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{{enzyme
| Name = acyl-[acyl-carrier-protein] desaturase
| EC_number = 1.14.19.2
| CAS_number = 37256-86-3
| IUBMB_EC_number = 1/14/19/2
| GO_code = 0045300
| image =2XZ0-image1.png
| width = 
| caption = Partially occupied Stearoyl Acyl Carrier Protein Desaturase from Ricinius Communis
}}
In [[enzymology]], an '''acyl-[acyl-carrier-protein] desaturase''' ({{EC number|1.14.19.2}}) is an [[enzyme]] that [[catalysis|catalyzes]] the [[chemical reaction]]

:stearoyl-[acyl-carrier-protein] + reduced acceptor + O<sub>2</sub> <math>\rightleftharpoons</math> oleoyl-[acyl-carrier-protein] + acceptor + 2 H<sub>2</sub>O

The systematic name of this enzyme class is '''acyl-[acyl-carrier-protein], hydrogen-donor:oxygen oxidoreductase'''. Other names in common use include '''stearyl acyl carrier protein desaturase''', and '''stearyl-ACP desaturase'''.  This enzyme participates in [[polyunsaturated fatty acid biosynthesis]].  It employs one [[cofactor (biochemistry)|cofactor]], [[ferredoxin]].<ref name="PUB16658913">{{cite journal|last=Jacobson|first=BS|author2=Jaworski J|author3= Stumpf PF|title=Fat Metabolism in Higher Plants LXII. Stearl-acyl Carrier Protein Desaturase from Spinach Chloroplasts|journal=Plant physiology|year=1974|volume=54|issue=4|pages=484–486|pmid=16658913|issn=0032-0889}}</ref>

==Reaction==

The 3 [[substrate (biochemistry)|substrates]] of this enzyme are [[stearoyl-(acyl-carrier-protein)]], [[reduced acceptor]], and [[oxygen|O<sub>2</sub>]], whereas its 3 [[product (chemistry)|products]] are [[oleoyl-(acyl-carrier-protein)]], [[Electron acceptor|acceptor]], and [[water|H<sub>2</sub>O]].<ref name ="PUB4300868">{{cite journal | author = Nagai J, Bloch K | year = 1968 | title = Enzymatic desaturation of stearyl acyl carrier protein | journal = J. Biol. Chem.  | volume = 243 | pages = 4626&ndash;33  | pmid = 4300868 | issue = 17}}</ref>

The precise mechanism of this class of enzymes is not known, however recent studies using the [[kinetic isotope effect]] suggest that the rate limiting step is the removal of a hydrogen from the carbon nearest the carboxylic acid group. The diiron cluster moves through to a peroxo intermediate which can then dehydrate the short-lived alcohol intermediate, liberating water.<ref name="PUB12413540">{{cite journal|last=Behrouzian|first=B|author2=Buist, BH|title=Fatty acid desaturation: variations on an oxidative theme|journal=Current Opinion in Chemical Biology|year=2002|volume=6|issue=5|page=577|doi=10.1016/S1367-5931(02)00365-4| pmid = 12413540}}</ref> There are a variety of specific enzymes within this class that attack using this mechanism, but do so at different points along the carbon chain of their respective fatty acids <ref name=PUB12413540 />

==Biological Function==

This enzyme belongs to the family of [[oxidoreductase]]s, specifically those acting on paired donors, with O<sub>2</sub> as oxidant and incorporation or reduction of oxygen. The oxygen incorporated need not be derived from O<sub>2</sub> with [[oxidation]] of a pair of donors resulting in the formation of H<sub>2</sub>O.

This family of enzymes is found only in the plastids of higher plant cells, unlike other desaturases such as [[acyl-lipid desaturase]]s and [[acyl-CoA desaturase]]s.<ref name="PUB10209131">{{cite journal|last=Murphy|first=D|title=Production of novel oils in plants|journal=Current Opinion in Biotechnology|year=1999|volume=10|issue=2|pages=175–180|doi=10.1016/S0958-1669(99)80031-7|pmid=10209131}}</ref> The regiospecific role of stearoyl-ACP desaturase is to initialise multiple [[Saturated compound|desaturations]] by acyl-lipid desaturases. [[Oleic acid]] is formed from this reaction is transported to either the [[thylakoid]] or cytoplasm to complete desaturation.<ref name=PUB9767077>{{cite journal|last=Los|first=D|author2=Murata,N|title=Structure and expression of fatty acid desaturases|journal=Biochimica et Biophysica Acta (BBA) - Lipids and Lipid Metabolism|year=1998|volume=1394|issue=1|pages=3–15|doi=10.1016/S0005-2760(98)00091-5|pmid=9767077}}</ref>

==Structural studies==

As of late 2007, 5 [[tertiary structure|structures]] have been solved for this class of enzymes, with [[Protein Data Bank|PDB]] accession codes {{PDB link|1OQ4}}, {{PDB link|1OQ7}}, {{PDB link|1OQ9}}, {{PDB link|1OQB}}, and {{PDB link|1ZA0}}.[[File:2xz1-image1.png|thumb|Fully bound Stearoyl-acyl-carrier-protein desaturase PDB [[Protein Data Bank|2XZ1]].]] {{PDB link|2XZ0}} and {{PDB link|2XZ1}} show the dramatic change in conformation of the enzyme when bound (2XZ1) and unbound (2XZ0). As a dimer, the fatty acid chain binds to a hydrophobic pocket at the interface of the two dimers,.<ref name="PUB 15012248" /> This central channel is mirrored by binding sites for the electron donors on either side.

The stabilisation of the diiron-oxo element required to catalyse the reaction has been of particular interest. Crystallographic studies<ref name=PUB8460163>{{cite journal|last=Fox|first=BG|author2=Sommerville,C. |author3=Munck, E|title=Stearoyl-acyl carrier protein delta 9 desaturase from Ricinus communis is a diiron-oxo protein|journal=Proceedings of the National Academy of Sciences|date=15 March 1993|volume=90|issue=6|pages=2486–2490|doi=10.1073/pnas.90.6.2486|pmid=8460163}}</ref> suggest that the iron groups are held in place by the desaturase using [[aspartate]] and [[glutamate]]. A structure of aspartate-X-X-histidine was found to be a common motif in several plant species.<ref name=PUB10394947>{{cite journal|last=Haralampidis|first=K|author2=D. Milioni |author3=J. Sanchez |author4=M. Baltrusch |author5=E. Heinz |author6=P. Hatzopoulos|title=emporal and transient expression of stearoyl-ACP carrier protein desaturase gene during olive fruit development|journal=Journal of Experimental Botany|year=1998|volume=49|issue=327|pages=1661–1669|doi=10.1093/jxb/49.327.1661|pmid=10394947}}</ref> Interestingly, this desaturase family can be further divided by the consensus motif used to hold the iron clusters in place. Of particular note are the "soluble" desaturases, which use carboxylic acid groups, whereas it is possible for some variants to use histidines instead. The histidine rich desaturases tend to be [[integral membrane protein]]s.<ref name ="PUB 15012248">{{cite journal|last=Shanklin|first=J|author2=Cahoon, E|title=Desaturation and related modifications of fatty acids|journal=Annual Review of Plant Physiology and Plant Molecular Biology|year=1998|volume=49|pages=611–641|doi=10.1146/annurev.arplant.49.1.611|pmid=15012248}}</ref>

Structural studies strongly suggest that the animal form of this enzyme ([[Stearoyl-acyl-carrier-protein desaturase]]) is evolutionarily divergent from the forms found in plants and fungi.<ref name="PUB00004734">{{cite journal |author=Shanklin J, Somerville C |title=Stearoyl-acyl-carrier-protein desaturase from higher plants is structurally unrelated to the animal and fungal homologs |journal=Proc. Natl. Acad. Sci. U.S.A. |volume=88 |issue=6 |pages=2510–2514 |year=1991 |pmid=2006187 |doi=10.1073/pnas.88.6.2510 |pmc=51262}}</ref> This is to be expected as the roles of the enzymes are different in both. For example, in insects, the desaturase is critical in the formation of [[ceramide]], and for complex signalling molecules ([[pheremones]]),<ref name=PUB12413540 /> while in fungi, the function of the enzyme, and concentration of unsaturated lipids is regulated in response to function of growth temperature by controlling membrane fluidity in cells.<ref name=PUB8246839>{{cite journal|last=Magnuson|first=K|author2=Jackowski, S. |author3= Rock, C. |author4= Cronan, J.|title=Regulation of fatty acid biosynthesis in Escherichia coli|journal=Microbiological Reviews|year=1993|volume=57|issue=3|pages=522–542|pmid=8246839}}</ref>

== Potential Industrial Relevance==

This enzyme class plays a critical role in the biosynthesis of unsaturated fatty acids in plants. The most interesting aspect of this class of enzymes is their specificity.<ref name=PUB12413540 /> A common theme in recent research has been to identify uncommon desaturases in various plants<ref name="PUB 15012248" /><ref name=PUB11027717>{{cite journal|last=Schultz|first=D|author2=Suh, M. |author3=Ohlrogge|title=Stearoyl-Acyl Carrier Protein and Unusual Acyl-Acyl Carrier Protein Desaturase Activities Are Differentially Influenced by Ferredoxin|journal=Plant Physiology|year=2000|volume=124|issue=2|pages=681–692|doi=10.1104/pp.124.2.681|pmid=11027717}}</ref>  and isolate their genetic code. In particular, this can then be inserted into model cells (such as ''Escherichia coli'') and up-regulated through metabolic engineering to skew the composition of oils produced by the model cells.<ref name=PUB8550538>{{cite journal|last=Cahoon|first=E|author2=Mills, L. |author3=Shanklin, J.|title=Modification of the fatty acid composition of Escherichia coli by coexpression of a plant acyl-acyl carrier protein desaturase and ferredoxin|journal=Journal of Bacteriology|year=1996|volume=178|issue=3|pages=936–939|pmid=8550538}}</ref>

This becomes a particularly lucrative endeavour if it becomes possible to successfully synthesise so-called [[Omega-3 fatty acid]]s or other [[nutraceutical]] products from basic saturated fatty acids, and extract them from their hosts.

==References==
{{reflist|1}}

[[Category:EC 1.14.19]]
[[Category:Enzymes of known structure]]