Essential fatty acid interactions

There is a wide variety of fatty acids found in nature. Two classes of fatty acids are considered essential, the omega-3 and omega-6 fatty acids. Essential fatty acids are necessary for humans but cannot be synthesized by the body and must therefore be obtained from food. Omega-3 and omega-6 are used in some cellular signaling pathways and are involved in mediating inflammation, protein synthesis, and metabolic pathways in the human body.

Arachidonic acid (AA) is a 20-carbon omega-6 essential fatty acid. It sits at the head of the "arachidonic acid cascade," which initiates 20 different signalling pathways that control a wide array of biological functions, including inflammation, cell growth, and the central nervous system. Most AA in the human body is derived from dietary linoleic acid (18:2 ω-6), which is found in nuts, seeds, vegetable oils, and animal fats.

Eicosanoids are signaling molecules derived from the essential fatty acids (EFAs). They are a major pathway by which the EFAs act in the body. There are four classes of eicosanoid and two or three series within each class. The plasma membranes of cells contain phospholipids, composed of a hydrophilic phosphate head and two hydrophobic fatty acid tails. Some of these fatty acids are 20-carbon polyunsaturated essential fatty acids (AA, EPA, or DGLA). EFAs are cleaved out of the phospholipid and released as free fatty acids. The EFA is oxygenated (by either of two pathways) and further modified, yielding the eicosanoids.

After oxidation, the eicosanoids are further modified, making a series. Members of a series are differentiated by a letter and are numbered by the number of double bonds, which does not change within a series. For example, cyclooxygenase action upon AA (with 4 double bonds) leads to the series-2 thromboxanes (TXA₂, TXB₂... ), each with two double bonds. Cyclooxygenase action on EPA (with 5 double bonds) leads to the series-3 thromboxanes (TXA₃, TXB₃, etc.), each with three double bonds. There are exceptions to this pattern, some of which indicate stereochemistry (PGF_2α).

Table (1) shows these sequences for AA (20:4 ω-6). The sequences for EPA (20:5 ω-3) and DGLA (20:3 ω-6) are analogous.

All prostanoids are substituted prostanoic acids. Cyberlipid Center's Prostenoid page illustrates the parent compound and the rings associated with each series letter.

The IUPAC and the IUBMB use the equivalent term icosanoid.

The arachidonic acid cascade is arguably the most elaborate signaling system neurobiologists have to deal with.