Fatty acid: Difference between revisions

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 [[Branched chain fatty acids|unbranched chain]] of an even number of carbon atoms, from 4 to 28.<ref name="iupac">{{cite journal |url=http://goldbook.iupac.org/F02330.html|title=IUPAC Compendium of Chemical Terminology|journal=Pure and Applied Chemistry|volume=67|issue=8–9|publisher=International Union of Pure and Applied Chemistry|year=1997 |pages=1307–1375|doi=10.1351/pac199567081307|access-date=2007-10-31|last1=Moss|first1=G. P.|last2=Smith|first2=P. A. S.|last3=Tavernier|first3=D.|s2cid=95004254|doi-access=free}}</ref> Fatty acids are a major component of the lipids (up to 70% by weight) in some species such as microalgae<ref>{{cite journal |last1=Chen |first1=Lin |title=Biodiesel production from algae oil high in free fatty acids by two-step catalytic conversion |journal=Bioresource Technology |date=2012 |volume=111 |pages=208–214 |doi=10.1016/j.biortech.2012.02.033 |pmid=22401712 |bibcode=2012BiTec.111..208C }}</ref> but in some other organisms are not found in their standalone form, but instead exist as three main classes of [[ester]]s: [[triglyceride]]s, [[phospholipid]]s, and [[cholesteryl ester]]s. In any of these forms, fatty acids are both important [[diet (nutrition)|dietary]] sources of fuel for animals and important structural components for [[cell (biology)|cells]].

The concept of fatty acid (''acide gras'') was introduced in 1813 by [[Michel Eugène Chevreul]],<ref>{{cite journal |last=Chevreul |first=M. E. |date=1813 |title=Sur plusieurs corps gras, et particulièrement sur leurs combinaisons avec les alcalis |journal=Annales de Chimie |volume=88 |pages=225-261225–261 |publisher=H. Perronneau |location=Paris |url=http://gallica.bnf.fr/ark:/12148/bpt6k65741176/f225.item |via=Gallica}}</ref><ref>{{cite book |last=Chevreul |first=M. E. |title=Recherches chimiques sur les corps gras d'origine animale |publisher=Levrault |location=Paris |date=1823 |url=https://archive.org/details/rechercheschimi00chevgoog |via=Internet Archive}}</ref><ref>{{cite web |last=Leray |first=Claude |title=Chronological history of lipid science |website=Cyberlipid Center |date=11 November 2017 |url=http://www.cyberlipid.org/history/history1.htm |url-status=dead |archive-url=https://web.archive.org/web/20171006012012/http://www.cyberlipid.org/history/history1.htm |archive-date=2017-10-06 }}</ref> though he initially used some variant terms: ''graisse acide'' and ''acide huileux'' ("acid fat" and "oily acid").<ref>{{cite encyclopedia |editor-last=Menten |editor-first=P. |title=Dictionnaire de chimie: Une approche étymologique et historique |publisher=De Boeck |location=Bruxelles |date=2013 |isbn=978-2-8041-8175-8 |url=https://books.google.com/books?id=NKTKDgAAQBAJ}}</ref>

In most naturally occurring unsaturated fatty acids, each double bond has three ([[omega-3 fatty acid|n-3n−3]]), six ([[omega-6 fatty acid|n-6n−6]]), or nine ([[omega-9 fatty acid|n-9n−9]]) carbon atoms after it, and all double bonds have a cis configuration. Most fatty acids in the ''trans'' configuration ([[trans fat]]s) are not found in nature and are the result of human processing (e.g., [[hydrogenation]]). Some trans fatty acids also occur naturally in the milk and meat of [[ruminant]]s (such as cattle and sheep). They are produced, by fermentation, in the rumen of these animals. They are also found in [[dairy product]]s from milk of ruminants, and may be also found in [[breast milk]] of women who obtained them from their diet.

|[[ArachidonicOleic acid]] || CH{{sub|3}}(CH{{sub|2}}){{sub|4}}'''CH=CH'''CH{{sub|2}}'''CH=CH'''CH{{sub|2}}'''CH=CH'''CH{{sub|27}}'''CH=CH'''(CH{{sub|2}}){{sub|37}}COOH^{[http://webbook.nist.gov/cgi/cbook.cgi?Name=Arachidonic+Acid&Units=SI NIST]} || ''cis'',''cis'',''cis'',''cis''-Δ{{sup|59}}Δ{{sup|8}},Δ{{sup|11}},Δ{{sup|14}} || 2018:41 || 2018:41(5,8,11,149) || [[omega-69 fatty acid|''n''−6−9]]

|[[EicosapentaenoicElaidic acid]] || CH{{sub|3}}(CH{{sub|2}}'''CH=CH'''CH){{sub|2}}'''CH=CH'''CH{{sub|2}}'''CH=CH'''CH{{sub|2}}'''CH=CH'''CH{{sub|27}}'''CH=CH'''(CH{{sub|2}}){{sub|37}}COOH || ''cis'',''cis'',''cis'',''cis'',''cistrans''-Δ{{sup|59}},Δ{{sup|8}},Δ{{sup|11}},Δ{{sup|14}},Δ{{sup|17}} || 2018:51 || 2018:51(5,8,11,14,179t) || [[omega-39 fatty acid|''n''−3−9]]

|[[Erucic acid]] || CH{{sub|3}}(CH{{sub|2}}){{sub|7}}'''CH=CH'''(CH{{sub|2}}){{sub|11}}COOH || ''cis''-Δ{{sup|13}} || 22:1 || 22:1(13) || [[omega-9 fatty acid|''n''−9]]

|'''''n''−''x''''' ('''''n'' minus ''x'''''; also '''ω−''x''''' or '''omega-omega−''x''''') '''nomenclature''' both provides names for individual compounds and classifies them by their likely biosynthetic properties in animals. A double bond is located on the ''x''^th carbon–carbon bond, [[#Numbering of the carbon atoms in a fatty acid|counting]] from the [[Methyl group|methyl]] end of the molecule backbone. For example, [[α-Linolenic acid|α-linolenic acid]] is classified as a [[omega-3 fatty acid|''n''−3]] or [[omega-3|omega−3]] fatty acid, and so it is likely to share a biosynthetic pathway with other compounds of this type. The ω−''x'', omega-omega−''x'', or "omega" notation is common in popular nutritional literature, but [[IUPAC nomenclature|IUPAC]] has deprecated it in favor of ''n''−''x'' notation in technical documents.<ref name="nomenclature-1979" /> The most commonly researched fatty acid biosynthetic pathways are [[omega-3 fatty acid|''n''−3]] and [[omega-6 fatty acid|''n''−6]].

Carbohydrates are converted into [[Pyruvic acid|pyruvate]] by [[glycolysis]] as the first important step in the conversion of carbohydrates into fatty acids.<ref name=stryer /> Pyruvate is then decarboxylated to form [[acetyl-CoA]] in the [[mitochondrion]]. However, this acetyl CoA needs to be transported into [[cytosol]] where the synthesis of fatty acids occurs. This cannot occur directly. To obtain cytosolic acetyl-CoA, [[Citric acid|citrate]] (produced by the condensation of acetyl-CoA with [[Oxaloacetic acid|oxaloacetate]]) is removed from the [[citric acid cycle]] and carried across the inner mitochondrial membrane into the cytosol.<ref name=stryer /> There it is cleaved by [[ATP citrate lyase]] into acetyl-CoA and oxaloacetate. The oxaloacetate is returned to the mitochondrion as [[malate]].<ref name= ferre>{{cite journal | doi = 10.1159/000100426 | title = SREBP-1c Transcription Factor and Lipid Homeostasis: Clinical Perspective | journal = Hormone Research | year = 2007 | first1 = P. | last1 = Ferre |first2=F. |last2=Foufelle | volume = 68 | issue = 2 | pages = 72–82| doi-broken-date = 1 November 2024 | pmid = 17344645 | quote = this process is outlined graphically in page 73| doi-access = free }}</ref> The cytosolic acetyl-CoA is carboxylated by [[acetyl-CoA carboxylase]] into [[malonyl-CoA]], the first committed step in the synthesis of fatty acids.<ref name= ferre /><ref name=Voet>{{cite book |last1=Voet |first1=Donald |first2=Judith G. |last2=Voet |first3=Charlotte W. |last3=Pratt |title=Fundamentals of Biochemistry |edition=2nd |publisher=John Wiley and Sons |year=2006 |pages=[https://archive.org/details/fundamentalsofbi00voet_0/page/547 547, 556] |isbn=978-0-471-21495-3 |url-access=registration |url=https://archive.org/details/fundamentalsofbi00voet_0/page/547 }}</ref>

Studies on the [[cell membrane]]s of [[mammal]]s and [[reptile]]s discovered that mammalian cell membranes are composed of a higher proportion of polyunsaturated fatty acids ([[docosahexaenoic acid|DHA]], [[omega-3 fatty acid|omega−3 fatty acid]]) than [[reptile]]s.<ref name=hulb1999/> Studies on bird fatty acid composition have noted similar proportions to mammals but with 1/3rd less omega-3omega−3 fatty acids as compared to [[omega-6 fatty acid|omega-6omega−6]] for a given body size.<ref name=hulb2002/> This fatty acid composition results in a more fluid cell membrane but also one that is permeable to various ions ({{chem2|H+}} & {{chem2|Na+}}), resulting in cell membranes that are more costly to maintain. This maintenance cost has been argued to be one of the key causes for the high metabolic rates and concomitant [[warm-blooded]]ness of mammals and birds.<ref name=hulb1999/> However polyunsaturation of cell membranes may also occur in response to chronic cold temperatures as well. In [[fish]] increasingly cold environments lead to increasingly high cell membrane content of both monounsaturated and polyunsaturated fatty acids, to maintain greater membrane fluidity (and functionality) at the lower [[temperature]]s.<ref name=hulb2003xa/><ref name=rayn1991/>

Fatty acids that are required for good health but cannot be made in sufficient quantity from other substrates, and therefore must be obtained from food, are called essential fatty acids. There are two series of essential fatty acids: one has a double bond [[Omega-3 fatty acid|three carbon atoms]] away from the methyl end; the other has a double bond [[Omega-6 fatty acid|six carbon atoms]] away from the methyl end. Humans lack the ability to introduce double bonds in fatty acids beyond carbons 9 and 10, as counted from the carboxylic acid side.<ref>{{cite book|last=Bolsover|first=Stephen R.|title=Cell Biology: A Short Course|url=https://books.google.com/books?id=3a6p9pA5gZ8C&pg=PA42+|date=15 February 2004|publisher=John Wiley & Sons|isbn=978-0-471-46159-3|pages=42ff|display-authors=etal}}</ref> Two essential fatty acids are [[linoleic acid]] (LA) and [[α-Linolenic acid|alpha-linolenic acid]] (ALA). These fatty acids are widely distributed in plant oils. The human body has a limited ability to convert ALA into the longer-chain [[omega-3 fatty acid]]s — [[eicosapentaenoic acid]] (EPA) and [[docosahexaenoic acid]] (DHA), which can also be obtained from fish. Omega-3Omega−3 and [[Omega-6 fatty acid|omega-6omega−6]] fatty acids are [[Biosynthesis|biosynthetic]] precursors to [[Cannabinoid#Endocannabinoids|endocannabinoids]] with [[Nociception|antinociceptive]], [[anxiolytic]], and [[Nervous system|neurogenic]] properties.<ref>{{Cite journal|last1=Ramsden|first1=Christopher E.|last2=Zamora|first2=Daisy|author-link3=Alexandros Makriyannis|last3=Makriyannis|first3=Alexandros|last4=Wood|first4=JodiAnne T.|last5=Mann|first5=J. Douglas|last6=Faurot|first6=Keturah R.|last7=MacIntosh|first7=Beth A.|last8=Majchrzak-Hong|first8=Sharon F.|last9=Gross|first9=Jacklyn R.|date=August 2015|title=Diet-induced changes in n-3 and n-6 derived endocannabinoids and reductions in headache pain and psychological distress|journal=The Journal of Pain|volume=16|issue=8|pages=707–716|doi=10.1016/j.jpain.2015.04.007|issn=1526-5900|pmc=4522350|pmid=25958314}}</ref>