Lipids are a broad group of naturally-occurring molecules which includes
fat
In nutrition, biology, and chemistry, fat usually means any ester of fatty acids, or a mixture of such compounds, most commonly those that occur in living beings or in food.
The term often refers specifically to triglycerides (triple est ...
s,
wax
Waxes are a diverse class of organic compounds that are lipophilic, malleable solids near ambient temperatures. They include higher alkanes and lipids, typically with melting points above about 40 °C (104 °F), melting to giv ...
es,
sterols,
fat-soluble vitamin
A vitamin is an organic molecule (or a set of molecules closely related chemically, i.e. vitamers) that is an essential micronutrient that an organism needs in small quantities for the proper functioning of its metabolism. Essential nutri ...
s (such as vitamins
A,
D,
E and
K),
monoglyceride
Monoglycerides (also: acylglycerols or monoacylglycerols) are a class of glycerides which are composed of a molecule of glycerol linked to a fatty acid via an ester bond. As glycerol contains both primary and secondary alcohol groups two differen ...
s,
diglycerides,
phospholipids, and others. The functions of lipids include storing energy,
signaling, and acting as structural components of
cell membranes.
Lipids have applications in the
cosmetic and
food industries, and in
nanotechnology.
Lipids may be broadly defined as
hydrophobic or
amphiphilic
An amphiphile (from the Greek αμφις amphis, both, and φιλíα philia, love, friendship), or amphipath, is a chemical compound possessing both hydrophilic (''water-loving'', polar) and lipophilic (''fat-loving'') properties. Such a compo ...
small molecules; the amphiphilic nature of some lipids allows them to form structures such as
vesicles, multilamellar/
unilamellar liposome
A unilamellar liposome is a spherical liposome, a vesicle, bounded by a single bilayer of an amphiphilic lipid or a mixture of such lipids, containing aqueous solution inside the chamber. Unilamellar liposomes are used to study biological systems a ...
s, or membranes in an aqueous environment. Biological lipids originate entirely or in part from two distinct types of biochemical subunits or "building-blocks":
ketoacyl and
isoprene groups.
Using this approach, lipids may be divided into eight categories:
fatty acyls,
glycerolipid
Lipids are a broad group of naturally-occurring molecules which includes fats, waxes, sterols, fat-soluble vitamins (such as vitamins A, D, E and K), monoglycerides, diglycerides, phospholipids, and others. The functions of lipids include s ...
s,
glycerophospholipid
Glycerophospholipids or phosphoglycerides are glycerol-based phospholipids. They are the main component of biological membranes. Two major classes are known: those for bacteria and eukaryotes and a separate family for archaea.
Structures
The t ...
s,
sphingolipid
Sphingolipids are a class of lipids containing a backbone of sphingoid bases, a set of aliphatic amino alcohols that includes sphingosine. They were discovered in brain extracts in the 1870s and were named after the mythological sphinx because o ...
s,
saccharolipid
Saccharolipids are chemical compounds containing fatty acids linked directly to a sugar backbone, forming structures that are compatible with membrane bilayers. In the saccharolipids, a monosaccharide substitutes for the glycerol backbone present ...
s, and
polyketide
Polyketides are a class of natural products derived from a precursor molecule consisting of a chain of alternating ketone (or reduced forms of a ketone) and methylene groups: (-CO-CH2-). First studied in the early 20th century, discovery, biosynth ...
s (derived from condensation of ketoacyl subunits); and sterol lipids and prenol lipids (derived from condensation of isoprene subunits).
Although the term "lipid" is sometimes used as a synonym for
fat
In nutrition, biology, and chemistry, fat usually means any ester of fatty acids, or a mixture of such compounds, most commonly those that occur in living beings or in food.
The term often refers specifically to triglycerides (triple est ...
s, fats are a subgroup of lipids called
triglycerides. Lipids also encompass molecules such as
fatty acids and their derivatives (including
tri-,
di-,
monoglyceride
Monoglycerides (also: acylglycerols or monoacylglycerols) are a class of glycerides which are composed of a molecule of glycerol linked to a fatty acid via an ester bond. As glycerol contains both primary and secondary alcohol groups two differen ...
s, and
phospholipids), as well as other
sterol-containing
metabolites such as
cholesterol.
Although humans and other mammals use various
biosynthetic pathway
Metabolism (, from el, μεταβολή ''metabolē'', "change") is the set of life-sustaining chemical reactions in organisms. The three main functions of metabolism are: the conversion of the energy in food to energy available to run cell ...
s both to break down and to synthesize lipids, some essential lipids can't be made this way and must be obtained from the diet.
History
In 1815,
Henri Braconnot
Henri Braconnot (29 May 1780 – 13 January 1855) was a French chemist and pharmacist.
He was born in Commercy, his father being a counsel at the local parliament. At the death of his father, in 1787, Henri began his instruction in an elementar ...
classified lipids (''graisses'') in two categories, ''suifs'' (solid greases or tallow) and ''huiles'' (fluid oils).
In 1823,
Michel Eugène Chevreul developed a more detailed classification, including oils, greases, tallow, waxes, resins, balsams and volatile oils (or essential oils).
The first synthetic triglyceride was reported by
Théophile-Jules Pelouze
Théophile-Jules Pelouze (also known as Jules Pelouze, Théophile Pelouze, Theo Pelouze, or T. J. Pelouze, ; 26 February 180731 May 1867) was a French chemist.
Life
He was born at Valognes, and died in Paris.
His father, Edmond Pelouze, was an ...
in 1844, when he produced
tributyrin by treating
butyric acid with
glycerin
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 ...
in the presence of concentrated
sulfuric acid. Several years later,
Marcellin Berthelot
Pierre Eugène Marcellin Berthelot (; 25 October 1827 – 18 March 1907) was a French chemist and Republican politician noted for the ThomsenBerthelot principle of thermochemistry. He synthesized many organic compounds from inorganic substa ...
, one of Pelouze's students, synthesized
tristearin
Stearin , or tristearin, or glyceryl tristearate is an odourless, white powder. It is a triglyceride derived from three units of stearic acid. Most triglycerides are derived from at least two and more commonly three different fatty acids. Like o ...
and
tripalmitin
Tripalmitin is a triglyceride derived from the fatty acid palmitic acid
Palmitic acid (hexadecanoic acid in IUPAC nomenclature) is a fatty acid with a 16-carbon chain. It is the most common saturated fatty acid found in animals, plants and mic ...
by reaction of the analogous
fatty acids with glycerin in the presence of gaseous
hydrogen chloride at high temperature.
In 1827,
William Prout
William Prout FRS (; 15 January 1785 – 9 April 1850) was an English chemist, physician, and natural theologian. He is remembered today mainly for what is called Prout's hypothesis.
Biography
Prout was born in Horton, Gloucestershire in ...
recognized fat ("oily" alimentary matters), along with protein ("albuminous") and carbohydrate ("saccharine"), as an important nutrient for humans and animals.
For a century, chemists regarded "fats" as only simple lipids made of fatty acids and glycerol (glycerides), but new forms were described later.
Theodore Gobley (1847) discovered phospholipids in mammalian brain and hen egg, called by him as "
lecithins".
Thudichum discovered in human brain some phospholipids (
cephalin
Phosphatidylethanolamine (PE) is a class of phospholipids found in biological membranes. They are synthesized by the addition of cytidine diphosphate- ethanolamine to diglycerides, releasing cytidine monophosphate. ''S''-Adenosyl methionine can ...
), glycolipids (
cerebroside
Cerebrosides is the common name for a group of glycosphingolipids called monoglycosylceramides which are important components in animal muscle and nerve cell membranes.
They consist of a ceramide with a single sugar residue at the 1-hydroxyl ...
) and sphingolipids (
sphingomyelin).
The terms lipoid, lipin, lipide and lipid have been used with varied meanings from author to author. In 1912, Rosenbloom and
Gies proposed the substitution of "lipoid" by "lipin". In 1920, Bloor introduced a new classification for "lipoids": simple lipoids (greases and waxes), compound lipoids (phospholipoids and glycolipoids), and the derived lipoids (fatty acids,
alcohols, sterols).
The word ''lipide'', which stems etymologically from Greek λίπος, ''lipos'' 'fat', was introduced in 1923 by the French pharmacologist
Gabriel Bertrand
Gabriel Bertrand (born 17 May 1867 in Paris, died 20 June 1962 in Paris) was a French pharmacologist, biochemist and bacteriologist.
Bertrand introduced into biochemistry both the term “ oxidase” and the concept of trace elements.
The lacc ...
. Bertrand included in the concept not only the traditional fats (glycerides), but also the "lipoids", with a complex constitution.
The word ''lipide'' was unanimously approved by the international commission of the ''Société de Chimie Biologique'' during the plenary session on July 3, 1923. The word ''lipide'' was later anglicized as ''lipid'' because of its pronunciation ('lɪpɪd). In French, the suffix ''-ide'', from Ancient Greek -ίδης (meaning 'son of' or 'descendant of'), is always pronounced (ɪd).
In 1947,
T. P. Hilditch defined "simple lipids" as greases and waxes (true waxes, sterols, alcohols).
Categories
Lipids have been classified into eight categories by the
Lipid MAPS
LIPID MAPS (Lipid Metabolites and Pathways Strategy) is a web portal designed to be a gateway to Lipidomics resources. The resource has spearheaded a classification of biological lipids, dividing them into eight general categories. LIPID MAPS pro ...
consortium
as follows:
Fatty acyls
Fatty acyls, a generic term for describing
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 ...
, their conjugates and derivatives, are a diverse group of molecules synthesized by chain-elongation of an
acetyl-CoA primer with
malonyl-CoA or
methylmalonyl-CoA Methylmalonyl-CoA is the thioester consisting of coenzyme A linked to methylmalonic acid. It is an important intermediate in the biosynthesis of succinyl-CoA, which plays an essential role in the tricarboxylic acid cycle (aka the Citric Acid Cyc ...
groups in a process called
fatty acid synthesis.
They are made of a
hydrocarbon chain that terminates with a
carboxylic acid group; this arrangement confers the molecule with a
polar,
hydrophilic
A hydrophile is a molecule or other molecular entity that is attracted to water molecules and tends to be dissolved by water.Liddell, H.G. & Scott, R. (1940). ''A Greek-English Lexicon'' Oxford: Clarendon Press.
In contrast, hydrophobes are ...
end, and a nonpolar,
hydrophobic end that is
insoluble in water. The fatty acid structure is one of the most fundamental categories of biological lipids and is commonly used as a building-block of more structurally complex lipids. The carbon chain, typically between four and 24 carbons long,
may be saturated or
unsaturated, and may be attached to
functional groups containing
oxygen
Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as ...
,
halogens,
nitrogen
Nitrogen is the chemical element with the symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of group 15 of the periodic table, often called the pnictogens. It is a common element in the universe, estimated at se ...
, and
sulfur. If a fatty acid contains a double bond, there is the possibility of either a ''cis'' or ''trans''
geometric isomerism
Geometry (; ) is, with arithmetic, one of the oldest branches of mathematics. It is concerned with properties of space such as the distance, shape, size, and relative position of figures. A mathematician who works in the field of geometry is ca ...
, which significantly affects the molecule's
configuration. ''Cis''-double bonds cause the fatty acid chain to bend, an effect that is compounded with more double bonds in the chain. Three double bonds in 18-carbon ''
linolenic acid'', the most abundant fatty-acyl chains of plant ''thylakoid membranes'', render these membranes highly ''fluid'' despite environmental low-temperatures,
and also makes linolenic acid give dominating sharp peaks in high resolution 13-C NMR spectra of chloroplasts. This in turn plays an important role in the structure and function of cell membranes.
Most naturally occurring fatty acids are of the ''cis'' configuration, although the ''trans'' form does exist in some natural and partially hydrogenated fats and oils.
Examples of biologically important fatty acids include the
eicosanoid
Eicosanoids are signaling molecules made by the enzymatic or non-enzymatic oxidation of arachidonic acid or other polyunsaturated fatty acids (PUFAs) that are, similar to arachidonic acid, around 20 carbon units in length. Eicosanoids are a s ...
s, derived primarily from
arachidonic acid and
eicosapentaenoic acid
Eicosapentaenoic acid (EPA; also icosapentaenoic acid) is an omega-3 fatty acid. In physiological literature, it is given the name 20:5(n-3). It also has the trivial name timnodonic acid. In chemical structure, EPA is a carboxylic acid with a 20-c ...
, that include
prostaglandins,
leukotriene
Leukotrienes are a family of eicosanoid inflammatory mediators produced in leukocytes by the oxidation of arachidonic acid (AA) and the essential fatty acid eicosapentaenoic acid (EPA) by the enzyme arachidonate 5-lipoxygenase.
Leukotrienes ...
s, and
thromboxane
Thromboxane is a member of the family of lipids known as eicosanoids. The two major thromboxanes are thromboxane A2 and thromboxane B2. The distinguishing feature of thromboxanes is a 6-membered ether-containing ring.
Thromboxane is named for i ...
s.
Docosahexaenoic acid
Docosahexaenoic acid (DHA) is an omega-3 fatty acid that is a primary structural component of the human brain, cerebral cortex, skin, and retina. In physiological literature, it is given the name 22:6(n-3). It can be synthesized from alpha-lin ...
is also important in biological systems, particularly with respect to sight.
Other major lipid classes in the fatty acid category are the fatty esters and fatty amides. Fatty esters include important biochemical intermediates such as
wax esters, fatty acid thioester
coenzyme A derivatives, fatty acid thioester
ACP derivatives and fatty acid carnitines. The fatty amides include
N-acyl ethanolamines, such as the
cannabinoid
Cannabinoids () are several structural classes of compounds found in the cannabis plant primarily and most animal organisms (although insects lack such receptors) or as synthetic compounds. The most notable cannabinoid is the phytocannabinoid tet ...
neurotransmitter
anandamide
Anandamide (ANA), also known as ''N''-arachidonoylethanolamine (AEA), is a fatty acid neurotransmitter. Anandamide was the first endocannabinoid to be discovered: it participates in the body's endocannabinoid system by binding to cannabinoid r ...
.
Glycerolipids
Glycerolipids are composed of mono-, di-, and tri-substituted
glycerols,
the best-known being the fatty acid
triesters of glycerol, called
triglycerides
A triglyceride (TG, triacylglycerol, TAG, or triacylglyceride) is an ester derived from glycerol and three fatty acids (from ''tri-'' and ''glyceride'').
Triglycerides are the main constituents of body fat in humans and other vertebrates, as we ...
. The word "triacylglycerol" is sometimes used synonymously with "triglyceride". In these compounds, the three hydroxyl groups of glycerol are each esterified, typically by different fatty acids. Because they function as an energy store, these lipids comprise the bulk of storage
fat
In nutrition, biology, and chemistry, fat usually means any ester of fatty acids, or a mixture of such compounds, most commonly those that occur in living beings or in food.
The term often refers specifically to triglycerides (triple est ...
in animal tissues. The hydrolysis of the
ester bonds of triglycerides and the release of glycerol and fatty acids from
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 ...
are the initial steps in metabolizing fat.
Additional subclasses of glycerolipids are represented by glycosylglycerols, which are characterized by the presence of one or more
sugar residues attached to glycerol via a
glycosidic linkage
A glycosidic bond or glycosidic linkage is a type of covalent bond that joins a carbohydrate (sugar) molecule to another group, which may or may not be another carbohydrate.
A glycosidic bond is formed between the hemiacetal or hemiketal group ...
. Examples of structures in this category are the digalactosyldiacylglycerols found in plant membranes
and seminolipid from mammalian
sperm cells
A spermatozoon (; also spelled spermatozoön; ; ) is a motile sperm cell, or moving form of the haploid cell that is the male gamete. A spermatozoon joins an ovum to form a zygote. (A zygote is a single cell, with a complete set of chromosomes, ...
.
Glycerophospholipids
Glycerophospholipids, usually referred to as
phospholipids (though
sphingomyelins are also classified as phospholipids), are ubiquitous in nature and are key components of the
lipid bilayer
The lipid bilayer (or phospholipid bilayer) is a thin polar membrane made of two layers of lipid molecules. These membranes are flat sheets that form a continuous barrier around all cells. The cell membranes of almost all organisms and many vir ...
of cells,
as well as being involved in
metabolism
Metabolism (, from el, μεταβολή ''metabolē'', "change") is the set of life-sustaining chemical reactions in organisms. The three main functions of metabolism are: the conversion of the energy in food to energy available to run c ...
and
cell signaling
In biology, cell signaling (cell signalling in British English) or cell communication is the ability of a cell to receive, process, and transmit signals with its environment and with itself. Cell signaling is a fundamental property of all cellula ...
.
Neural tissue (including the brain) contains relatively high amounts of glycerophospholipids, and alterations in their composition has been implicated in various neurological disorders.
Glycerophospholipids may be subdivided into distinct classes, based on the nature of the polar headgroup at the ''sn''-3 position of the glycerol backbone in
eukaryotes and eubacteria, or the ''sn''-1 position in the case of
archaebacteria
Archaea ( ; singular archaeon ) is a domain of single-celled organisms. These microorganisms lack cell nuclei and are therefore prokaryotes. Archaea were initially classified as bacteria, receiving the name archaebacteria (in the Archaebact ...
.
Examples of glycerophospholipids found in
biological membranes are
phosphatidylcholine
Phosphatidylcholines (PC) are a class of phospholipids that incorporate choline as a headgroup.
They are a major component of biological membranes and can be easily obtained from a variety of readily available sources, such as egg yolk or soybea ...
(also known as PC, GPCho or
lecithin),
phosphatidylethanolamine (PE or GPEtn) and
phosphatidylserine
Phosphatidylserine (abbreviated Ptd-L-Ser or PS) is a phospholipid and is a component of the cell membrane. It plays a key role in cell cycle signaling, specifically in relation to apoptosis. It is a key pathway for viruses to enter cells via ap ...
(PS or GPSer). In addition to serving as a primary component of cellular membranes and binding sites for intra- and intercellular proteins, some glycerophospholipids in eukaryotic cells, such as
phosphatidylinositols and
phosphatidic acid Phosphatidic acids are anionic phospholipids important to cell signaling and direct activation of lipid-gated ion channels. Hydrolysis of phosphatidic acid gives rise to one molecule each of glycerol and phosphoric acid and two molecules of fatty ac ...
s are either precursors of or, themselves, membrane-derived
second messengers.
Typically, one or both of these hydroxyl groups are acylated with long-chain fatty acids, but there are also alkyl-linked and 1Z-alkenyl-linked (
plasmalogen) glycerophospholipids, as well as dialkylether variants in archaebacteria.
Sphingolipids
Sphingolipid
Sphingolipids are a class of lipids containing a backbone of sphingoid bases, a set of aliphatic amino alcohols that includes sphingosine. They were discovered in brain extracts in the 1870s and were named after the mythological sphinx because o ...
s are a complicated family of compounds
that share a common structural feature, a
sphingoid base backbone that is synthesized
''de novo'' from the amino acid
serine and a long-chain fatty acyl CoA, then converted into
ceramide
Ceramides are a family of waxy lipid molecules. A ceramide is composed of N-acetyl sphingosine and a fatty acid. Ceramides are found in high concentrations within the cell membrane of eukaryotic cells, since they are component lipids that make ...
s, phosphosphingolipids, glycosphingolipids and other compounds. The major sphingoid base of mammals is commonly referred to as
sphingosine
Sphingosine (2-amino-4-trans-octadecene-1,3-diol) is an 18-carbon amino alcohol with an unsaturated hydrocarbon chain, which forms a primary part of sphingolipids, a class of cell membrane lipids that include sphingomyelin, an important phos ...
. Ceramides (N-acyl-sphingoid bases) are a major subclass of sphingoid base derivatives with an
amide-linked fatty acid. The fatty acids are typically saturated or mono-unsaturated with chain lengths from 16 to 26 carbon atoms.
The major phosphosphingolipids of mammals are
sphingomyelins (ceramide phosphocholines),
whereas insects contain mainly ceramide phosphoethanolamines
and fungi have phytoceramide phosphoinositols and
mannose-containing headgroups.
The glycosphingolipids are a diverse family of molecules composed of one or more sugar residues linked via a
glycosidic bond
A glycosidic bond or glycosidic linkage is a type of covalent bond that joins a carbohydrate (sugar) molecule to another group, which may or may not be another carbohydrate.
A glycosidic bond is formed between the hemiacetal or hemiketal group ...
to the sphingoid base. Examples of these are the simple and complex glycosphingolipids such as
cerebroside
Cerebrosides is the common name for a group of glycosphingolipids called monoglycosylceramides which are important components in animal muscle and nerve cell membranes.
They consist of a ceramide with a single sugar residue at the 1-hydroxyl ...
s and
gangliosides.
Sterols
Sterols, such as
cholesterol and its derivatives, are an important component of membrane lipids,
along with the glycerophospholipids and sphingomyelins. Other examples of sterols are the
bile acids and their conjugates,
which in mammals are oxidized derivatives of cholesterol and are synthesized in the liver. The plant equivalents are the
phytosterols
Phytosterols are phytosteroids, similar to cholesterol, that serve as structural components of biological membranes of plants. They encompass plant sterols and stanols. More than 250 sterols and related compounds have been identified. Free phytos ...
, such as
β-sitosterol,
stigmasterol
Stigmasterol – a plant sterol (''phytosterol'') – is among the most abundant of plant sterols, having a major function to maintain the structure and physiology of cell membranes. In the European Union, it is a food additive listed with E num ...
, and
brassicasterol; the latter compound is also used as a
biomarker
In biomedical contexts, a biomarker, or biological marker, is a measurable indicator of some biological state or condition. Biomarkers are often measured and evaluated using blood, urine, or soft tissues to examine normal biological processes, p ...
for
algal growth.
The predominant sterol in
fungal cell membranes is
ergosterol.
Sterols are
steroids in which one of the hydrogen atoms is substituted with a
hydroxyl group, at position 3 in the carbon chain. They have in common with steroids the same fused four-ring core structure. Steroids have different biological roles as
hormone
A hormone (from the Greek participle , "setting in motion") is a class of signaling molecules in multicellular organisms that are sent to distant organs by complex biological processes to regulate physiology and behavior. Hormones are require ...
s and
signaling molecules. The eighteen-carbon (C18) steroids include the
estrogen
Estrogen or oestrogen is a category of sex hormone responsible for the development and regulation of the female reproductive system and secondary sex characteristics. There are three major endogenous estrogens that have estrogenic hormonal ac ...
family whereas the C19 steroids comprise the
androgen
An androgen (from Greek ''andr-'', the stem of the word meaning "man") is any natural or synthetic steroid hormone that regulates the development and maintenance of male characteristics in vertebrates by binding to androgen receptors. This in ...
s such as
testosterone and
androsterone
Androsterone, or 3α-hydroxy-5α-androstan-17-one, is an endogenous steroid hormone, neurosteroid, and putative pheromone. It is a weak androgen with a potency that is approximately 1/7 that of testosterone. Androsterone is a metabolite of test ...
. The C21 subclass includes the
progestogens as well as the
glucocorticoids and
mineralocorticoids
Mineralocorticoids are a class of corticosteroids, which in turn are a class of steroid hormones. Mineralocorticoids are produced in the adrenal cortex and influence salt and water balances (electrolyte balance and fluid balance). The primary min ...
.
The
secosteroids, comprising various forms of
vitamin D, are characterized by cleavage of the B ring of the core structure.
Prenols
Prenol
Prenol, or 3-methyl-2-buten-1-ol, is a natural alcohol. It is one of the most simple terpenoids. It is a clear colorless oil that is reasonably soluble in water and miscible with most common organic solvents. It has a fruity odor and is used oc ...
lipids are synthesized from the five-carbon-unit precursors
isopentenyl diphosphate and
dimethylallyl diphosphate, which are produced mainly via the
mevalonic acid (MVA) pathway.
The simple isoprenoids (linear alcohols, diphosphates, etc.) are formed by the successive addition of C5 units, and are classified according to number of these
terpene units. Structures containing greater than 40 carbons are known as polyterpenes.
Carotenoids are important simple isoprenoids that function as
antioxidants and as precursors of
vitamin A.
Another biologically important class of molecules is exemplified by the
quinones and
hydroquinones, which contain an isoprenoid tail attached to a quinonoid core of non-isoprenoid origin.
Vitamin E and
vitamin K, as well as the
ubiquinone
Coenzyme Q, also known as ubiquinone and marketed as CoQ10, is a coenzyme family that is ubiquitous in animals and most bacteria (hence the name ubiquinone). In humans, the most common form is coenzyme Q10 or ubiquinone-10.
It is a 1,4-benzoq ...
s, are examples of this class. Prokaryotes synthesize polyprenols (called
bactoprenols) in which the terminal isoprenoid unit attached to oxygen remains unsaturated, whereas in animal polyprenols (
dolichol
Dolichol refers to any of a group of long-chain mostly unsaturated organic compounds that are made up of varying numbers of isoprene units terminating in an α-saturated isoprenoid group, containing an alcohol functional group.
Functions
Dolicho ...
s) the terminal isoprenoid is reduced.
Saccharolipids
Saccharolipid
Saccharolipids are chemical compounds containing fatty acids linked directly to a sugar backbone, forming structures that are compatible with membrane bilayers. In the saccharolipids, a monosaccharide substitutes for the glycerol backbone present ...
s describe compounds in which fatty acids are linked to a sugar backbone, forming structures that are compatible with membrane bilayers. In the saccharolipids, a
monosaccharide substitutes for the glycerol backbone present in glycerolipids and glycerophospholipids. The most familiar saccharolipids are the acylated
glucosamine
Glucosamine (C6H13NO5) is an amino sugar and a prominent precursor in the biochemical synthesis of glycosylated proteins and lipids. Glucosamine is part of the structure of two polysaccharides, chitosan and chitin. Glucosamine is one of the most ...
precursors of the
Lipid A component of the
lipopolysaccharides in
Gram-negative bacteria
Gram-negative bacteria are bacteria that do not retain the crystal violet stain used in the Gram staining method of bacterial differentiation. They are characterized by their cell envelopes, which are composed of a thin peptidoglycan cell wall ...
. Typical lipid A molecules are
disaccharides
A disaccharide (also called a double sugar or ''biose'') is the sugar formed when two monosaccharides are joined by glycosidic linkage. Like monosaccharides, disaccharides are simple sugars soluble in water. Three common examples are sucrose, lac ...
of glucosamine, which are derivatized with as many as seven fatty-acyl chains. The minimal lipopolysaccharide required for growth in
''E. coli'' is Kdo
2-Lipid A, a hexa-acylated disaccharide of glucosamine that is glycosylated with two 3-deoxy-D-manno-octulosonic acid (Kdo) residues.
Polyketides
Polyketides are synthesized by polymerization of
acetyl and
propionyl subunits by classic enzymes as well as iterative and multimodular enzymes that share mechanistic features with the
fatty acid synthases. They comprise many
secondary metabolite
Secondary metabolites, also called specialised metabolites, toxins, secondary products, or natural products, are organic compounds produced by any lifeform, e.g. bacteria, fungi, animals, or plants, which are not directly involved in the norma ...
s and
natural products
A natural product is a natural compound or substance produced by a living organism—that is, found in nature. In the broadest sense, natural products include any substance produced by life. Natural products can also be prepared by chemical sy ...
from animal, plant, bacterial, fungal and marine sources, and have great structural diversity.
Many
polyketide
Polyketides are a class of natural products derived from a precursor molecule consisting of a chain of alternating ketone (or reduced forms of a ketone) and methylene groups: (-CO-CH2-). First studied in the early 20th century, discovery, biosynth ...
s are cyclic molecules whose backbones are often further modified by
glycosylation,
methylation,
hydroxylation,
oxidation, or other processes. Many commonly used
anti-microbial,
anti-parasitic
Antiparasitics are a class of medications which are indicated for the treatment of parasitic diseases, such as those caused by helminths, amoeba, ectoparasites, parasitic fungi, and protozoa, among others. Antiparasitics target the parasitic agents ...
, and
anti-cancer
Cancer is a group of diseases involving abnormal cell growth with the potential to invade or spread to other parts of the body. These contrast with benign tumors, which do not spread. Possible signs and symptoms include a lump, abnormal ble ...
agents are polyketides or polyketide derivatives, such as
erythromycins,
tetracyclines,
avermectin
The avermectins are a series of drugs and pesticides used to treat parasitic worms and insect pests. They are a group of 16-membered macrocyclic lactone derivatives with potent anthelmintic and insecticidal properties. These naturally occurring c ...
s, and antitumor
epothilone
Epothilones are a class of potential cancer drugs. Like taxanes, they prevent cancer cells from dividing by interfering with tubulin, but in early trials, epothilones have better efficacy and milder adverse effects than taxanes.
, epothilones A t ...
s.
Biological functions
Component of biological membranes
Eukaryotic
Eukaryotes () are organisms whose Cell (biology), cells have a cell nucleus, nucleus. All animals, plants, fungi, and many unicellular organisms, are Eukaryotes. They belong to the group of organisms Eukaryota or Eukarya, which is one of the ...
cells feature the compartmentalized membrane-bound
organelles that carry out different biological functions. The
glycerophospholipids are the main structural component of
biological membranes
A biological membrane, biomembrane or cell membrane is a selectively permeable membrane that separates the interior of a cell from the external environment or creates intracellular compartments by serving as a boundary between one part of the c ...
, as the cellular
plasma membrane and the intracellular membranes of
organelles; in animal cells, the plasma membrane physically separates the
intracellular
This glossary of biology terms is a list of definitions of fundamental terms and concepts used in biology, the study of life and of living organisms. It is intended as introductory material for novices; for more specific and technical definitions ...
components from the
extracellular environment. The glycerophospholipids are
amphipathic
An amphiphile (from the Greek αμφις amphis, both, and φιλíα philia, love, friendship), or amphipath, is a chemical compound possessing both hydrophilic (''water-loving'', polar) and lipophilic (''fat-loving'') properties. Such a compoun ...
molecules (containing both
hydrophobic and
hydrophilic
A hydrophile is a molecule or other molecular entity that is attracted to water molecules and tends to be dissolved by water.Liddell, H.G. & Scott, R. (1940). ''A Greek-English Lexicon'' Oxford: Clarendon Press.
In contrast, hydrophobes are ...
regions) that contain a glycerol core linked to two fatty acid-derived "tails" by
ester linkages and to one "head" group by a
phosphate
In chemistry, a phosphate is an anion, salt, functional group or ester derived from a phosphoric acid. It most commonly means orthophosphate, a derivative of orthophosphoric acid .
The phosphate or orthophosphate ion is derived from phosph ...
ester linkage. While glycerophospholipids are the major component of biological membranes, other non-glyceride lipid components such as
sphingomyelin and
sterols (mainly
cholesterol in animal cell membranes) are also found in biological membranes.
In plants and algae, the galactosyldiacylglycerols,
[Heinz E. (1996). "Plant glycolipids: structure, isolation and analysis", pp. 211–332 in ''Advances in Lipid Methodology'', Vol. 3. W.W. Christie (ed.). Oily Press, Dundee. ] and sulfoquinovosyldiacylglycerol,
which lack a phosphate group, are important components of membranes of chloroplasts and related organelles and are the most abundant lipids in photosynthetic tissues, including those of higher plants, algae and certain bacteria.
Plant thylakoid membranes have the largest lipid component of a non-bilayer forming monogalactosyl diglyceride (MGDG), and little phospholipids; despite this unique lipid composition, chloroplast thylakoid membranes have been shown to contain a dynamic lipid-bilayer matrix as revealed by magnetic resonance and electron microscope studies.
A biological membrane is a form of
lamellar phase lipid bilayer
The lipid bilayer (or phospholipid bilayer) is a thin polar membrane made of two layers of lipid molecules. These membranes are flat sheets that form a continuous barrier around all cells. The cell membranes of almost all organisms and many vir ...
. The formation of lipid bilayers is an energetically preferred process when the
glycerophospholipids described above are in an aqueous environment.
This is known as the hydrophobic effect. In an aqueous system, the polar heads of lipids align towards the polar, aqueous environment, while the hydrophobic tails minimize their contact with water and tend to cluster together, forming a
vesicle; depending on the
concentration of the lipid, this biophysical interaction may result in the formation of
micelles,
liposomes, or
lipid bilayer
The lipid bilayer (or phospholipid bilayer) is a thin polar membrane made of two layers of lipid molecules. These membranes are flat sheets that form a continuous barrier around all cells. The cell membranes of almost all organisms and many vir ...
s. Other aggregations are also observed and form part of the polymorphism of
amphiphile (lipid) behavior.
Phase behavior is an area of study within
biophysics
Biophysics is an interdisciplinary science that applies approaches and methods traditionally used in physics to study biological phenomena. Biophysics covers all scales of biological organization, from molecular to organismic and populations. ...
and is the subject of current academic research.
Micelles and bilayers form in the polar medium by a process known as the
hydrophobic effect
The hydrophobic effect is the observed tendency of nonpolar substances to aggregate in an aqueous solution and exclude water molecules. The word hydrophobic literally means "water-fearing", and it describes the segregation of water and nonpolar ...
.
When dissolving a lipophilic or amphiphilic substance in a polar environment, the polar molecules (i.e., water in an aqueous solution) become more ordered around the dissolved lipophilic substance, since the polar molecules cannot form
hydrogen bonds to the lipophilic areas of the
amphiphile. So in an aqueous environment, the water molecules form an ordered "
clathrate" cage around the dissolved lipophilic molecule.
The formation of lipids into
protocell membranes represents a key step in models of
abiogenesis
In biology, abiogenesis (from a- 'not' + Greek bios 'life' + genesis 'origin') or the origin of life is the natural process by which life has arisen from non-living matter, such as simple organic compounds. The prevailing scientific hypothes ...
, the origin of life.
Energy storage
Triglycerides, stored in adipose tissue, are a major form of energy storage both in animals and plants. They are a major source of energy in aerobic respiration. The complete oxidation of fatty acids releases about 38 kJ/g (9
kcal/g), compared with only 17 kJ/g (4 kcal/g) for the oxidative breakdown of
carbohydrate
In organic chemistry, a carbohydrate () is a biomolecule consisting of carbon (C), hydrogen (H) and oxygen (O) atoms, usually with a hydrogen–oxygen atom ratio of 2:1 (as in water) and thus with the empirical formula (where ''m'' may or m ...
s and
protein
Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, res ...
s. The
adipocyte, or fat cell, is designed for continuous synthesis and breakdown of triglycerides in animals, with breakdown controlled mainly by the activation of hormone-sensitive enzyme
lipase.
Migratory birds that must fly long distances without eating use triglycerides to fuel their flights.
Signaling
Evidence has emerged showing that
lipid signaling is a vital part of the
cell signaling
In biology, cell signaling (cell signalling in British English) or cell communication is the ability of a cell to receive, process, and transmit signals with its environment and with itself. Cell signaling is a fundamental property of all cellula ...
.
Lipid signaling may occur via activation of
G protein-coupled
G protein-coupled receptors (GPCRs), also known as seven-(pass)-transmembrane domain receptors, 7TM receptors, heptahelical receptors, serpentine receptors, and G protein-linked receptors (GPLR), form a large group of evolutionarily-related p ...
or
nuclear receptors, and members of several different lipid categories have been identified as signaling molecules and
cellular messengers.
These include
sphingosine-1-phosphate
Sphingosine-1-phosphate (S1P) is a signaling sphingolipid, also known as lysosphingolipid. It is also referred to as a bioactive lipid mediator. Sphingolipids at large form a class of lipids characterized by a particular aliphatic aminoalcoho ...
, a sphingolipid derived from ceramide that is a potent messenger molecule involved in regulating calcium mobilization,
cell growth, and apoptosis;
diacylglycerol
A diglyceride, or diacylglycerol (DAG), is a glyceride consisting of two fatty acid chains covalently bonded to a glycerol molecule through ester linkages. Two possible forms exist, 1,2-diacylglycerols and 1,3-diacylglycerols. DAGs can act as s ...
(DAG) and the
phosphatidylinositol phosphates (PIPs), involved in calcium-mediated activation of
protein kinase C;
the
prostaglandins, which are one type of fatty-acid derived eicosanoid involved in
inflammation
Inflammation (from la, inflammatio) is part of the complex biological response of body tissues to harmful stimuli, such as pathogens, damaged cells, or irritants, and is a protective response involving immune cells, blood vessels, and molec ...
and
immunity;
the steroid hormones such as
estrogen
Estrogen or oestrogen is a category of sex hormone responsible for the development and regulation of the female reproductive system and secondary sex characteristics. There are three major endogenous estrogens that have estrogenic hormonal ac ...
,
testosterone and
cortisol, which modulate a host of functions such as reproduction, metabolism and blood pressure; and the
oxysterol
An oxysterol is a derivative of cholesterol obtained by oxidation involving enzymes and / or pro-oxidants. Such compounds play important roles in various biological processes such as cholesterol homeostasis, lipid metabolism (sphingolipids, fatty ...
s such as 25-hydroxy-cholesterol that are
liver X receptor agonists.
Phosphatidylserine lipids are known to be involved in signaling for the phagocytosis of apoptotic cells or pieces of cells. They accomplish this by being exposed to the extracellular face of the cell membrane after the inactivation of
flippases which place them exclusively on the cytosolic side and the activation of scramblases, which scramble the orientation of the phospholipids. After this occurs, other cells recognize the phosphatidylserines and phagocytosize the cells or cell fragments exposing them.
Other functions
The "fat-soluble" vitamins (
A,
D,
E and
K) – which are isoprene-based lipids – are essential nutrients stored in the liver and fatty tissues, with a diverse range of functions.
Acyl-carnitines are involved in the transport and metabolism of fatty acids in and out of mitochondria, where they undergo
beta oxidation
In biochemistry and metabolism, beta-oxidation is the 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 acid cyc ...
. Polyprenols and their phosphorylated derivatives also play important transport roles, in this case the transport of
oligosaccharides across membranes. Polyprenol phosphate sugars and polyprenol diphosphate sugars function in extra-cytoplasmic glycosylation reactions, in extracellular polysaccharide biosynthesis (for instance,
peptidoglycan
Peptidoglycan or murein is a unique large macromolecule, a polysaccharide, consisting of sugars and amino acids that forms a mesh-like peptidoglycan layer outside the plasma membrane, the rigid cell wall (murein sacculus) characteristic of most ba ...
polymerization in bacteria), and in eukaryotic protein N-
glycosylation.
Cardiolipins are a subclass of glycerophospholipids containing four acyl chains and three glycerol groups that are particularly abundant in the inner mitochondrial membrane.
They are believed to activate enzymes involved with
oxidative phosphorylation.
Lipids also form the basis of steroid hormones.
Metabolism
The major dietary lipids for humans and other animals are animal and plant triglycerides, sterols, and membrane phospholipids. The process of lipid metabolism synthesizes and degrades the lipid stores and produces the structural and functional lipids characteristic of individual tissues.
Biosynthesis
In animals, when there is an oversupply of dietary carbohydrate, the excess carbohydrate is converted to triglycerides. This involves the synthesis of fatty acids from
acetyl-CoA and the
esterification
In chemistry, an ester is a 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. Glycerides ar ...
of fatty acids in the production of triglycerides, a process called
lipogenesis.
Fatty acids are made by
fatty acid synthases that polymerize and then reduce acetyl-CoA units. The acyl chains in the fatty acids are extended by a cycle of reactions that add the acetyl group, reduce it to an alcohol,
dehydrate it to an
alkene group and then reduce it again to an
alkane group. The enzymes of fatty acid biosynthesis are divided into two groups, in animals and fungi all these fatty acid synthase reactions are carried out by a single multifunctional protein,
while in plant
plastids and bacteria separate enzymes perform each step in the pathway.
The fatty acids may be subsequently converted to triglycerides that are packaged in
lipoproteins
A lipoprotein is a biochemical assembly whose primary function is to transport hydrophobic lipid (also known as fat) molecules in water, as in blood plasma or other extracellular fluids. They consist of a triglyceride and cholesterol center, su ...
and secreted from the liver.
The synthesis of
unsaturated fatty acids involves a
desaturation reaction, whereby a double bond is introduced into the fatty acyl chain. For example, in humans, the desaturation of
stearic acid by
stearoyl-CoA desaturase-1
Stearoyl-CoA desaturase (Δ-9-desaturase) is an endoplasmic reticulum enzyme that catalyzes the rate-limiting step in the formation of monounsaturated fatty acids (MUFAs), specifically oleate and palmitoleate from stearoyl-CoA and palmitoyl-C ...
produces
oleic acid. The doubly unsaturated fatty acid
linoleic acid as well as the triply unsaturated
α-linolenic acid
''alpha''-Linolenic acid (ALA), also known as α-Linolenic acid (from Greek language, Greek ''alpha'' meaning "first" and ''linon'' meaning flax), is an omega-3 fatty acid, ''n''−3, or omega-3, essential fatty acid. ALA is found in many seeds ...
cannot be synthesized in mammalian tissues, and are therefore
essential fatty acid
Essential fatty acids, or EFAs, are fatty acids that humans and other animals must ingest because the body requires them for good health but cannot synthesize them.
Only two fatty acids are known to be essential for humans: alpha-linolenic ac ...
s and must be obtained from the diet.
Triglyceride synthesis takes place in the
endoplasmic reticulum by metabolic pathways in which acyl groups in fatty acyl-CoAs are transferred to the hydroxyl groups of glycerol-3-phosphate and diacylglycerol.
Terpenes and
isoprenoids, including the
carotenoids, are made by the assembly and modification of
isoprene units donated from the reactive precursors
isopentenyl pyrophosphate
Isopentenyl pyrophosphate (IPP, isopentenyl diphosphate, or IDP) is an isoprenoid precursor. IPP is an intermediate in the classical, HMG-CoA reductase pathway (commonly called the mevalonate pathway) and in the ''non-mevalonate'' MEP pathway of i ...
and
dimethylallyl pyrophosphate.
These precursors can be made in different ways. In animals and
archaea, the
mevalonate pathway
The mevalonate pathway, also known as the isoprenoid pathway or HMG-CoA reductase pathway is an essential metabolic pathway present in eukaryotes, archaea, and some bacteria. The pathway produces two five-carbon building blocks called isopentenyl ...
produces these compounds from acetyl-CoA,
while in plants and bacteria the
non-mevalonate pathway uses pyruvate and
glyceraldehyde 3-phosphate
Glyceraldehyde 3-phosphate, also known as triose phosphate or 3-phosphoglyceraldehyde and abbreviated as G3P, GA3P, GADP, GAP, TP, GALP or PGAL, is a metabolite that occurs as an intermediate in several central pathways of all organisms.Nelson, D ...
as substrates.
One important reaction that uses these activated isoprene donors is
steroid biosynthesis. Here, the isoprene units are joined together to make
squalene
Squalene is an organic compound. It is a triterpenoid with the formula C30H50. It is a colourless oil, although impure samples appear yellow. It was originally obtained from shark liver oil (hence its name, as ''Squalus'' is a genus of sharks). A ...
and then folded up and formed into a set of rings to make
lanosterol
Lanosterol is a tetracyclic triterpenoid and is the compound from which all animal and fungal steroids are derived. By contrast plant steroids are produced via cycloartenol.
Role in biosynthesis of other steroids
Elaboration of lanosterol under en ...
.
Lanosterol can then be converted into other steroids such as
cholesterol and
ergosterol.
Degradation
Beta oxidation
In biochemistry and metabolism, beta-oxidation is the 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 acid cyc ...
is the metabolic process by which fatty acids are broken down in the
mitochondria or in
peroxisomes
A peroxisome () is a membrane-bound organelle, a type of microbody, found in the cytoplasm of virtually all eukaryotic cells. Peroxisomes are oxidative organelles. Frequently, molecular oxygen serves as a co-substrate, from which hydrogen per ...
to generate
acetyl-CoA. For the most part, fatty acids are oxidized by a mechanism that is similar to, but not identical with, a reversal of the process of fatty acid synthesis. That is, two-carbon fragments are removed sequentially from the carboxyl end of the acid after steps of
dehydrogenation,
hydration, and
oxidation to form a
beta-keto acid, which is split by
thiolysis Thiolysis is a reaction with a thiol (R-SH) that cleaves one compound into two. Thiolysis involves the addition of coenzyme A to one of the products. This reaction is similar to hydrolysis, which involves water instead of a thiol. This reaction is s ...
. The acetyl-CoA is then ultimately converted into
ATP, CO
2, and H
2O using the
citric acid cycle and the
electron transport chain. Hence the citric acid cycle can start at acetyl-CoA when fat is being broken down for energy if there is little or no glucose available. The energy yield of the complete oxidation of the fatty acid palmitate is 106 ATP.
Unsaturated and odd-chain fatty acids require additional enzymatic steps for degradation.
Nutrition and health
Most of the fat found in food is in the form of triglycerides, cholesterol, and phospholipids. Some dietary fat is necessary to facilitate absorption of fat-soluble vitamins (
A,
D,
E, and
K) and
carotenoids
Carotenoids (), also called tetraterpenoids, are yellow, orange, and red organic pigments that are produced by plants and algae, as well as several bacteria, and fungi. Carotenoids give the characteristic color to pumpkins, carrots, parsnips, co ...
.
Humans and other mammals have a dietary requirement for certain essential fatty acids, such as
linoleic acid (an
omega-6 fatty acid
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 in the ''n''-6 position, that is, the sixth bond, counting from ...
) and
alpha-linolenic acid
''alpha''-Linolenic acid (ALA), also known as α-Linolenic acid (from Greek ''alpha'' meaning "first" and ''linon'' meaning flax), is an ''n''−3, or omega-3, essential fatty acid. ALA is found in many seeds and oils, including flaxseed, wa ...
(an omega-3 fatty acid) because they cannot be synthesized from simple precursors in the diet.
Both of these fatty acids are 18-carbon
polyunsaturated fatty acids differing in the number and position of the double bonds. Most
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 f ...
s are rich in linoleic acid (
safflower
Safflower (''Carthamus tinctorius'') is a highly branched, herbaceous, thistle-like annual plant in the family Asteraceae. It is commercially cultivated for vegetable oil extracted from the seeds and was used by the early Spanish colonies along ...
,
sunflower, and
corn oils). Alpha-linolenic acid is found in the green leaves of plants and in some seeds, nuts, and legumes (in particular
flax,
rapeseed
Rapeseed (''Brassica napus ''subsp.'' napus''), also known as rape, or oilseed rape, is a bright-yellow flowering member of the family Brassicaceae (mustard or cabbage family), cultivated mainly for its oil-rich seed, which naturally contains a ...
,
walnut
A walnut is the edible seed of a drupe of any tree of the genus ''Juglans'' (family Juglandaceae), particularly the Persian or English walnut, '' Juglans regia''.
Although culinarily considered a "nut" and used as such, it is not a true ...
, and
soy
The soybean, soy bean, or soya bean (''Glycine max'') is a species of legume native to East Asia, widely grown for its edible bean, which has numerous uses.
Traditional unfermented food uses of soybeans include soy milk, from which tofu and ...
).
Fish oil
Fish oil is oil derived from the tissues of oily fish. Fish oils contain the omega-3 fatty acids eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), precursors of certain eicosanoids that are known to reduce inflammation in the b ...
s are particularly rich in the longer-chain omega-3 fatty acids
eicosapentaenoic acid
Eicosapentaenoic acid (EPA; also icosapentaenoic acid) is an omega-3 fatty acid. In physiological literature, it is given the name 20:5(n-3). It also has the trivial name timnodonic acid. In chemical structure, EPA is a carboxylic acid with a 20-c ...
(EPA) and
docosahexaenoic acid
Docosahexaenoic acid (DHA) is an omega-3 fatty acid that is a primary structural component of the human brain, cerebral cortex, skin, and retina. In physiological literature, it is given the name 22:6(n-3). It can be synthesized from alpha-lin ...
(DHA).
Many studies have shown positive health benefits associated with consumption of omega-3 fatty acids on infant development, cancer, cardiovascular diseases, and various mental illnesses (such as depression, attention-deficit hyperactivity disorder, and dementia).
In contrast, it is now well-established that consumption of
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 pr ...
s, such as those present in
partially hydrogenated vegetable oils, are a risk factor for
cardiovascular disease. Fats that are good for one may be turned into trans fats by improper cooking methods that result in overcooking the lipids.
A few studies have suggested that total dietary fat intake is linked to an increased risk of obesity
and diabetes;
however, a number of very large studies, including the Women's Health Initiative Dietary Modification Trial, an eight-year study of 49,000 women, the Nurses' Health Study, and the Health Professionals Follow-up Study, revealed no such links.
None of these studies suggested any connection between percentage of calories from fat and risk of cancer, heart disease, or weight gain. The Nutrition Source,
a website maintained by the department of nutrition at the
T. H. Chan School of Public Health at
Harvard University
Harvard University is a private Ivy League research university in Cambridge, Massachusetts. Founded in 1636 as Harvard College and named for its first benefactor, the Puritan clergyman John Harvard, it is the oldest institution of high ...
, summarizes the current evidence on the effect of dietary fat: "Detailed research—much of it done at Harvard—shows that the total amount of fat in the diet isn't really linked with weight or disease."
See also
*
*
*
*
*
*
*
*
*
* , a class of natural products composed of long aliphatic chains and phenolic rings that occur in plants, fungi and bacteria
References
Bibliography
*
*
*
*
External links
Introductory
List of lipid-related web sitesNature Lipidomics Gateway– Round-up and summaries of recent lipid research
Lipid Library– General reference on lipid chemistry and biochemistry
Cyberlipid.org– Resources and history for lipids.
– Modeling of Lipid Membranes
Lipids, Membranes and Vesicle Trafficking– The Virtual Library of Biochemistry, Molecular Biology and Cell Biology
Nomenclature
IUPAC nomenclature of lipids
Databases
– Comprehensive lipid and lipid-associated gene/protein databases.
LipidBank– Japanese database of lipids and related properties, spectral data and references.
General
ApolloLipids– Provides dyslipidemia and cardiovascular disease prevention and treatment information as well as continuing medical education programs
National Lipid Association– Professional medical education organization for health care professionals who seek to prevent morbidity and mortality stemming from dyslipidemias and other cholesterol-related disorders.
{{Portal bar, Food, Biology
*
Underwater diving physiology