In chemistry, particularly in biochemistry, a fatty acid is a carboxylic acid with an aliphatic chain, which is either saturated or unsaturated. Most naturally occurring fatty acids have an unbranched chain of an even number of carbon atoms, from 4 to 28. Fatty acids are a major component of the lipids (up to 70% by weight) in some species such as microalgae but in some other organisms are not found in their standalone form, but instead exist as three main classes of esters: triglycerides, phospholipids, and cholesteryl esters. In any of these forms, fatty acids are both important dietary sources of fuel for animals and important structural components for cells.
History The concept of fatty acid (acide gras) was introduced in 1813 by Michel Eugène Chevreul, though he initially used some variant terms: graisse acide and acide huileux ("acid fat" and "oily acid").
Types of fatty acids
Fatty acids are classified in many ways: by length, by saturation vs unsaturation, by even vs odd carbon content, and by linear vs branched.
Length of fatty acids Short-chain fatty acids (SCFAs) are fatty acids with aliphatic tails of five or fewer carbons (e.g. butyric acid). Medium-chain fatty acids (MCFAs) are fatty acids with aliphatic tails of 6 to 12 carbons, which can form medium-chain triglycerides. Long-chain fatty acids (LCFAs) are fatty acids with aliphatic tails of 13 to 21 carbons. Very long chain fatty acids (VLCFAs) are fatty acids with aliphatic tails of 22 or more carbons.
Saturated fatty acids
Saturated fatty acids have no C=C double bonds. They have the formula CH(CH)COOH, where n is some positive integer. An important saturated fatty acid is stearic acid (n = 16), which when neutralized with sodium hydroxide is the most common form of soap.
+ Examples of saturated fatty acids Common name Chemical structure C :D Propionic acidCHCHCOOH3:0Butyric acidCH(CH)COOH4:0 Caprylic acid CH(CH)COOH 8:0 Capric acid CH(CH)COOH 10:0 Lauric acid CH(CH)COOH 12:0 Myristic acid CH(CH)COOH 14:0 Palmitic acid CH(CH)COOH 16:0 Stearic acid CH(CH)COOH 18:0 Arachidic acid CH(CH)COOH 20:0 Behenic acid CH(CH)COOH 22:0 Lignoceric acid CH(CH)COOH 24:0 Cerotic acid CH(CH)COOH 26:0
Unsaturated fatty acids
Unsaturated fatty acids have one or more C=C double bonds. The C=C double bonds can give either cis or trans isomers.
cis A cis configuration means that the two hydrogen atoms adjacent to the double bond stick out on the same side of the chain. The rigidity of the double bond freezes its conformation and, in the case of the cis isomer, causes the chain to bend and restricts the conformational freedom of the fatty acid. The more double bonds the chain has in the cis configuration, the less flexibility it has. When a chain has many cis bonds, it becomes quite curved in its most accessible conformations. For example, oleic acid, with one double bond, has a "kink" in it, whereas linoleic acid, with two double bonds, has a more pronounced bend. α-Linolenic acid, with three double bonds, favors a hooked shape. The effect of this is that, in restricted environments, such as when fatty acids are part of a phospholipid in a lipid bilayer or triglycerides in lipid droplets, cis bonds limit the ability of fatty acids to be closely packed, and therefore can affect the melting temperature of the membrane or of the fat. Cis unsaturated fatty acids, however, increase cellular membrane fluidity, whereas trans unsaturated fatty acids do not. trans A trans configuration, by contrast, means that the adjacent two hydrogen atoms lie on opposite sides of the chain. As a result, they do not cause the chain to bend much, and their shape is similar to straight saturated fatty acids.
In most naturally occurring unsaturated fatty acids, each double bond has three (n−3), six (n−6), or nine (n−9) carbon atoms after it, and all double bonds have a cis configuration. Most fatty acids in the trans configuration (trans fats) are not found in nature and are the result of human processing (e.g., hydrogenation). Some trans fatty acids also occur naturally in the milk and meat of ruminants (such as cattle and sheep). They are produced, by fermentation, in the rumen of these animals. They are also found in dairy products from milk of ruminants, and may be also found in breast milk of women who obtained them from their diet.
The geometric differences between the various types of unsaturated fatty acids, as well as between saturated and unsaturated fatty acids, play an important role in biological processes, and in the construction of biological structures (such as cell membranes).
+ Examples of unsaturated fatty acids Common name Chemical structure Δ C:D IUPAC n−xOmega−3:Eicosapentaenoic acidCHCHCH=CHCHCH=CHCHCH=CHCHCH=CHCHCH=CH(CH)COOHcis,cis,cis,cis,cis-Δ,Δ,Δ,Δ,Δ20:520:5(5,8,11,14,17)n−3α-Linolenic acidCHCHCH=CHCHCH=CHCHCH=CH(CH)COOHcis,cis,cis-Δ,Δ,Δ18:318:3(9,12,15)n−3Docosahexaenoic acidCHCHCH=CHCHCH=CHCHCH=CHCHCH=CHCHCH=CHCHCH=CH(CH)COOHcis,cis,cis,cis,cis,cis-Δ,Δ,Δ,Δ,Δ,Δ22:622:6(4,7,10,13,16,19)n−3Omega−6:Arachidonic acidCH(CH)CH=CHCHCH=CHCHCH=CHCHCH=CH(CH)COOHNIST cis,cis,cis,cis-ΔΔ,Δ,Δ20:420:4(5,8,11,14)n−6Linoleic acidCH(CH)CH=CHCHCH=CH(CH)COOHcis,cis-Δ,Δ18:218:2(9,12)n−6Linolelaidic acidCH(CH)CH=CHCHCH=CH(CH)COOHtrans,trans-Δ,Δ18:218:2(9t,12t)n−6Omega−9:Oleic acid CH(CH)CH=CH(CH)COOH cis-Δ 18:1 18:1(9) n−9Elaidic acid CH(CH)CH=CH(CH)COOH trans-Δ 18:1 18:1(9t) n−9Erucic acid CH(CH)CH=CH(CH)COOH cis-Δ 22:1 22:1(13) n−9Omega−5, 7, and 10:Myristoleic acid CH(CH)CH=CH(CH)COOH cis-Δ 14:1 14:1(9) n−5Palmitoleic acid CH(CH)CH=CH(CH)COOH cis-Δ 16:1 16:1(9) n−7Vaccenic acidCH(CH)CH=CH(CH)COOHtrans-Δ18:118:1(11t)n−7Sapienic acid CH(CH)CH=CH(CH)COOH cis-Δ 16:1 16:1(6) n−10
Even- vs odd-chained fatty acids Most naturally occurring fatty acids are even-chained, e.g. stearic (C18:0) and oleic (C18:1), meaning they are composed of an even number of carbon atoms; odd-chained fatty acids (OCFA) also occur, albeit far less frequently. The most common OCFA are the saturated C15 and C17 derivatives, pentadecanoic acid and heptadecanoic acid respectively, which are found in dairy products. On a molecular level, OCFAs are biosynthesized and metabolized slightly differently from the even-chained relatives.
Branching Most common fatty acids are straight-chain compounds, with no additional carbon atoms bonded as side groups to the main hydrocarbon chain. Branched-chain fatty acids contain one or more methyl groups bonded to the hydrocarbon chain.
Nomenclature
Carbon atom numbering
Most naturally occurring fatty acids have an unbranched chain of carbon atoms, with a carboxyl group (–COOH) at one end, and a methyl group (–CH3) at the other end.
The position of each carbon atom in the backbone of a fatty acid is usually indicated by counting from 1 at the −COOH end. Carbon number x is often abbreviated C-x (or sometimes Cx), with x = 1, 2, 3, etc. This is the numbering scheme recommended by the IUPAC.
Another convention uses letters of the Greek alphabet in sequence, starting with the first carbon after the carboxyl group. Thus carbon α (alpha) is C-2, carbon β (beta) is C-3, and so forth.
Although fatty acids can be of diverse lengths, in this second convention the last carbon in the chain is always labelled as ω (omega), which is the last letter in the Greek alphabet. A third numbering convention counts the carbons from that end, using the labels "ω", "ω−1", "ω−2". Alternatively, the label "ω−x" is written "n−x", where the "n" is meant to represent the number of carbons in the chain.
In either numbering scheme, the position of a double bond in a fatty acid chain is always specified by giving the label of the carbon closest to the carboxyl end. Thus, in an 18 carbon fatty acid, a double bond between C-12 (or ω−6) and C-13 (or ω−5) is said to be "at" position C-12 or ω−6. The IUPAC naming of the acid, such as "octadec-12-enoic acid" (or the more pronounceable variant "12-octadecanoic acid") is always based on the "C" numbering.
The notation Δx,y,... is traditionally used to specify a fatty acid with double bonds at positions x,y,.... (The capital Greek letter "Δ" (delta) corresponds to Roman "D", for Double bond). Thus, for example, the 20-carbon arachidonic acid is Δ5,8,11,14, meaning that it has double bonds between carbons 5 and 6, 8 and 9, 11 and 12, and 14 and 15.
In the context of human diet and fat metabolism, unsaturated fatty acids are often classified by the position of the double bond closest between to the ω carbon (only), even in the case of multiple double bonds such as the essential fatty acids. Thus linoleic acid (18 carbons, Δ9,12), γ-linolenic acid (18-carbon, Δ6,9,12), and arachidonic acid (20-carbon, Δ5,8,11,14) are all classified as "ω−6" fatty acids; meaning that their formula ends with –CH=CH–––––.
Fatty acids with an odd number of carbon atoms are called odd-chain fatty acids, whereas the rest are even-chain fatty acids. The difference is relevant to gluconeogenesis.
Naming of fatty acids The following table describes the most common systems of naming fatty acids.