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 storing energy,
signaling, and acting as structural components of
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 interior of all cells from the outside environment (t ...
s.
Lipids have applications in the
cosmetic
Cosmetic may refer to:
*Cosmetics, or make-up, substances to enhance the beauty of the human body, apart from simple cleaning
*Cosmetic, an adjective describing beauty, aesthetics, or appearance, especially concerning the human body
*Cosmetic, a t ...
and
food industries, and in
nanotechnology.
Lipids may be broadly defined as
hydrophobic or
amphiphilic small molecules; the amphiphilic nature of some lipids allows them to form structures such as
vesicles
Vesicle may refer to:
; In cellular biology or chemistry
* Vesicle (biology and chemistry), a supramolecular assembly of lipid molecules, like a cell membrane
* Synaptic vesicle
; In human embryology
* Vesicle (embryology), bulge-like features o ...
, multilamellar/
unilamellar liposomes, 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,
glycerolipids,
glycerophospholipids,
sphingolipids,
saccharolipids, and
polyketides (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
fats, fats are a subgroup of lipids called
triglycerides. Lipids also encompass molecules such as
fatty acids and their derivatives (including
tri-
Numeral or number prefixes are prefixes derived from numerals or occasionally other numbers. In English and many other languages, they are used to coin numerous series of words. For example:
* unicycle, bicycle, tricycle (1-cycle, 2-cycle, 3-c ...
,
di-,
monoglycerides, and
phospholipids), as well as other
sterol-containing
metabolites such as
cholesterol
Cholesterol is any of a class of certain organic molecules called lipids. It is a sterol (or modified steroid), a type of lipid. Cholesterol is biosynthesized by all animal cells and is an essential structural component of animal cell memb ...
.
Although humans and other mammals use various
biosynthetic pathways 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 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 in 1844, when he produced
tributyrin
Tributyrin is a triglyceride naturally present in butter. It is an ester composed of butyric acid and glycerol. Among other things, it is used as an ingredient in making margarine. It is present in butter and can be described as a liquid fat wi ...
by treating
butyric acid with
glycerin in the presence of concentrated
sulfuric acid. Several years later,
Marcellin Berthelot, one of Pelouze's students, synthesized
tristearin and
tripalmitin by reaction of the analogous
fatty acids with glycerin in the presence of gaseous
hydrogen chloride at high temperature.
In 1827,
William Prout 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), glycolipids (
cerebroside) 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. 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 consortium
as follows:
Fatty acyls
Fatty acyls, a generic term for describing
fatty acids, 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 groups in a process called
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. Most of the acetyl-CoA which is co ...
.
They are made of a
hydrocarbon chain
In organic chemistry, hydrocarbons ( compounds composed solely of carbon and hydrogen) are divided into two classes: aromatic compounds and aliphatic compounds (; G. ''aleiphar'', fat, oil). Aliphatic compounds can be saturated, like hexane, o ...
that terminates with a
carboxylic acid group; this arrangement confers the molecule with a
polar
Polar may refer to:
Geography
Polar may refer to:
* Geographical pole, either of two fixed points on the surface of a rotating body or planet, at 90 degrees from the equator, based on the axis around which a body rotates
*Polar climate, the cli ...
,
hydrophilic end, and a nonpolar,
hydrophobic end that is
insoluble
In chemistry, solubility is the ability of a substance, the solute, to form a solution with another substance, the solvent. Insolubility is the opposite property, the inability of the solute to form such a solution.
The extent of the solubi ...
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 ...
,
halogens,
nitrogen, and
sulfur. If a fatty acid contains a double bond, there is the possibility of either a ''cis'' or ''trans''
geometric isomerism, 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
Linolenic acid is a type of naturally-occurring fatty acid. It can refer to either of two octadecatrienoic acids (i.e. with an 18-carbon chain and three double bonds, which are found in the '' cis'' configuration), or a mixture of the two. Linol ...
'', 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
eicosanoids, derived primarily from
arachidonic acid and
eicosapentaenoic acid, that include
prostaglandins,
leukotrienes, and
thromboxanes.
Docosahexaenoic acid 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 neurotransmitter
anandamide.
Glycerolipids
Glycerolipids are composed of mono-, di-, and tri-substituted
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 ...
s,
the best-known being the fatty acid
triesters of glycerol, called
triglycerides. 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 animal tissues. The hydrolysis of the
ester
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 ...
bonds of triglycerides and the release of glycerol and fatty acids from
adipose tissue 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. Examples of structures in this category are the digalactosyldiacylglycerols found in plant membranes
and seminolipid from mammalian
sperm cells.
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 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 ...
and
cell signaling.
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.
Examples of glycerophospholipids found in
biological membranes are
phosphatidylcholine (also known as PC, GPCho or
lecithin),
phosphatidylethanolamine (PE or GPEtn) and
phosphatidylserine (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
phosphatidylinositol
Phosphatidylinositol (or Inositol Phospholipid) consists of a family of lipids as illustrated on the right, where red is x, blue is y, and black is z, in the context of independent variation, a class of the phosphatidylglycerides. In such molecul ...
s and
phosphatidic acids 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
Sphingolipids are a complicated family of compounds
that share a common structural feature, a
sphingoid base
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 ...
backbone that is synthesized
''de novo'' from the amino acid
serine and a long-chain fatty acyl CoA, then converted into
ceramides, phosphosphingolipids, glycosphingolipids and other compounds. The major sphingoid base of mammals is commonly referred to as
sphingosine. Ceramides (N-acyl-sphingoid bases) are a major subclass of sphingoid base derivatives with an
amide
In organic chemistry, an amide, also known as an organic amide or a carboxamide, is a compound with the general formula , where R, R', and R″ represent organic groups or hydrogen atoms. The amide group is called a peptide bond when it i ...
-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
Mannose is a sugar monomer of the aldohexose series of carbohydrates. It is a C-2 epimer of glucose. Mannose is important in human metabolism, especially in the glycosylation of certain proteins. Several congenital disorders of glycosylat ...
-containing headgroups.
The glycosphingolipids are a diverse family of molecules composed of one or more sugar residues linked via a
glycosidic bond to the sphingoid base. Examples of these are the simple and complex glycosphingolipids such as
cerebrosides and
ganglioside
A ganglioside is a molecule composed of a glycosphingolipid (ceramide and oligosaccharide) with one or more sialic acids (e.g. ''N''-acetylneuraminic acid, NANA) linked on the sugar chain. NeuNAc, an acetylated derivative of the carbohydrate ...
s.
Sterols
Sterols, such as
cholesterol
Cholesterol is any of a class of certain organic molecules called lipids. It is a sterol (or modified steroid), a type of lipid. Cholesterol is biosynthesized by all animal cells and is an essential structural component of animal cell memb ...
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, such as
β-sitosterol,
stigmasterol, and
brassicasterol
Brassicasterol (24-methyl cholest-5,22-dien-3β-ol) is a 28- carbon sterol synthesised by several unicellular algae ( phytoplankton) and some terrestrial plants, like rape. This compound has frequently been used as a biomarker for the presence of ...
; the latter compound is also used as a
biomarker for
algal
Algae (; singular alga ) is an informal term for a large and diverse group of photosynthetic eukaryotic organisms. It is a polyphyletic grouping that includes species from multiple distinct clades. Included organisms range from unicellular micr ...
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
hormones and
signaling molecules
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 cellular ...
. 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
androgens 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 tes ...
. The C21 subclass includes the
progestogens
Progestogens, also sometimes written progestagens or gestagens, are a class of natural or synthetic steroid hormones that bind to and activate the progesterone receptors (PR). Progesterone is the major and most important progestogen in the bo ...
as well as the
glucocorticoids and
mineralocorticoids.
The
secosteroid
A secosteroid () is a type of steroid with a "broken" ring. The word ''secosteroid ''derives from the Latin verb ''secare'' meaning "to cut", and 'steroid'. Secosteroids are alternatively described as a subclass of steroids; ; or derived from st ...
s, comprising various forms of
vitamin D
Vitamin D is a group of Lipophilicity, fat-soluble secosteroids responsible for increasing intestinal absorption of calcium, magnesium, and phosphate, and many other biological effects. In humans, the most important compounds in this group ar ...
, are characterized by cleavage of the B ring of the core structure.
Prenols
Prenol 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
hydroquinone
Hydroquinone, also known as benzene-1,4-diol or quinol, is an aromatic organic compound that is a type of phenol, a derivative of benzene, having the chemical formula C6H4(OH)2. It has two hydroxyl groups bonded to a benzene ring in a ''pa ...
s, which contain an isoprenoid tail attached to a quinonoid core of non-isoprenoid origin.
Vitamin E and
vitamin K, as well as the
ubiquinones, 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 (
dolichols) the terminal isoprenoid is reduced.
Saccharolipids
Saccharolipids 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 precursors of the
Lipid A component of the
lipopolysaccharide
Lipopolysaccharides (LPS) are large molecules consisting of a lipid and a polysaccharide that are bacterial toxins. They are composed of an O- antigen, an outer core, and an inner core all joined by a covalent bond, and are found in the out ...
s in
Gram-negative bacteria. Typical lipid A molecules are
disaccharides 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
Propionic acid (, from the Greek words πρῶτος : ''prōtos'', meaning "first", and πίων : ''píōn'', meaning "fat"; also known as propanoic acid) is a naturally occurring carboxylic acid with chemical formula CH3CH2CO2H. It is a liqu ...
subunits by classic enzymes as well as iterative and multimodular enzymes that share mechanistic features with the
fatty acid synthases. They comprise many
secondary metabolites and
natural products from animal, plant, bacterial, fungal and marine sources, and have great structural diversity.
Many
polyketides are cyclic molecules whose backbones are often further modified by
glycosylation,
methylation,
hydroxylation,
oxidation
Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or ...
, or other processes. Many commonly used
anti-microbial
An antimicrobial is an agent that kills microorganisms or stops their growth. Antimicrobial medicines can be grouped according to the microorganisms they act primarily against. For example, antibiotics are used against bacteria, and antifungals ...
,
anti-parasitic, and
anti-cancer agents are polyketides or polyketide derivatives, such as
erythromycins,
tetracyclines,
avermectins, and antitumor
epothilones.
Biological functions
Component of biological membranes
Eukaryotic cells feature the compartmentalized membrane-bound
organelles that carry out different biological functions. The
glycerophospholipids
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 ...
are the main structural component of
biological membranes, as the cellular
plasma membrane and the intracellular membranes of
organelles; in animal cells, the plasma membrane physically separates the
intracellular components from the
extracellular environment. The glycerophospholipids are
amphipathic molecules (containing both
hydrophobic and
hydrophilic regions) that contain a glycerol core linked to two fatty acid-derived "tails" by
ester
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 ...
linkages and to one "head" group by a
phosphate ester linkage. While glycerophospholipids are the major component of biological membranes, other non-glyceride lipid components such as
sphingomyelin and
sterols (mainly
cholesterol
Cholesterol is any of a class of certain organic molecules called lipids. It is a sterol (or modified steroid), a type of lipid. Cholesterol is biosynthesized by all animal cells and is an essential structural component of animal cell memb ...
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 Lamellar phase refers generally to packing of polar-headed long chain nonpolar-tail molecules in an environment of bulk polar liquid, as sheets of bilayers separated by bulk liquid. In biophysics, polar lipids (mostly, phospholipids, and rarely, gl ...
lipid bilayer. The formation of lipid bilayers is an energetically preferred process when the
glycerophospholipids
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 ...
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
micelle
A micelle () or micella () (plural micelles or micellae, respectively) is an aggregate (or supramolecular assembly) of surfactant amphipathic lipid molecules dispersed in a liquid, forming a colloidal suspension (also known as associated coll ...
s,
liposomes, or
lipid bilayers. Other aggregations are also observed and form part of the polymorphism of
amphiphile (lipid) behavior.
Phase behavior is an area of study within
biophysics and is the subject of current academic research.
Micelles and bilayers form in the polar medium by a process known as the
hydrophobic effect.
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
A clathrate is a chemical substance consisting of a lattice that traps or contains molecules. The word ''clathrate'' is derived from the Latin (), meaning ‘with bars, latticed’. Most clathrate compounds are polymeric and completely envelo ...
" cage around the dissolved lipophilic molecule.
The formation of lipids into
protocell membranes represents a key step in models of
abiogenesis, 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 o ...
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.
Lipid signaling may occur via activation of
G protein-coupled or
nuclear receptors, and members of several different lipid categories have been identified as signaling molecules and
cellular messengers.
These include
sphingosine-1-phosphate, a sphingolipid derived from ceramide that is a potent messenger molecule involved in regulating calcium mobilization,
cell growth, and apoptosis;
diacylglycerol (DAG) and the
phosphatidylinositol
Phosphatidylinositol (or Inositol Phospholipid) consists of a family of lipids as illustrated on the right, where red is x, blue is y, and black is z, in the context of independent variation, a class of the phosphatidylglycerides. In such molecul ...
phosphates (PIPs), involved in calcium-mediated activation of
protein kinase C;
the
prostaglandins
The prostaglandins (PG) are a group of physiologically active lipid compounds called eicosanoids having diverse hormone-like effects in animals. Prostaglandins have been found in almost every tissue in humans and other animals. They are deri ...
, which are one type of fatty-acid derived eicosanoid involved in
inflammation and
immunity
Immunity may refer to:
Medicine
* Immunity (medical), resistance of an organism to infection or disease
* ''Immunity'' (journal), a scientific journal published by Cell Press
Biology
* Immune system
Engineering
* Radiofrequence immunity de ...
;
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
oxysterols such as 25-hydroxy-cholesterol that are
liver X receptor
The liver X receptor (LXR) is a member of the nuclear receptor family of transcription factors and is closely related to nuclear receptors such as the PPARs, FXR and RXR. Liver X receptors (LXRs) are important regulators of cholesterol, ...
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. 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 polymerization in bacteria), and in eukaryotic protein N-
glycosylation.
Cardiolipin
Cardiolipin (IUPAC name 1,3-bis(''sn''-3’-phosphatidyl)-''sn''-glycerol) is an important component of the inner mitochondrial membrane, where it constitutes about 20% of the total lipid composition. It can also be found in the membranes of most ...
s 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 of fatty acids in the production of triglycerides, a process called
lipogenesis
In biochemistry, lipogenesis is the conversion of fatty acids and glycerol into fats, or a metabolic process through which acetyl-CoA is converted to triglyceride for storage in fat. Lipogenesis encompasses both fatty acid and triglyceride s ...
.
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
In physiology, dehydration is a lack of total body water, with an accompanying disruption of metabolic processes. It occurs when free water loss exceeds free water intake, usually due to exercise, disease, or high environmental temperature. Mil ...
it to an
alkene
In organic chemistry, an alkene is a hydrocarbon containing a carbon–carbon double bond.
Alkene is often used as synonym of olefin, that is, any hydrocarbon containing one or more double bonds.H. Stephen Stoker (2015): General, Organic ...
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 and secreted from the liver.
The synthesis of
unsaturated fatty acid
In chemistry, particularly in biochemistry, a fatty acid is a carboxylic acid with an aliphatic chain, which is either saturated or unsaturated. Most naturally occurring fatty acids have an unbranched chain of an even number of carbon atoms, f ...
s 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 produces
oleic acid. The doubly unsaturated fatty acid
linoleic acid as well as the triply unsaturated
α-linolenic acid cannot be synthesized in mammalian tissues, and are therefore
essential fatty acids 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
The terpenoids, also known as isoprenoids, are a class of naturally occurring organic chemicals derived from the 5-carbon compound isoprene and its derivatives called terpenes, diterpenes, etc. While sometimes used interchangeably with "terpenes" ...
, including the
carotenoids, are made by the assembly and modification of
isoprene units donated from the reactive precursors
isopentenyl pyrophosphate and
dimethylallyl pyrophosphate
Dimethylallyl pyrophosphate (DMAPP; or alternatively, dimethylallyl diphosphate (DMADP); also isoprenyl pyrophosphate) is an isoprenoid precursor. It is a product of both the mevalonate pathway and the MEP pathway of isoprenoid precursor biosynt ...
.
These precursors can be made in different ways. In animals and
archaea, the
mevalonate pathway produces these compounds from acetyl-CoA,
while in plants and bacteria the
non-mevalonate pathway uses pyruvate and
glyceraldehyde 3-phosphate as substrates.
One important reaction that uses these activated isoprene donors is
steroid biosynthesis
A steroid is a biologically active organic compound with four rings arranged in a specific molecular configuration. Steroids have two principal biological functions: as important components of cell membranes that alter membrane fluidity; and a ...
. Here, the isoprene units are joined together to make
squalene and then folded up and formed into a set of rings to make
lanosterol.
Lanosterol can then be converted into other steroids such as
cholesterol
Cholesterol is any of a class of certain organic molecules called lipids. It is a sterol (or modified steroid), a type of lipid. Cholesterol is biosynthesized by all animal cells and is an essential structural component of animal cell memb ...
and
ergosterol.
Degradation
Beta oxidation is the metabolic process by which fatty acids are broken down in the
mitochondria or in
peroxisomes 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
In chemistry, dehydrogenation is a chemical reaction that involves the removal of hydrogen, usually from an organic molecule. It is the reverse of hydrogenation. Dehydrogenation is important, both as a useful reaction and a serious problem. A ...
,
hydration Hydration may refer to:
* Hydrate, a substance that contains water
* Hydration enthalpy, energy released through hydrating a substance
* Hydration reaction, a chemical addition reaction where a hydroxyl group and proton are added to a compound
* ...
, and
oxidation
Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or ...
to form a
beta-keto acid, which is split by
thiolysis. The acetyl-CoA is then ultimately converted into
ATP, CO
2, and H
2O using 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 oxidation of acetyl-CoA derived from carbohydrates, fats, and prot ...
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.
Humans and other mammals have a dietary requirement for certain essential fatty acids, such as
linoleic acid (an
omega-6 fatty acid) and
alpha-linolenic acid (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
Polyunsaturated fatty acids (PUFAs) are fatty acids that contain more than one double bond in their backbone. This class includes many important compounds, such as essential fatty acids and those that give drying oils their characteristic pr ...
differing in the number and position of the double bonds. Most
vegetable oils are rich in linoleic acid (
safflower,
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,
walnut, and
soy).
Fish oils are particularly rich in the longer-chain omega-3 fatty acids
eicosapentaenoic acid (EPA) and
docosahexaenoic acid (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 fats, such as those present in
partially hydrogenated vegetable oil
Trans fat regulation, that aims to limit the amount of " trans fat" — fat containing ''trans'' fatty acids — in industrial food products, has been enacted in many countries. These regulations were motivated by numerous studies that pointed ...
s, 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 highe ...
, 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
*
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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.
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*
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