Fatty acids (FA) are carboxylic acids with a hydrocarbon chain of varying length. The carboxyl group and the hydrocarbon chain determine the physical and chemical properties of the molecule.
In most cases the hydrocarbon chain is unbranched and with an even number of carbon atoms. It can contain only single bonds between carbon atoms, as in the case of saturated fatty acids, or at least one double or triple bonds between carbon atoms, as in the case of unsaturated fatty acids.
Fatty acids are members of the class of compounds known as lipids, and can be classified on the basis of chemical and physiological criteria, such as the length of the hydrocarbon chain, the presence or absence of double and triple bonds, the position of the double bonds with respect to the methyl end of the chain, the geometric isomerism or cis-trans isomerism of double bonds, or the ability to synthesize them.
In nature, they are rarely found in free form, and in that case they are known as free fatty acids (FFAs) or nonesterified fatty acids (NEFAs). In humans, there is a small amount of FFAs in the bloodstream, resulting from lipolysis; about three-fourths are bound to albumin, whereas one-fourth is bound to lipoproteins.
Most commonly, fatty acids are bound through ester bonds to other organic molecules such as glycerol, glycerol 3-phosphate and sterols, to form more complex lipids, such as triglycerides, which are a major energy store in mammals, phospholipids, which are the major components of cell membranes, or sterol esters such as cholesterol esters.
They have important functions in cells. They can be oxidized to provide energy, they are components of cell membranes, being component of phospholipids, and are the precursors of bioactive lipid mediators.
Structure and properties of fatty acids
Fatty acids are carboxylic acids with the general formula R–COOH, where R– is an hydrocarbon chain.
A carboxylic acid is an organic compound that contains a carboxyl functional group, −COOH, in which a carbon atom is bonded to an oxygen atom by a double bond, to form a carbonyl group, –C=O, and to an hydroxyl group, –OH, by a single bond. Carboxylic acids are weak acids. Their acidic nature results from the hydrogen of the carboxyl group and will be greater the shorter the hydrocarbon chain.
The R– group is attached to the fourth bond of the carboxylic carbon. R– group of formic acid, the simplest carboxyl acid, is a hydrogen atom, whereas in the other carboxyl acids it is an hydrocarbon chain that can be:
- linear or branched;
- with carbocyclic units;
- with an even or odd number of carbon atoms;
- without double/triple bonds between carbon atoms, therefore saturated;
- with double/triple bonds between carbon atoms, therefore unsaturated.
Fatty acids found in foods almost always have a carbon chain with an even number of carbon atoms and a length of 4 to 24 atoms; moreover the chain can be saturated or unsaturated.
Branched FA are common in Gram-positive bacteria, and are present in low concentration in milk and meat lipids of ruminants.
Polarity
The carboxyl group is a polar acid group, whereas the hydrocarbon chain is the nonpolar region of the molecule. The hydrophobicity of the chain increases as the length increases, and this determines the degree of solubility of the fatty acid in polar and non-polar solvents.
In fatty acids with short carbon chain, the polarity of the carboxyl group wins over the hydrophobicity of the carbon chain, the molecule has a polar nature and is soluble in polar solvents such as water and ethanol. Butyric acid is an example of a fatty acid soluble in polar solvents.
As the length of the carbon chain increases, the solubility of the fatty acid in polar solvents decreases. Starting from caproic acid, whose chain is two carbons longer than that of butyric acid, the polarity decreases, and, for chain length greater than 16-18 carbon atoms, therefore from palmitic acid and stearic acid forward, the molecules are completely insoluble in water. Examples are arachidic acid, behenic acid and lignoceric acid, three saturated FA with carbon chains of 20-, 22- and 24 carbon atoms.
Melting and boiling point
The characteristics of the carbon chain also affect polarity, melting point, and boiling point.
Saturated FA have higher melting and boiling temperatures as their molecular weight and, consequently, their chain length grow. The configuration of saturated linear chains is mainly linear. Due to the tight packing of the molecules, many intermolecular hydrophobic connections can form, stabilising the structure and giving it an almost crystalline appearance. As a result, the melting and boiling points rise.
Unsaturated FA have lower melting and boiling points than their comparable saturated FA. This results from variations in the geometry of the carbon chain brought on by double bonds.
The double bond can be thought of as a plane on which the carbon chain joins and continues, or enters and departs, due to its hard planar structure, which prevents rotations between the two carbon atoms, which are possible in the single bond. The molecule may exhibit cis-trans isomerism or geometric isomerism if a portion of it has a stiff planar structure. The double bond is in cis configuration if the chain’s entry and exit from the plain happen on the same side, whereas it is in trans configuration if they happen on the opposite sides.
Because of the bend or “kink” that the cis configuration creates in the carbon chain, it packs less tightly than saturated FA, resulting in fewer intermolecular hydrophobic interactions. Less energy is required to shift the molecules apart as a result, and the structure is less stable. As a result, cis fatty acids will have lower melting and boiling points than the comparable saturated FA.
Trans arrangement straightens the carbon chain, giving it a form akin to that of a saturated FA, and possesses geometry comparable to that of the single bond. Because they have comparable melting and boiling temperatures to saturated FA, molecules with exclusively trans double bonds pack with a similar efficiency.
Saturated and trans fatty acids, for example, form more rigid structures than cis fatty acids, which affect the fluidity of biological membranes in which they are present. Similarly, the geometry of cis and trans double bonds and saturated and unsaturated fatty acids differ, and this affects their biological functions.
Classification of fatty acids
There are many fatty acid classification schemes. As was previously demonstrated, they may be categorised according to their polarity or according to the structural elements of the carbon chain.
A straightforward method is based on the chain’s even or odd number of carbon atoms.
Other methods are based on the presence or absence of branches or cyclic structures in the chain, the length of the carbon chain, the number of double or triple bonds, the position of the first double bond in relation to the methyl end of the chain, the geometric isomerism of the double bonds, or the presence or absence of double or triple bonds. They can also be categorised according to how important they are to humans. Here are some illustrations.
Length of the carbon chain
Chain length in fatty acids with linear chains ranges from one to thirty carbon atoms and beyond.
They fall into the following categories depending on the length of the chain:
- short-chain fatty acids (SCFAs), chain length ranges from 1 to 5 carbons;
- medium-chain fatty acids (MCFAs), chain length ranges from 6 to 12 carbons;
- long-chain fatty acids (LCFAs), chain length ranges from 13 to 21 carbons;
- very long chain fatty acids (VLCFAs), chain length is equal or greater than 22 carbons.
Saturated and unsaturated fatty acids
Based on the presence or absence of double or triple bonds in the carbon chain, they are classified into:
- saturated fatty acids, there is no double/triple bond;
- unsaturated fatty acids, there is at least one double/triple bond.
Based on the number of double/triple bonds in the carbon chain, they are classified into:
- monounsaturated fatty acids, there is only one double/triple bond;
- polyunsaturated fatty acids, there is more than one double/triple bond.
The initial double bond’s location in relation to the chain’s terminal methyl terminus can be used to further categorise unsaturated FA.
- Omega-3 polyunsaturated fatty acids, such as alpha-linolenic acid, stearidonic acid, eicosapentaenoic acid (EPA), and docosahexaenoic acid (DHA), have their initial double bonds three carbon atoms from the methyl end.
- Omega-6 polyunsaturated fatty acids, such as linoleic acid, gamma-linolenic acid, dihomo-gamma-linolenic acid, and adrenic acid, have their initial double bonds six carbon atoms from the methyl end.
- Omega-7 fatty acids, such as palmitoleic acid, when the initial double bond is seven carbon atoms from the methyl end.
- Omega-9 fatty acids, such as oleic acid, erucic acid, nervonic acid, and Mead acid, when the initial double bond is nine carbon atoms from the methyl end.
- Gadoleic acid is an example of an omega-11 FA, where the initial double bond is located eleven carbon atoms from the methyl end.
When the carbon chain has at least one triple bond, the fatty acid is referred to as an acetylenic fatty acid.
The existence of conjugated double bond systems can also be used to categorise polyunsaturated fatty acids.
- Conjugated fatty acids: two or more double bonds are not separated by one or more methylene groups (–CH2–).
- Unconjugated fatty acids: the double bonds in the chain are methylene-interrupted. Mainstream polyunsaturated fatty acids are unconjugated fatty acids.
Cis and trans isomers
Based on the geometric isomerism, unsaturated FA with double bonds may be divided into the following categories:
- cis fatty acids, which are more prevalent in diet and nature;
- Less prevalent are trans fatty acids.
In nature, trans fatty acids (TFA) are produced by bacteria in ruminants’ rumens from unsaturated fatty acids (FA) consumed by animals, and they may be found in their milk and meat. However, trans fatty acids are mostly a byproduct of the hydrogenation of unsaturated fatty acid-rich oils, which are particularly harmful to human health.
Examples of cis-trans isomerism include oleic acid and elaidinic or elaidic acid; elaidic acid is the trans-isomer of oleic acid. Brassidic acid and vaccenic acid are other trans FA that can be present in food.
Essential fatty acids
The physiological classification of fatty acids is based on the body’s capacity to produce them. Alpha-linolenic acid and linoleic acid are two unsaturated fatty acids that are categorised as necessary fatty acids since animals cannot synthesis them because they lack the enzymes delta-12 desaturase (EC 1.14.19.6) and delta-15 desaturase (EC 1.14.19.13). They must thus be taken with meals.
Functions
In the cell, fatty acids play a variety of tasks.
They are a source of energy. They are oxidised to make ATP after being released from intracellular triglycerides or, if released from adipose tissue triglycerides, in the cells of other tissues and organs, and are mostly transported in the circulation by albumins.
The primary sites of oxidation are the liver, heart, and skeletal muscle. Although the oxidation of short-chain saturated FA produces less energy than that of proteins and carbohydrates—for example, acetic acid produces 3.5 kcal/g of energy, propionic acid produces 5.0 kcal/g, butyric acid produces 6.0 kcal/g, and caproic acid produces 7.5 kcal/g—cells still obtain more energy from their oxidation.
They are the building blocks of strong bioactive lipid mediators that participate in signalling pathways. Unsaturated fatty acids (FA) from the bloodstream and membrane phospholipids, such as arachidonic and docosahexaenoic acids, can be metabolised to produce powerful pro-resolving lipid mediators like Lipoxins, Maresins, and D-series Resolvins as well as pro-inflammatory lipid mediators like prostaglandins and leukotrienes.
Being a part of phospholipids, a crucial component of all biological membranes, they play a structural function in the creation of cell membranes.
