Fatty acid


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 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 was introduced in 1813 by Michel Eugène Chevreul, though he initially used some variant terms: graisse acide and acide huileux.

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

Saturated fatty acids have no C=C double bonds. They have the formula CHCOOH, where n is some positive integer. An important saturated fatty acid is stearic acid, which when neutralized with sodium hydroxide is the most common form of soap.
Common nameChemical structureC :D
Propionic acidCHCHCOOH3:0
Butyric acidCHCOOH4:0
Caprylic acidCHCOOH8:0
Capric acidCHCOOH10:0
Lauric acidCHCOOH12:0
Myristic acidCHCOOH14:0
Palmitic acidCHCOOH16:0
Stearic acidCHCOOH18:0
Arachidic acidCHCOOH20:0
Behenic acidCHCOOH22:0
Lignoceric acidCHCOOH24:0
Cerotic acidCHCOOH26:0

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, six, or nine carbon atoms after it, and all double bonds have a cis configuration. Most fatty acids in the trans configuration are not found in nature and are the result of human processing. Some trans fatty acids also occur naturally in the milk and meat of ruminants. 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.

Even- vs odd-chained fatty acids

Most naturally occurring fatty acids are even-chained, e.g. stearic and oleic, meaning they are composed of an even number of carbon atoms; odd-chained fatty acids 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 at one end, and a methyl group 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, 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 is C-2, carbon β 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, 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 and C-13 is said to be "at" position C-12 or ω−6. The IUPAC naming of the acid, such as "octadec-12-enoic 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,..... 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, even in the case of multiple double bonds such as the essential fatty acids. Thus linoleic acid, γ-linolenic acid, and arachidonic acid 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.
NomenclatureExamplesExplanation
TrivialPalmitoleic acidTrivial names are non-systematic historical names, which are the most frequent naming system used in literature. Most common fatty acids have trivial names in addition to their systematic names. These names frequently do not follow any pattern, but they are concise and often unambiguous.
Systematiccis-9-octadec-9-enoic acid
-octadec-9-enoic acid		Systematic names derive from the standard IUPAC Rules for the Nomenclature of Organic Chemistry, published in 1979, along with a recommendation published specifically for lipids in 1977. Carbon atom numbering begins from the carboxylic end of the molecule backbone. Double bonds are labelled with cis-/trans- notation or E-/Z- notation, where appropriate. This notation is generally more verbose than common nomenclature, but has the advantage of being more technically clear and descriptive.
Δxcis-Δ9, cis-Δ12 octadecadienoic acidIn Δx 'nomenclature, each double bond is indicated by Δx'', where the double bond begins at the xth carbon–carbon bond, counting from carboxylic end of the molecule backbone. Each double bond is preceded by a cis- or trans- prefix, indicating the configuration of the molecule around the bond. For example, linoleic acid is designated "cis-Δ9, cis-Δ12 octadecadienoic acid". This nomenclature has the advantage of being less verbose than systematic nomenclature, but is no more technically clear or descriptive.
n−''x
n''−3
n−x nomenclature both provides names for individual compounds and classifies them by their likely biosynthetic properties in animals. A double bond is located on the xth carbon–carbon bond, counting from the methyl end of the molecule backbone. For example, α-linolenic acid is classified as a n−3 or omega−3 fatty acid, and so it is likely to share a biosynthetic pathway with other compounds of this type. The ω−x, omega−x, or "omega" notation is common in popular nutritional literature, but IUPAC has deprecated it in favor of n−''x notation in technical documents. The most commonly researched fatty acid biosynthetic pathways are n''−3 and n−6.
Lipid numbers18:3
18:3n3
18:3, cis,''cis,cis-Δ9,Δ12,Δ15
18:3		Lipid numbers' take the form C:''D, where C'' is the number of carbon atoms in the fatty acid and D is the number of double bonds in the fatty acid. If D is more than one, the double bonds are assumed to be interrupted by units, i.e., at intervals of 3 carbon atoms along the chain. For instance, α-linolenic acid is an 18:3 fatty acid and its three double bonds are located at positions Δ9, Δ12, and Δ15. This notation can be ambiguous, as some different fatty acids can have the same C:''D numbers. Consequently, when ambiguity exists this notation is usually paired with either a Δx'' or n−''x'' term. For instance, although α-linolenic acid and γ-linolenic acid are both 18:3, they may be unambiguously described as 18:3n3 and 18:3n6 fatty acids, respectively. For the same purpose, IUPAC recommends using a list of double bond positions in parentheses, appended to the C:D notation. For instance, IUPAC recommended notations for α- and γ-linolenic acid are 18:3 and 18:3, respectively.