2011). Essentials of medical biochemistry: with clinical cases. Academic Press.
3. Lunn, J., & Theobald, HE (2006). The health effects of dietary unsaturated fatty acids. Nutrition Bulletin, 31 (3), 178-224.
4. Nelson, DL, Lehninger, AL, & Cox, MM (2008). Lehninger principles of biochemistry. Macmillan.
5. Stoker, HS (2012). General, organic, and biological chemistry. Nelson Education.