Editing Lipid (section)

== Categories ==
Lipids have been classified into eight categories by the [[LIPID MAPS|Lipid MAPS]] consortium<ref name="Fahy_2009"/> as follows:

===Fatty acyls===
{{Main|Fatty acid}}
[[File:Prostacyclin-2D-skeletal.png|thumb|[[Prostacyclin|I<sub>2</sub> – Prostacyclin]] (an example of a [[prostaglandin]], an eicosanoid fatty acid)]]
[[Image:Leukotriene B4.svg|right|thumb|[[Leukotriene B4|LTB<sub>4</sub>]] (an example of a [[leukotriene]], an eicosanoid fatty acid)]]

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]].<ref name="Vance_2002"/><ref name="Brown_2007"/> They are made of a [[hydrocarbon chain]] that terminates with a [[carboxylic acid]] group; this arrangement confers the molecule with a [[chemical polarity|polar]], [[hydrophilic]] 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,<ref name="Hunt_1995"/> may be saturated or [[unsaturated compound|unsaturated]], and may be attached to [[functional group]]s containing [[oxygen]], [[halogen]]s, [[nitrogen]], and [[sulfur]]. If a fatty acid contains a double bond, there is the possibility of either a ''cis'' or ''trans'' [[cis–trans isomerism|geometric isomerism]], which significantly affects the molecule's [[molecular configuration|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,<ref name="YashRoy_1987"/> 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.<ref name = "Devlin_1997" />{{rp|193–5}} 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.<ref name="Hunter_2006"/>

Examples of biologically important fatty acids include the [[eicosanoid]]s, derived primarily from [[arachidonic acid]] and [[eicosapentaenoic acid]], that include [[prostaglandin]]s, [[leukotriene]]s, and [[thromboxane]]s. [[Docosahexaenoic acid]] is also important in biological systems, particularly with respect to sight.<ref name="The Lipid Chronicles">{{cite web|title=A Long Lipid, a Long Name: Docosahexaenoic Acid|url=http://www.samuelfurse.com/2011/12/a-long-name-a-long-lipid-docosahexaenoic-acid/ | vauthors = Furse S |work=The Lipid Chronicles|date=2011-12-02}}</ref><ref>{{cite web|title=DHA for Optimal Brain and Visual Functioning|url=http://www.dhaomega3.org/Overview/DHA-for-Optimal-Brain-and-Visual-Functioning|publisher=DHA/EPA Omega-3 Institute}}</ref> 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 ester]]s, fatty acid thioester [[coenzyme A]] derivatives, fatty acid thioester [[Acyl carrier protein|ACP]] derivatives and fatty acid carnitines. The fatty amides include [[N-acylethanolamine|N-acyl ethanolamines]], such as the [[cannabinoid]] neurotransmitter [[anandamide]].<ref name="Fezza_2008"/>

===Glycerolipids===
[[Image:Fat triglyceride shorthand formula.PNG|thumb|upright=1.3|Example of an unsaturated fat triglyceride (C<sub>55</sub>H<sub>98</sub>O<sub>6</sub>). Left part: [[glycerol]]; right part, from top to bottom: [[palmitic acid]], [[oleic acid]], [[alpha-linolenic acid]].]]

Glycerolipids are composed of mono-, di-, and tri-substituted [[glycerol]]s,<ref name="Coleman_2004"/> the best-known being the fatty acid [[Ester|triesters]] of glycerol, called [[triglyceride]]s. 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 bonds of triglycerides and the release of glycerol and fatty acids from [[adipose tissue]] are the initial steps in metabolizing fat.<ref name = "van_Holde_1996" />{{rp|630–1}}

Additional subclasses of glycerolipids are represented by glycosylglycerols, which are characterized by the presence of one or more [[monosaccharide|sugar residues]] attached to glycerol via a [[glycosidic linkage]]. Examples of structures in this category are the digalactosyldiacylglycerols found in plant membranes<ref name="Hölzl_2007"/> and seminolipid from mammalian [[sperm cells]].<ref name="Honke_2004"/>

===Glycerophospholipids===
{{Main|Glycerophospholipid}}
[[File:Phosphatidyl-ethanolamine.svg|thumb|300px|[[Phosphatidylethanolamine]]]]

Glycerophospholipids, usually referred to as [[phospholipid]]s (though [[sphingomyelin]]s are also classified as phospholipids), are ubiquitous in nature and are key components of the [[lipid bilayer]] of cells,<ref name="The Structure of a Membrane">{{cite news|title=The Structure of a Membrane|url=http://www.samuelfurse.com/2011/11/the-structure-of-a-membrane/|work=The Lipid Chronicles|access-date=2011-12-31|date=2011-11-05}}</ref> as well as being involved in [[metabolism]] and [[cell signaling]].<ref name="Berridge_1989"/> Neural tissue (including the brain) contains relatively high amounts of glycerophospholipids, and alterations in their composition has been implicated in various neurological disorders.<ref name="pmid10878232">{{cite journal | vauthors = Farooqui AA, Horrocks LA, Farooqui T | title = Glycerophospholipids in brain: their metabolism, incorporation into membranes, functions, and involvement in neurological disorders | journal = Chemistry and Physics of Lipids | volume = 106 | issue = 1 | pages = 1–29 | date = June 2000 | pmid = 10878232 | doi = 10.1016/S0009-3084(00)00128-6 }}</ref> 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 [[eukaryote]]s and eubacteria, or the ''sn''-1 position in the case of [[archaebacteria]].<ref name="Ivanova_2007"/>

Examples of glycerophospholipids found in [[biological membrane]]s 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]]s and [[phosphatidic acid]]s are either precursors of or, themselves, membrane-derived [[second messenger system|second messengers]].<ref name = "van_Holde_1996" />{{rp|844}} 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.<ref name="Paltauf_1994"/>

===Sphingolipids===
{{Main|Sphingolipid}}
[[File:Sphingomyelin-horizontal-2D-skeletal.png|thumb|300px|[[Sphingomyelin]]]]

Sphingolipids are a complicated family of compounds<ref name="Merrill">{{cite book | veditors = Vance JE, Vance EE | vauthors = Merrill AH, Sandoff K | title=Biochemistry of Lipids, Lipoproteins and Membranes | date = 2002 | publisher = Elsevier | location = Amsterdam | isbn = 978-0-444-51138-6 | edition = 4th | chapter = Chapter 14: Sphingolipids: Metabolism and Cell Signaling | chapter-url = http://bio.ijs.si/~krizaj/group/Predavanja%202011/Biochemistry%20Lipids%20Lipoproteins%20and%20Membranes/14.pdf | pages = 373–407  }}</ref> that share a common structural feature, a [[sphingoid base]] backbone that is synthesized [[de novo synthesis|''de novo'']] from the amino acid [[serine]] and a long-chain fatty acyl CoA, then converted into [[ceramide]]s, 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]]-linked fatty acid. The fatty acids are typically saturated or mono-unsaturated with chain lengths from 16 to 26 carbon atoms.<ref name = "Devlin_1997" />{{rp|421–2}}

The major phosphosphingolipids of mammals are [[sphingomyelin]]s (ceramide phosphocholines),<ref name="Hori_1993"/> whereas insects contain mainly ceramide phosphoethanolamines<ref name="Wiegandt_1992"/> and fungi have phytoceramide phosphoinositols and [[mannose]]-containing headgroups.<ref name="Guan_2008"/> 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 [[cerebroside]]s and [[ganglioside]]s.

===Sterols===
[[File:Cholesterol.svg|thumb|280px|alt=Chemical diagram|Chemical structure of [[cholesterol]]]]
{{Main|Sterol}}
Sterols, such as [[cholesterol]] and its derivatives, are an important component of membrane lipids,<ref name="Bach_2003" /> along with the glycerophospholipids and sphingomyelins. Other examples of sterols are the [[bile acid]]s and their conjugates,<ref name="Russell_2003" /> 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]]; the latter compound is also used as a [[biomarker]] for [[algae|algal]] growth.<ref name="Villinski_2008"/> The predominant sterol in [[fungal]] cell membranes is [[ergosterol]].<ref name="Deacon 2005"/>

Sterols are [[steroid]]s 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]]s and [[signaling molecules]]. The eighteen-carbon (C18) steroids include the [[estrogen]] family whereas the C19 steroids comprise the [[androgen]]s such as [[testosterone]] and [[androsterone]]. The C21 subclass includes the [[progestogens]] as well as the [[glucocorticoid]]s and [[mineralocorticoids]].<ref name = "Stryer_2007" />{{rp|749}} The [[secosteroid]]s, comprising various forms of [[vitamin D]], are characterized by cleavage of the B ring of the core structure.<ref name="Bouillon_2006"/>

===Prenols===
[[File:Geraniol structure.png|thumb|Prenol lipid (2''E''-geraniol)]]
[[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.<ref name="Kuzuyama_2003"/> 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. [[Carotenoid]]s are important simple isoprenoids that function as [[antioxidant]]s and as precursors of [[vitamin A]].<ref name="Rao_2007"/> Another biologically important class of molecules is exemplified by the [[quinone]]s and [[hydroquinone]]s, which contain an isoprenoid tail attached to a quinonoid core of non-isoprenoid origin.<ref name="Brunmark_1989"/> [[Vitamin E]] and [[vitamin K]], as well as the [[ubiquinone]]s, are examples of this class. Prokaryotes synthesize polyprenols (called [[bactoprenol]]s) in which the terminal isoprenoid unit attached to oxygen remains unsaturated, whereas in animal polyprenols ([[dolichol]]s) the terminal isoprenoid is reduced.<ref name="Swiezewska_2005"/>

===Saccharolipids===
[[File:Kdo2-lipidA.png|thumb|right|300px|Structure of the saccharolipid Kdo<sub>2</sub>-lipid A.<ref name="Raetz_2006"/> [[Glucosamine]] residues in blue, [[3-Deoxy-D-manno-oct-2-ulosonic acid|Kdo]] residues in red, [[acyl]] chains in black and [[phosphate]] groups in green.]]

[[Saccharolipid]]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]] precursors of the [[lipid A|Lipid&nbsp;A]] component of the [[lipopolysaccharide]]s in [[Gram-negative bacteria]]. Typical lipid&nbsp;A molecules are [[disaccharides]] of glucosamine, which are derivatized with as many as seven fatty-acyl chains. The minimal lipopolysaccharide required for growth in [[Escherichia coli|''E. coli'']] is Kdo<sub>2</sub>-Lipid A, a hexa-acylated disaccharide of glucosamine that is glycosylated with two 3-deoxy-D-manno-octulosonic acid (Kdo) residues.<ref name="Raetz_2006"/>

===Polyketides===
[[Polyketide|Polyketides]] are synthesized by polymerization of [[acetyl]] and [[Propionyl-CoA|propionyl]] subunits by classic enzymes as well as iterative and multimodular enzymes that share mechanistic features with the [[fatty acid synthase]]s. They comprise many [[secondary metabolite]]s and [[natural products]] from animal, plant, bacterial, fungal and marine sources, and have great structural diversity.<ref name="Walsh_2004"/><ref name="Caffrey_2008"/> Many [[polyketide]]s are cyclic molecules whose backbones are often further modified by [[glycosylation]], [[methylation]], [[hydroxylation]], [[oxidation]], or other processes. Many commonly used [[antimicrobial]], [[antiparasitic]], and [[anticancer]] agents are polyketides or polyketide derivatives, such as [[erythromycin]]s, [[tetracycline antibiotics|tetracyclines]], [[avermectin]]s, and antitumor [[epothilone]]s.<ref name="Minto_2008"/>