Fatty acids

Fats, oils and lipids consist of a large number of organic compounds, including fatty acids (FA), monoacylglycerols (MG), diacylglycerols (DG), triacylglycerols (TG), phospholipids (PL), eicosanoids, resolvins, docosanoids, sterols, sterol esters, carotenoids, vitamins A and E, fatty alcohols, hydrocarbons and wax esters. Classically, lipids were defined as substances that are soluble in organic solvents.

Fatty acids, both free and as part of complex lipids, play a number of key roles in metabolism – major metabolic fuel (storage and transport of energy), as essential components of all membranes, and as gene regulators.

In addition, dietary lipids provide polyunsaturated fatty acids (PUFAs) that are precursors of powerful locally acting metabolites, i.e. the eicosanoids. As part of complex lipids, fatty acids are also important for thermal and electrical insulation, and for mechanical protection.

Most of fatty acids are obtained form the hydrolysis of hard animal fats (tallow), coconut, palm kernel and soybean oils and from the fractional distillation of crude tall oil. Over the years, the production of edible oil fats and oils has soared, with vegetable sources now dominating the filed formally held by butter and lard.

The most common fats (solid) or oils (liquid) are glycerolipids, which are essentially composed of triacylglycerols. The triacylglycerols are accompanied by minor amounts of phospholipids, monoacylglycerols, diacylglycerols and sterols/sterol esters. Fatty acids constitute the main components of these lipid entities and are required in human nutrition as a source of energy, and for metabolic and structural activities.

Fatty acids are carbon chains with a methyl group at one end of the molecule (designated omega, ω) and a carboxyl group at the other end. The carbon atom next to the carboxyl group is called the α carbon, and the subsequent one the β carbon.

Fatty acids are classified according to the presence and number of double bonds in their carbon chain:

Saturated fatty acids
Saturated fatty acids are ‘filled’ (saturated) with hydrogen. Most saturated fatty acids are straight hydrocarbon chains with an even number of carbon atoms. The most common fatty acids contain 12–22 carbon atoms.

Unsaturated fatty acids
Monounsaturated fatty acids have one carbon–carbon double bond, which can occur in different positions. The most common monoenes have a chain length of 16–22 and a double bond with the cis configuration. This means that the hydrogen atoms on either side of the double bond are oriented in the same direction.

Physically, most of the fatty acids are liquid at room temperature. The different properties are to a large extend related to saturation or unsaturation presence. Generally, solid fats are indicated by a dominance of saturated fatty acids and liquid oils are evidence of a high level of unsaturated fatty acids.
Fatty acids

Unsaturated fatty acids: Arachidonic acid

Arachidonic acid, which is polyunsaturated fatty acid, is used in the animal body for a number of biological functions. Arachidonic acid (AA), all-cis-5, 8, 11, 14-eicosatetraenoic acid (where eicos or eikosi in Greek refers to the number 20), is an omega-6 polyunsaturated fatty acid (PUFA).

Its chemical formula is C20H32O2, 20:4(ω -6), where 20:4 refers to its 20 carbon atom chain with four double bonds, and (ω -6) refers to the position of the first double bond from the last, omega carbon atom. Arachidonic acid has an average mass of 304.467 g/mol and usually assumes a hairpin structure.

It is a component of phospholipid membranes and is a precursor to an extensive group of compounds called eicosanoids. Three of them: prostaglandins, leukotrienes and thromboxanes are largely responsible for the regulation of inflammatory processes.

Arachidonic acid can be stored in membrane phospholipids and released from nuclear envelop or plasma membrane by cytosolic phospholipase A2 (cPLA2), either constitutively or in respond to a variety of cell specific stimuli, including growth factors, hormones, cytokines, signaling molecules, or cell trauma.

Arachidonic acid can be provided to humans and mammals by an exogenous source supplied either by the direct consumption of dietary food that contains high level of arachidonic acid, whole eggs, salmon, tuna, a wide range of lean meat and its visible meat fats, or through the parent molecule, linoleic acid (LA; 18:2n-6).

Unsaturated fatty acids: Arachidonic acid