fatty acids
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In
chemistry Chemistry is the scientific study of the properties and behavior of matter. It is a natural science that covers the elements that make up matter to the compounds made of atoms, molecules and ions: their composition, structure, properties ...

chemistry
, particularly in
biochemistry Biochemistry or biological chemistry is the study of chemical processes within and relating to living organisms. A sub-discipline of both chemistry and biology, biochemistry may be divided into three fields: structural biology, enzymology a ...

biochemistry
, a fatty acid is a
carboxylic acid In organic chemistry, a carboxylic acid is an organic acid that contains a carboxyl group () attached to an R-group. The general formula of a carboxylic acid is or , with substituent, R referring to the alkyl, alkenyl, aryl, or other group. ...
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
ester In chemistry, an ester is a chemical compound, compound derived from an oxoacid (organic or inorganic) in which at least one hydroxyl group () is replaced by an alkoxy group (), as in the substitution reaction of a carboxylic acid and an Alcohol ...

ester
s:
triglyceride
triglyceride
s,
phospholipid
phospholipid
s, 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
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 (SCFA) are fatty acids with aliphatic tails of five or fewer
carbon Carbon () is a chemical element with the chemical symbol, symbol C and atomic number 6. It is nonmetallic and tetravalence, tetravalent—its atom making four electrons available to form covalent bond, covalent chemical bonds. It belongs to gro ...

carbon
s (e.g.
butyric acid
butyric acid
). * Medium-chain fatty acids (MCFA) are fatty acids with aliphatic tails of 6 to 12 carbons, which can form medium-chain triglycerides. * Long-chain fatty acids (LCFA) are fatty acids with aliphatic tails of 13 to 21 carbons. * Very long chain fatty acids (VLCFA) 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, for different ''n''. An important saturated fatty acid is
stearic acid Stearic acid ( , ) is a saturated fatty acid with an 18-carbon chain. The IUPAC name is octadecanoic acid. It is a waxy solid and its chemical formula is C17H35CO2H. Its name comes from the Greek language, Greek word στέαρ "''stéar''", whi ...

stearic acid
(''n'' = 16), which when neutralized with
sodium hydroxide Sodium hydroxide, also known as lye and caustic soda, is an inorganic compound with the formula NaOH. It is a white solid ionic compound consisting of sodium cations and hydroxide anions . Sodium hydroxide is a highly corrosive, caustic base (c ...

sodium hydroxide
is the most common form of
soap Soap is a salt (chemistry), salt of a fatty acid used in a variety of cleansing and lubricating products. In a domestic setting, soaps are surfactants usually used for washing, bathing, and other types of housekeeping. In industrial settings, ...

soap
.


Unsaturated fatty acids

Unsaturated fatty acids have one or more C=C
double bond
double bond
s. 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
oleic acid
, with one double bond, has a "kink" in it, whereas
linoleic acid
linoleic acid
, with two double bonds, has a more pronounced bend.
α-Linolenic acid
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 fat Trans fat, also called trans-unsaturated fatty acids, or trans fatty acids, is a type of unsaturated fat that naturally occurs in small amounts in meat and milk fat. It became widely produced as an unintentional byproduct in the industrial pro ...
s) are not found in nature and are the result of human processing (e.g.,
hydrogenation
hydrogenation
). Some trans fatty acids also occur naturally in the milk and meat of
ruminant Ruminants ( suborder Ruminantia) are hoofed herbivorous grazing or browsing mammals that are able to acquire nutrients from plant-based food by fermenting it in a specialized stomach prior to digestion, principally through microbial actions. ...
s (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 Breast milk (sometimes spelled as breastmilk) or mother's milk is milk produced by mammary glands located in the breast of a human female. Breast milk is the primary source of nutrition for newborns, containing fat, protein, carbohydrates (lactos ...
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).


Even- vs odd-chained fatty acids

Most fatty acids are even-chained, e.g. stearic (C18) and oleic (C18), meaning they are composed of an even number of carbon atoms. Some fatty acids have odd numbers of carbon atoms; they are referred to as odd-chained fatty acids (OCFA). 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.


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 In organic chemistry, a methyl group is an alkyl derived from methane, containing one carbon atom chemical bond, bonded to three hydrogen atoms, having chemical formula . In chemical formula, formulas, the group is often skeletal formula#Pseudoe ...

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 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 The Greek alphabet has been used to write the Greek language since the late 9th or early 8th century BCE. It is derived from the earlier Phoenician alphabet, and was the earliest known alphabetic script to have distinct letters for vowels as we ...

Greek alphabet
in sequence, starting with the first carbon ''after'' the carboxyl group. Thus carbon α (
alpha Alpha (uppercase , lowercase ; grc, ἄλφα, ', or ell, άλφα, álfa) is the first Letter (alphabet), letter of the Greek alphabet. In the system of Greek numerals, it has a value of one. Alpha is derived from the Phoenician alphabet, P ...

alpha
) is C-2, carbon β (
beta
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
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.A common mistake is to say that the last carbon is "ω−1".
Another common mistake is to say that the position of a bond in omega-notation is the number of the carbon closest to the END.
For double bonds, these two mistakes happen to compensate each other; so that a "ω−3" fatty acid indeed has the double bond between the 3rd and 4th carbons from the end, counting the methyl as 1.
However, for substitutions and other purposes, they don't: a hydroxyl "at ω−3" is on carbon 15 (4th from the end), not 16. See for example this article.
Note also that the "−" in the omega-notation is a minus sign, and "ω−3" should in principle be read "omega minus three". However, it is very common (especially in non-scientific literature) to write it "ω-3" (with a hyphen/dash) and read it as "omega-three". See for example Karen Dooley (2008)
Omega-three fatty acids and diabetes
In either numbering scheme, the position of a
double bond
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 Δ''x'',''y'',... is traditionally used to specify a fatty acid with double bonds at positions ''x'',''y'',.... (The capital Greek letter "Δ" ( delta) corresponds to "D", for Double bond). Thus, for example, the 20-carbon
arachidonic acid Arachidonic acid (AA, sometimes ARA) is a polyunsaturated omega-6 fatty acid 20:4(ω-6), or 20:4(5,8,11,14). It is structurally related to the saturated arachidic acid found in Cupuaçu, cupuaçu butter. Its name derives from the New Latin word ...

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 to the ω carbon (only), even in the case of multiple double bonds such as the essential fatty acids. Thus (18 carbons, Δ9,12), (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 In science, a formula is a concise way of expressing information symbolically, as in a mathematical formula or a ''chemical formula''. The informal use of the terminology, term ''formula'' in science refers to the Commensurability (philosophy o ...
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.


Free fatty acids

When circulating in the
plasma Plasma or plasm may refer to: Science * Plasma (physics) Plasma () 1, where \nu_ is the electron gyrofrequency and \nu_ is the electron collision rate. It is often the case that the electrons are magnetized while the ions are not. Magnetized ...
(plasma fatty acids), not in their
ester In chemistry, an ester is a chemical compound, compound derived from an oxoacid (organic or inorganic) in which at least one hydroxyl group () is replaced by an alkoxy group (), as in the substitution reaction of a carboxylic acid and an Alcohol ...

ester
, fatty acids are known as non-esterified fatty acids (NEFAs) or free fatty acids (FFAs). FFAs are always bound to a
transport protein A transport protein (variously referred to as a transmembrane pump, transporter, escort protein, acid transport protein, cation transport protein, or anion transport protein) is a protein that serves the function of moving other materials within ...
, such as
albumin Albumin is a protein family, family of globular proteins, the most common of which are the serum albumins. All the proteins of the albumin family are water-solubility, soluble, moderately soluble in concentrated salt solutions, and experience h ...
. FFAs also form from food oils and fats by hydrolysis, contributing to the characteristic rancid odor. An analogous process happens in
biodiesel Biodiesel is a form of diesel fuel derived from plants or animals and consisting of long-chain fatty acid esters. It is typically made by chemically reacting lipids such as animal fat (tallow), soybean oil, or some other vegetable oil with ...
with risk of part corrosion.


Production


Industrial

Fatty acids are usually produced industrially by the
hydrolysis Hydrolysis (; ) is any chemical reaction in which a molecule of water breaks one or more chemical bonds. The term is used broadly for substitution reaction, substitution, elimination reaction, elimination, and solvation reactions in which water ...
of s, with the removal of
glycerol Glycerol (), also called glycerine in British English and glycerin in American English, is a simple triol compound. It is a colorless, odorless, viscous liquid that is sweet-tasting and non-toxic. The glycerol backbone is found in lipids known ...
(see oleochemicals).
Phospholipid Phospholipids, are a class of lipids whose molecule has a hydrophile, hydrophilic "head" containing a phosphate group and two hydrophobic "tails" derived from fatty acids, joined by an Alcohol (chemistry), alcohol residue (usually a glycerol mol ...
s represent another source. Some fatty acids are produced synthetically by hydrocarboxylation of alkenes.


By animals

In animals, fatty acids are formed from carbohydrates predominantly in the
liver The liver is a major Organ (anatomy), organ only found in vertebrates which performs many essential biological functions such as detoxification of the organism, and the Protein biosynthesis, synthesis of proteins and biochemicals necessary for ...
,
adipose tissue Adipose tissue, body fat, or simply fat is a loose connective tissue composed mostly of adipocytes. In addition to adipocytes, adipose tissue contains the stromal vascular fraction (SVF) of cells including preadipocytes, fibroblasts, vascular en ...
, and the
mammary glands A mammary gland is an exocrine gland in human Humans (''Homo sapiens'') are the most abundant and widespread species of primate, characterized by bipedality, bipedalism and exceptional cognitive skills due to a large and complex Human ...
during lactation. Carbohydrates are converted into
pyruvate Pyruvic acid (CH3COCOOH) is the simplest of the keto acids, alpha-keto acids, with a carboxylic acid and a ketone functional group. Pyruvate, the conjugate acid, conjugate base, CH3COCOO−, is an intermediate in several metabolic pathways throug ...
by
glycolysis Glycolysis is the metabolic pathway that converts glucose () into pyruvic acid, pyruvate (). The Thermodynamic free energy, free energy released in this process is used to form the high-energy molecules adenosine triphosphate (ATP) and NADH, red ...
as the first important step in the conversion of carbohydrates into fatty acids. Pyruvate is then decarboxylated to form
acetyl-CoA Acetyl-CoA (acetyl coenzyme A) is a molecule that participates in many biochemical reactions in protein, carbohydrate and lipid metabolism Metabolism (, from el, μεταβολή ''metabolē'', "change") is the set of life-sustaining che ...
in the
mitochondrion A mitochondrion (; ) is an organelle found in the Cell (biology), cells of most Eukaryotes, such as animals, plants and Fungus, fungi. Mitochondria have a double lipid bilayer, membrane structure and use aerobic respiration to generate adenosi ...
. However, this acetyl CoA needs to be transported into
cytosol The cytosol, also known as cytoplasmic matrix or groundplasm, is one of the liquids found inside cell (biology), cells (Fluid compartments#Intracellular compartment, intracellular fluid (ICF)). It is separated into compartments by membranes. For ...
where the synthesis of fatty acids occurs. This cannot occur directly. To obtain cytosolic acetyl-CoA,
citrate Citric acid is an organic compound with the chemical formula HOC(CO2H)(CH2CO2H)2. It is a Transparency and translucency, colorless Weak acid, weak organic acid. It occurs naturally in Citrus, citrus fruits. In biochemistry, it is an intermedi ...
(produced by the condensation of acetyl-CoA with
oxaloacetate Oxaloacetic acid (also known as oxalacetic acid or OAA) is a crystalline organic compound with the chemical formula HO2CC(O)CH2CO2H. Oxaloacetic acid, in the form of its conjugate base oxaloacetate, is a metabolic intermediate in many processes ...
) is removed from the
citric acid cycle The citric acid cycle (CAC)—also known as the Krebs cycle or the TCA cycle (tricarboxylic acid cycle)—is a series of chemical reactions to release stored energy through the Redox, oxidation of acetyl-CoA derived from carbohydrates, fats, and p ...
and carried across the inner mitochondrial membrane into the cytosol. There it is cleaved by ATP citrate lyase into acetyl-CoA and oxaloacetate. The oxaloacetate is returned to the mitochondrion as
malate Malic acid is an organic compound with the molecular formula . It is a dicarboxylic acid that is made by all living organisms, contributes to the sour taste of fruits, and is used as a food additive. Malic acid has two stereoisomeric forms (L ...
. The cytosolic acetyl-CoA is carboxylated by acetyl CoA carboxylase into malonyl-CoA, the first committed step in the synthesis of fatty acids. Malonyl-CoA is then involved in a repeating series of reactions that lengthens the growing fatty acid chain by two carbons at a time. Almost all natural fatty acids, therefore, have even numbers of carbon atoms. When synthesis is complete the free fatty acids are nearly always combined with glycerol (three fatty acids to one glycerol molecule) to form s, the main storage form of fatty acids, and thus of energy in animals. However, fatty acids are also important components of the s that form the phospholipid bilayers out of which all the membranes of the cell are constructed (the
cell wall A cell wall is a structural layer surrounding some types of cells, just outside the cell membrane The cell membrane (also known as the plasma membrane (PM) or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biolog ...
, and the membranes that enclose all the
organelles In cell biology Cell biology (also cellular biology or cytology) is a branch of biology that studies the Anatomy, structure, Physiology, function, and behavior of cell (biology), cells. All living organisms are made of cells. A cell is the bas ...
within the cells, such as the
nucleus Nucleus (plural, : nuclei) is a Latin word for the seed inside a fruit. It most often refers to: *Atomic nucleus, the very dense central region of an atom *Cell nucleus, a central organelle of a eukaryotic cell, containing most of the cell's DNA ...
, the
mitochondria A mitochondrion (; ) is an organelle found in the cells of most Eukaryotes, such as animals, plants and fungi. Mitochondria have a double membrane structure and use aerobic respiration to generate adenosine triphosphate (ATP), which i ...
,
endoplasmic reticulum The endoplasmic reticulum (ER) is, in essence, the transportation system of the eukaryotic cell, and has many other important functions such as protein folding. It is a type of organelle made up of two subunits – rough endoplasmic reticulum ...
, and the
Golgi apparatus The Golgi apparatus (), also known as the Golgi complex, Golgi body, or simply the Golgi, is an organelle found in most eukaryotic Cell (biology), cells. Part of the endomembrane system in the cytoplasm, it protein targeting, packages proteins ...
). The "uncombined fatty acids" or "free fatty acids" found in the circulation of animals come from the breakdown (or
lipolysis Lipolysis is the metabolic pathway through which lipid triglycerides are hydrolysis, hydrolyzed into a glycerol and free fatty acids. It is used to mobilize stored energy during fasting or exercise, and usually occurs in Adipose tissue, fat adip ...
) of stored triglycerides. Because they are insoluble in water, these fatty acids are transported bound to plasma
albumin Albumin is a protein family, family of globular proteins, the most common of which are the serum albumins. All the proteins of the albumin family are water-solubility, soluble, moderately soluble in concentrated salt solutions, and experience h ...
. The levels of "free fatty acids" in the blood are limited by the availability of albumin binding sites. They can be taken up from the blood by all cells that have mitochondria (with the exception of the cells of the
central nervous system The central nervous system (CNS) is the part of the nervous system consisting primarily of the brain and spinal cord. The CNS is so named because the brain integrates the received information and coordinates and influences the activity of all par ...
). Fatty acids can only be broken down in mitochondria, by means of beta-oxidation followed by further combustion in the
citric acid cycle The citric acid cycle (CAC)—also known as the Krebs cycle or the TCA cycle (tricarboxylic acid cycle)—is a series of chemical reactions to release stored energy through the Redox, oxidation of acetyl-CoA derived from carbohydrates, fats, and p ...
to CO and water. Cells in the central nervous system, although they possess mitochondria, cannot take free fatty acids up from the blood, as the
blood–brain barrier The blood–brain barrier (BBB) is a highly selective semipermeable membrane, semipermeable border of endothelium, endothelial cells that prevents solutes in the circulating blood from ''non-selectively'' crossing into the extracellular fluid of ...
is impervious to most free fatty acids, excluding
short-chain fatty acid Short-chain fatty acids (SCFAs) are fatty acid In chemistry, particularly in biochemistry, a fatty acid is a carboxylic acid with an aliphatic chain, which is either saturated and unsaturated compounds#Organic chemistry, saturated or unsaturated. ...
s and medium-chain fatty acids. These cells have to manufacture their own fatty acids from carbohydrates, as described above, in order to produce and maintain the phospholipids of their cell membranes, and those of their organelles.


Variation between animal species

Studies on the
cell membrane The cell membrane (also known as the plasma membrane (PM) or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane that separates and protects the cytoplasm, interior of all Cell (biology), cells from th ...
s of
mammal Mammals () are a group of vertebrate animals constituting the class Mammalia (), characterized by the presence of mammary glands which in females produce milk for feeding (nursing) their young, a neocortex (a region of the brain), fu ...
s and
reptile Reptiles, as most commonly defined are the animals in the Class (biology), class Reptilia ( ), a paraphyletic grouping comprising all sauropsid, sauropsids except birds. Living reptiles comprise turtles, crocodilians, Squamata, squamates (lizar ...
s discovered that mammalian cell membranes are composed of a higher proportion of polyunsaturated fatty acids ( DHA,
omega-3 fatty acid Omega−3 fatty acids, also called Omega-3 oils, ω−3 fatty acids or ''n''−3 fatty acids, are polyunsaturated fatty acids (PUFAs) characterized by the presence of a double bond, three atoms away from the terminal methyl group in their chem ...
) than
reptile Reptiles, as most commonly defined are the animals in the Class (biology), class Reptilia ( ), a paraphyletic grouping comprising all sauropsid, sauropsids except birds. Living reptiles comprise turtles, crocodilians, Squamata, squamates (lizar ...
s. Studies on bird fatty acid composition have noted similar proportions to mammals but with 1/3rd less omega-3 fatty acids as compared to
omega-6 Omega-6 fatty acids (also referred to as ω-6 fatty acids or ''n''-6 fatty acids) are a family of polyunsaturated fatty acids that have in common a final carbon-carbon double bond#Bond order, double bond in the Fatty acid#Nomenclature, ''n''-6 p ...
for a given body size. This fatty acid composition results in a more fluid cell membrane but also one that is permeable to various ions ( & ), resulting in cell membranes that are more costly to maintain. This maintenance cost has been argued to be one of the key causes for the high metabolic rates and concomitant
warm-blooded Warm-blooded is an informal term referring to animal species which can maintain a body temperature higher than their environment. In particular, homeothermy, homeothermic species maintain a stable body temperature by regulating metabolic proce ...
ness of mammals and birds. However polyunsaturation of cell membranes may also occur in response to chronic cold temperatures as well. In
fish Fish are Aquatic animal, aquatic, craniate, gill-bearing animals that lack Limb (anatomy), limbs with Digit (anatomy), digits. Included in this definition are the living hagfish, lampreys, and Chondrichthyes, cartilaginous and bony fish as we ...
increasingly cold environments lead to increasingly high cell membrane content of both monounsaturated and polyunsaturated fatty acids, to maintain greater membrane fluidity (and functionality) at the lower
temperature Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measurement, measured with a thermometer. Thermometers are calibrated in various Conversion of units of temperature, temp ...
s.


Fatty acids in dietary fats

The following table gives the fatty acid,
vitamin E Vitamin E is a group of eight fat soluble compounds that include four tocopherols and four tocotrienols. Vitamin E deficiency, which is rare and usually due to an underlying problem with digesting dietary fat rather than from a diet low in vitami ...
and
cholesterol Cholesterol is any of a class of certain organic compound, organic molecules called lipids. It is a sterol (or chemical modification, modified steroid), a type of lipid. Cholesterol is biosynthesis, biosynthesized by all animal Cell (biology)# ...
composition of some common dietary fats.


Reactions of fatty acids

Fatty acids exhibit reactions like other carboxylic acids, i.e. they undergo
esterification In chemistry Chemistry is the scientific study of the properties and behavior of matter. It is a natural science that covers the elements that make up matter to the compounds made of atoms, molecules and ions: their composition, stru ...
and acid-base reactions.


Acidity

Fatty acids do not show a great variation in their acidities, as indicated by their respective p''K''a. Nonanoic acid, for example, has a p''K'' of 4.96, being only slightly weaker than acetic acid (4.76). As the chain length increases, the solubility of the fatty acids in water decreases, so that the longer-chain fatty acids have minimal effect on the pH of an aqueous solution. Near neutral pH, fatty acids exist at their conjugate bases, i.e. oleate, etc. Solutions of fatty acids in
ethanol Ethanol (abbr. EtOH; also called ethyl alcohol, grain alcohol, drinking alcohol, or simply alcohol) is an organic compound. It is an Alcohol (chemistry), alcohol with the chemical formula . Its formula can be also written as or (an ethyl ...
can be titrated with
sodium hydroxide Sodium hydroxide, also known as lye and caustic soda, is an inorganic compound with the formula NaOH. It is a white solid ionic compound consisting of sodium cations and hydroxide anions . Sodium hydroxide is a highly corrosive, caustic base (c ...

sodium hydroxide
solution using
phenolphthalein Phenolphthalein ( ) is a chemical compound with the chemical formula, formula carbon, C20hydrogen, H14oxygen, O4 and is often written as "HIn", "HPh", "phph" or simply "Ph" in shorthand notation. Phenolphthalein is often used as an indicator in ...
as an indicator. This analysis is used to determine the free fatty acid content of fats; i.e., the proportion of the triglycerides that have been hydrolyzed. Neutralization of fatty acids, one form of
saponification Saponification is a process of converting esters into soap, soaps and Alcohol (chemistry), alcohols by the action of aqueous alkali (for example, aqueous sodium hydroxide solutions). Soaps are salt (chemistry), salts of fatty acids, which in turn a ...
(soap-making), is a widely practiced route to metallic soaps.


Hydrogenation and hardening

Hydrogenation Hydrogenation is a chemical reaction A chemical reaction is a process that leads to the IUPAC nomenclature for organic transformations, chemical transformation of one set of chemical substances to another. Classically, chemical reactions e ...
of unsaturated fatty acids is widely practiced. Typical conditions involve 2.0–3.0 MPa of H pressure, 150 °C, and nickel supported on silica as a catalyst. This treatment affords saturated fatty acids. The extent of hydrogenation is indicated by the
iodine number Iodine is a chemical element with the Symbol (chemistry), symbol I and atomic number 53. The heaviest of the stable halogens, it exists as a semi-lustrous, non-metallic solid at standard conditions that melts to form a deep violet liquid at , ...
. Hydrogenated fatty acids are less prone toward
rancidification Rancidification is the process of complete or incomplete autoxidation or hydrolysis of fats and oils when exposed to air, light, moisture, or bacterial action, producing short-chain aldehydes, ketones and free fatty acids. When these processes oc ...
. Since the saturated fatty acids are higher melting than the unsaturated precursors, the process is called hardening. Related technology is used to convert vegetable oils into
margarine Margarine (, also , ) is a Spread (food), spread used for flavoring, baking, and cooking. It is most often used as a substitute for butter. Although originally made from animal fats, most margarine consumed today is made from vegetable oil. The ...
. The hydrogenation of triglycerides (vs fatty acids) is advantageous because the carboxylic acids degrade the nickel catalysts, affording nickel soaps. During partial hydrogenation, unsaturated fatty acids can be isomerized from ''cis'' to ''trans'' configuration. More forcing hydrogenation, i.e. using higher pressures of H and higher temperatures, converts fatty acids into
fatty alcohol Fatty alcohols (or long-chain alcohols) are usually high-molecular-weight, straight-chain primary alcohols, but can also range from as few as 4–6 carbons to as many as 22–26, derived from natural fats and oils. The precise chain length varies ...
s. Fatty alcohols are, however, more easily produced from fatty acid
ester In chemistry, an ester is a chemical compound, compound derived from an oxoacid (organic or inorganic) in which at least one hydroxyl group () is replaced by an alkoxy group (), as in the substitution reaction of a carboxylic acid and an Alcohol ...

ester
s. In the Varrentrapp reaction certain unsaturated fatty acids are cleaved in molten alkali, a reaction which was, at one point of time, relevant to structure elucidation.


Auto-oxidation and rancidity

Unsaturated fatty acids and their esters undergo auto-oxidation, which involves replacement of a C-H bond with C-O bond. The process requires oxygen (air) and is accelerated by the presence of traces of metals, which serve as catalysts. Doubly unsaturated fatty acids are particularly prone to this reaction. Vegetable oils resist this process to a small degree because they contain antioxidants, such as
tocopherol Tocopherols (; TCP) are a class of organic compounds, organic chemical compounds (more precisely, various Methyl group, methylated phenols), many of which have vitamin E activity. Because the vitamin activity was first identified in 1936 from a die ...
. Fats and oils often are treated with chelating agents such as
citric acid Citric acid is an organic compound with the chemical formula HOC(CO2H)(CH2CO2H)2. It is a Transparency and translucency, colorless Weak acid, weak organic acid. It occurs naturally in Citrus, citrus fruits. In biochemistry, it is an intermedi ...
to remove the metal catalysts.


Ozonolysis

Unsaturated fatty acids are susceptible to degradation by ozone. This reaction is practiced in the production of
azelaic acid Azelaic acid (AzA) is an organic compound with the formula HOOC(CH2)7carboxylic acid, COOH. This saturated dicarboxylic acid exists as a white powder. It is found in wheat, rye, and barley. It is a precursor to diverse industrial products includi ...
((CH)(COH)) from .


Circulation


Digestion and intake

Short- and medium-chain fatty acids are absorbed directly into the blood via intestine capillaries and travel through the portal vein just as other absorbed nutrients do. However, long-chain fatty acids are not directly released into the intestinal capillaries. Instead they are absorbed into the fatty walls of the intestine villi and reassemble again into s. The triglycerides are coated with
cholesterol Cholesterol is any of a class of certain organic compound, organic molecules called lipids. It is a sterol (or chemical modification, modified steroid), a type of lipid. Cholesterol is biosynthesis, biosynthesized by all animal Cell (biology)# ...
and protein (protein coat) into a compound called a
chylomicron Chylomicrons (from the Greek χυλός, chylos, meaning ''juice'' (of plants or animals), and micron, meaning ''small particle''), also known as ultra low-density lipoproteins (ULDL), are lipoprotein, lipoprotein particles that consist of trigl ...
. From within the cell, the chylomicron is released into a lymphatic capillary called a
lacteal A lacteal is a Lymph capillary, lymphatic capillary that absorbs dietary fats in the Intestinal villus, villi of the small intestine. Triglycerides are emulsified by bile and hydrolyzed by the enzyme lipase, resulting in a mixture of fatty acids, ...
, which merges into larger lymphatic vessels. It is transported via the lymphatic system and the
thoracic duct In human anatomy Anatomy () is the branch of biology concerned with the study of the structure of organisms and their parts. Anatomy is a branch of natural science that deals with the structural organization of living things. It is an old ...
up to a location near the heart (where the arteries and veins are larger). The thoracic duct empties the chylomicrons into the bloodstream via the left
subclavian vein The subclavian vein is a paired large vein, one on either side of the body, that is responsible for draining blood from the upper extremities, allowing this blood to return to the heart. The left subclavian vein plays a key role in the absorption ...
. At this point the chylomicrons can transport the triglycerides to tissues where they are stored or metabolized for energy.


Metabolism

Fatty acids are broken down to CO and water by the intra-cellular
mitochondria A mitochondrion (; ) is an organelle found in the cells of most Eukaryotes, such as animals, plants and fungi. Mitochondria have a double membrane structure and use aerobic respiration to generate adenosine triphosphate (ATP), which i ...
through
beta oxidation In biochemistry and metabolism, beta-oxidation is the catabolism, catabolic process by which fatty acid molecules are broken down in the cytosol in prokaryotes and in the mitochondria in eukaryotes to generate acetyl-CoA, which enters the citric ...
and the
citric acid cycle The citric acid cycle (CAC)—also known as the Krebs cycle or the TCA cycle (tricarboxylic acid cycle)—is a series of chemical reactions to release stored energy through the Redox, oxidation of acetyl-CoA derived from carbohydrates, fats, and p ...
. In the final step (
oxidative phosphorylation Oxidative phosphorylation (UK , US ) or electron transport-linked phosphorylation or terminal oxidation is the metabolic pathway in which Cell (biology), cells use enzymes to Redox, oxidize nutrients, thereby releasing chemical energy in order t ...
), reactions with oxygen release a lot of energy, captured in the form of large quantities of ATP. Many cell types can use either
glucose Glucose is a simple sugar with the Chemical formula#Molecular formula, molecular formula . Glucose is overall the most abundant monosaccharide, a subcategory of carbohydrates. Glucose is mainly made by plants and most algae during photosynthesis f ...
or fatty acids for this purpose, but fatty acids release more energy per gram. Fatty acids (provided either by ingestion or by drawing on triglycerides stored in fatty tissues) are distributed to cells to serve as a fuel for muscular contraction and general metabolism.


Essential fatty acids

Fatty acids that are required for good health but cannot be made in sufficient quantity from other substrates, and therefore must be obtained from food, are called essential fatty acids. There are two series of essential fatty acids: one has a double bond three carbon atoms away from the methyl end; the other has a double bond six carbon atoms away from the methyl end. Humans lack the ability to introduce double bonds in fatty acids beyond carbons 9 and 10, as counted from the carboxylic acid side. Two essential fatty acids are (LA) and
alpha-linolenic acid ''alpha''-Linolenic acid (ALA), also known as α-Linolenic acid (from Greek ''alpha'' meaning "first" and ''linon'' meaning flax Flax, also known as common flax or linseed, is a flowering plant, ''Linum usitatissimum'', in the family Lina ...
(ALA). These fatty acids are widely distributed in plant oils. The human body has a limited ability to convert ALA into the longer-chain
omega-3 fatty acid Omega−3 fatty acids, also called Omega-3 oils, ω−3 fatty acids or ''n''−3 fatty acids, are polyunsaturated fatty acids (PUFAs) characterized by the presence of a double bond, three atoms away from the terminal methyl group in their chem ...
s —
eicosapentaenoic acid Eicosapentaenoic acid (EPA; also icosapentaenoic acid) is an omega-3 fatty acid Omega−3 fatty acids, also called Omega-3 oils, ω−3 fatty acids or ''n''−3 fatty acids, are polyunsaturated fatty acids (PUFAs) characterized by the presenc ...
(EPA) and
docosahexaenoic acid Docosahexaenoic acid (DHA) is an omega-3 fatty acid Omega−3 fatty acids, also called Omega-3 oils, ω−3 fatty acids or ''n''−3 fatty acids, are polyunsaturated fatty acids (PUFAs) characterized by the presence of a double bond, three ...
(DHA), which can also be obtained from fish. Omega-3 and
omega-6 Omega-6 fatty acids (also referred to as ω-6 fatty acids or ''n''-6 fatty acids) are a family of polyunsaturated fatty acids that have in common a final carbon-carbon double bond#Bond order, double bond in the Fatty acid#Nomenclature, ''n''-6 p ...
fatty acids are
biosynthetic Biosynthesis is a multi-step, enzyme-Catalysis, catalyzed process where substrate (chemistry), substrates are converted into more complex Product (chemistry), products in living organisms. In biosynthesis, simple Chemical compound, compounds are mo ...
precursors to
endocannabinoids Cannabinoids () are several structural classes of compounds found in the Cannabis, cannabis plant primarily and most animal organisms (although insects lack such receptors) or as synthetic compounds. The most notable cannabinoid is the phytocannab ...
with antinociceptive,
anxiolytic An anxiolytic (; also antipanic or antianxiety agent) is a medication or other intervention that reduces anxiety (mood), anxiety. This effect is in contrast to anxiogenic agents which increase anxiety. Anxiolytic medications are used for the tre ...
, and neurogenic properties.


Distribution

Blood fatty acids adopt distinct forms in different stages in the blood circulation. They are taken in through the intestine in
chylomicron Chylomicrons (from the Greek χυλός, chylos, meaning ''juice'' (of plants or animals), and micron, meaning ''small particle''), also known as ultra low-density lipoproteins (ULDL), are lipoprotein, lipoprotein particles that consist of trigl ...
s, but also exist in very low density lipoproteins (VLDL) and low density lipoproteins (LDL) after processing in the liver. In addition, when released from adipocytes, fatty acids exist in the blood as
free fatty acids In chemistry, particularly in biochemistry, a fatty acid is a carboxylic acid with an aliphatic chain, which is either saturated and unsaturated compounds#Organic chemistry, saturated or unsaturated. Most naturally occurring fatty acids have an B ...
. It is proposed that the blend of fatty acids exuded by mammalian skin, together with
lactic acid Lactic acid is an organic acid. It has a molecular formula . It is white in the solid state and it is miscibility, miscible with water. When in the dissolved state, it forms a colorless solution. Production includes both artificial synthesis as ...
and
pyruvic acid Pyruvic acid (CH3COCOOH) is the simplest of the keto acids, alpha-keto acids, with a carboxylic acid and a ketone functional group. Pyruvate, the conjugate acid, conjugate base, CH3COCOO−, is an intermediate in several metabolic pathways throug ...
, is distinctive and enables animals with a keen sense of smell to differentiate individuals.


Skin

The
stratum corneum The stratum corneum (Latin language, Latin for 'horny layer') is the outermost layer of the epidermis (skin), epidermis. The human stratum corneum comprises several levels of flattened corneocytes that are divided into two layers: the ''stratum ...
the outermost layer of the
epidermis The epidermis is the outermost of the three layers that comprise the skin, the inner layers being the dermis and Subcutaneous tissue, hypodermis. The epidermis layer provides a barrier to infection from environmental pathogens and regulates the ...
is composed of terminally differentiated and enucleated
corneocyte Corneocytes are terminally cellular differentiation, differentiated keratinocytes and compose most of the stratum corneum, the outermost layer of the epidermis (skin), epidermis. They are regularly replaced through desquamation and renewal from lo ...
s within a lipid matrix. Together with
cholesterol Cholesterol is any of a class of certain organic compound, organic molecules called lipids. It is a sterol (or chemical modification, modified steroid), a type of lipid. Cholesterol is biosynthesis, biosynthesized by all animal Cell (biology)# ...
and
ceramide Ceramides are a family of waxy lipid molecules. A ceramide is composed of N-acetyl sphingosine and a fatty acid In chemistry, particularly in biochemistry, a fatty acid is a carboxylic acid with an aliphatic chain, which is either saturated ...
s, free fatty acids form a water-impermeable barrier that prevents evaporative water loss. Generally, the epidermal lipid matrix is composed of an equimolar mixture of ceramides (about 50% by weight), cholesterol (25%), and free fatty acids (15%). Saturated fatty acids 16 and 18 carbons in length are the dominant types in the epidermis, while unsaturated fatty acids and saturated fatty acids of various other lengths are also present. The relative abundance of the different fatty acids in the epidermis is dependent on the body site the skin is covering. There are also characteristic epidermal fatty acid alterations that occur in
psoriasis Psoriasis is a long-lasting, noncontagious autoimmune disease characterized by skin plaque, raised areas of abnormal skin. These areas are erythema, red, pink, or purple, Xeroderma, dry, itchy, and scaly. Psoriasis varies in severity from small ...
,
atopic dermatitis Atopic dermatitis (AD), also known as atopic eczema, is a long-term type of inflammation of the skin ( dermatitis). It results in itchy, red, swollen, and cracked skin. Clear fluid may come from the affected areas, which often thickens over tim ...
, and other inflammatory conditions.


Analysis

The chemical analysis of fatty acids in lipids typically begins with an interesterification step that breaks down their original esters (triglycerides, waxes, phospholipids etc.) and converts them to
methyl In organic chemistry, a methyl group is an alkyl derived from methane, containing one carbon atom chemical bond, bonded to three hydrogen atoms, having chemical formula . In chemical formula, formulas, the group is often skeletal formula#Pseudoe ...
esters, which are then separated by gas chromatography. or analyzed by
gas chromatography Gas chromatography (GC) is a common type of chromatography used in analytical chemistry for Separation process, separating and analyzing compounds that can be vaporized without Chemical decomposition, decomposition. Typical uses of GC include tes ...
and mid-
infrared spectroscopy Infrared spectroscopy (IR spectroscopy or vibrational spectroscopy) is the measurement of the interaction of infrared radiation with matter by absorption spectroscopy, absorption, emission spectrum, emission, or reflection (physics), reflection. ...
. Separation of unsaturated isomers is possible by silver ion complemented thin-layer chromatography. Other separation techniques include
high-performance liquid chromatography High-performance liquid chromatography (HPLC), formerly referred to as high-pressure liquid chromatography, is a technique in analytical chemistry used to separate, identify, and quantify each component in a mixture. It relies on pumps to pa ...
(with short columns packed with
silica gel Silica gel is an amorphous and porous form of silicon dioxide (silica), consisting of an irregular tridimensional framework of alternating silicon and oxygen Oxygen is the chemical element with the chemical symbol, symbol O and at ...
with bonded phenylsulfonic acid groups whose hydrogen atoms have been exchanged for silver ions). The role of silver lies in its ability to form complexes with unsaturated compounds.


Industrial uses

Fatty acids are mainly used in the production of
soap Soap is a salt (chemistry), salt of a fatty acid used in a variety of cleansing and lubricating products. In a domestic setting, soaps are surfactants usually used for washing, bathing, and other types of housekeeping. In industrial settings, ...

soap
, both for cosmetic purposes and, in the case of metallic soaps, as lubricants. Fatty acids are also converted, via their methyl esters, to
fatty alcohol Fatty alcohols (or long-chain alcohols) are usually high-molecular-weight, straight-chain primary alcohols, but can also range from as few as 4–6 carbons to as many as 22–26, derived from natural fats and oils. The precise chain length varies ...
s and fatty amines, which are precursors to surfactants, detergents, and lubricants. Other applications include their use as
emulsifiers An emulsion is a mixture of two or more liquids that are normally immiscible (unmixable or unblendable) owing to liquid-liquid phase separation. Emulsions are part of a more general class of two-phase systems of matter called colloids. Alt ...
, texturizing agents, wetting agents, anti-foam agents, or stabilizing agents. Esters of fatty acids with simpler alcohols (such as methyl-, ethyl-, n-propyl-, isopropyl- and butyl esters) are used as emollients in cosmetics and other personal care products and as synthetic lubricants. Esters of fatty acids with more complex alcohols, such as
sorbitol Sorbitol (), less commonly known as glucitol (), is a sugar alcohol Sugar alcohols (also called polyhydric alcohols, polyalcohols, alditols or glycitols) are organic compounds, typically derivative (chemistry), derived from sugars, containing o ...
,
ethylene glycol Ethylene glycol (IUPAC nomenclature, IUPAC name: ethane-1,2-diol) is an organic compound (a Vicinal (chemistry), vicinal diol) with the formula . It is mainly used for two purposes, as a raw material in the manufacture of polyester fibers and fo ...
, diethylene glycol, and
polyethylene glycol Polyethylene glycol (PEG; ) is a polyether compound derived from petroleum Petroleum, also known as crude oil, or simply oil, is a naturally occurring yellowish-black liquid mixture of mainly hydrocarbons, and is found in geological for ...
are consumed in food, or used for personal care and water treatment, or used as synthetic lubricants or fluids for metal working.


See also

* Fatty acid synthase *
Fatty acid synthesis In biochemistry, fatty acid synthesis is the creation of fatty acids from acetyl-CoA and NADPH through the action of enzymes called fatty acid synthases. This process takes place in the cytoplasm of the Cell (biology), cell. Most of the acetyl-Co ...
* Fatty aldehyde * List of saturated fatty acids * List of unsaturated fatty acids * List of carboxylic acids *
Vegetable oil Vegetable oils, or vegetable fats, are oils extracted from seeds or from other parts of fruits. Like animal fats, vegetable fats are ''mixtures'' of triglycerides. Soybean oil, grape seed oil, and cocoa butter are examples of seed oils, or fats ...


References


External links


Lipid Library

''Prostaglandins, Leukotrienes & Essential Fatty Acids'' journal


{{DEFAULTSORT:Fatty Acid Commodity chemicals E-number additives