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acyl- desaturase
Partially occupied Stearoyl Acyl Carrier Protein Desaturase from Ricinius Communis
Identifiers
EC no.	1.14.19.2
CAS no.	37256-86-3
Databases
IntEnz	IntEnz view
BRENDA	BRENDA entry
ExPASy	NiceZyme view
KEGG	KEGG entry
MetaCyc	metabolic pathway
PRIAM	profile
PDB structures	RCSB PDB PDBe PDBsum
Gene Ontology	AmiGO / QuickGO
Search PMC articles PubMed articles NCBI proteins

In enzymology, an acyl- desaturase (EC 1.14.19.2) is an enzyme that catalyzes the chemical reaction

stearoyl- + reduced acceptor + O₂ ${\displaystyle \rightleftharpoons }$ oleoyl- + acceptor + 2 H₂O

The systematic name of this enzyme class is acyl-, 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, ferredoxin.

Reaction

The 3 substrates of this enzyme are stearoyl-(acyl-carrier-protein), reduced acceptor, and O₂, whereas its 3 products are oleoyl-(acyl-carrier-protein), acceptor, and H₂O.

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. 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

Biological Function

This enzyme belongs to the family of oxidoreductases, specifically those acting on paired donors, with O₂ as oxidant and incorporation or reduction of oxygen. The oxygen incorporated need not be derived from O₂ with oxidation of a pair of donors resulting in the formation of H₂O.

This family of enzymes is found only in the plastids of higher plant cells, unlike other desaturases such as acyl-lipid desaturases and acyl-CoA desaturases. The regiospecific role of stearoyl-ACP desaturase is to initialise multiple desaturations by acyl-lipid desaturases. Oleic acid is formed from this reaction is transported to either the thylakoid or cytoplasm to complete desaturation.

Structural studies

As of late 2007, 5 structures have been solved for this class of enzymes, with PDB accession codes 1OQ4, 1OQ7, 1OQ9, 1OQB, and 1ZA0.

2XZ0 and 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,. 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 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. 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 proteins.

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. 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), 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.

Potential Industrial Relevance

This enzyme class plays a critical role in the biosynthesis of unsaturated fatty acids in plants, and are very specific to their substrates. A common theme in recent research has been to identify uncommon desaturases in various plants 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.

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

References

1. Jacobson, BS; Jaworski J; Stumpf PF (1974). "Fat Metabolism in Higher Plants LXII. Stearl-acyl Carrier Protein Desaturase from Spinach Chloroplasts". Plant Physiology. 54 (4): 484–486. doi:10.1104/pp.54.4.484. ISSN 0032-0889. PMC 367438. PMID 16658913.
2. Nagai J, Bloch K (1968). "Enzymatic desaturation of stearyl acyl carrier protein". J. Biol. Chem. 243 (17): 4626–33. doi:10.1016/S0021-9258(18)93235-7. PMID 4300868.
3. ^ Behrouzian, B; Buist, BH (2002). "Fatty acid desaturation: variations on an oxidative theme". Current Opinion in Chemical Biology. 6 (5): 577–82. doi:10.1016/S1367-5931(02)00365-4. PMID 12413540.
4. Murphy, D (1999). "Production of novel oils in plants". Current Opinion in Biotechnology. 10 (2): 175–180. doi:10.1016/S0958-1669(99)80031-7. PMID 10209131.
5. Los, D; Murata, N (1998). "Structure and expression of fatty acid desaturases". Biochimica et Biophysica Acta (BBA) - Lipids and Lipid Metabolism. 1394 (1): 3–15. doi:10.1016/S0005-2760(98)00091-5. PMID 9767077.
6. ^ Shanklin, J; Cahoon, E (1998). "Desaturation and related modifications of fatty acids". Annual Review of Plant Physiology and Plant Molecular Biology. 49: 611–641. doi:10.1146/annurev.arplant.49.1.611. PMID 15012248.
7. Fox, BG; Sommerville, C.; Munck, E (15 March 1993). "Stearoyl-acyl carrier protein delta 9 desaturase from Ricinus communis is a diiron-oxo protein". Proceedings of the National Academy of Sciences. 90 (6): 2486–2490. Bibcode:1993PNAS...90.2486F. doi:10.1073/pnas.90.6.2486. PMC 46112. PMID 8460163.
8. Haralampidis, K; D. Milioni; J. Sanchez; M. Baltrusch; E. Heinz; P. Hatzopoulos (1998). "Temporal and transient expression of stearoyl-ACP carrier protein desaturase gene during olive fruit development". Journal of Experimental Botany. 49 (327): 1661–1669. doi:10.1093/jxb/49.327.1661.
9. Shanklin J, Somerville C (1991). "Stearoyl-acyl-carrier-protein desaturase from higher plants is structurally unrelated to the animal and fungal homologs". Proc. Natl. Acad. Sci. U.S.A. 88 (6): 2510–2514. Bibcode:1991PNAS...88.2510S. doi:10.1073/pnas.88.6.2510. PMC 51262. PMID 2006187.
10. Magnuson, K; Jackowski, S.; Rock, C.; Cronan, J. (1993). "Regulation of fatty acid biosynthesis in Escherichia coli". Microbiological Reviews. 57 (3): 522–542. doi:10.1128/MMBR.57.3.522-542.1993. PMC 372925. PMID 8246839.
11. Schultz, D; Suh, M.; Ohlrogge (2000). "Stearoyl-Acyl Carrier Protein and Unusual Acyl-Acyl Carrier Protein Desaturase Activities Are Differentially Influenced by Ferredoxin". Plant Physiology. 124 (2): 681–692. doi:10.1104/pp.124.2.681. PMC 59173. PMID 11027717.
12. Cahoon, E; Mills, L.; Shanklin, J. (1996). "Modification of the fatty acid composition of Escherichia coli by coexpression of a plant acyl-acyl carrier protein desaturase and ferredoxin". Journal of Bacteriology. 178 (3): 936–939. doi:10.1128/jb.178.3.936-939.1996. PMC 177750. PMID 8550538.

• Biology

• EC 1.14.19
• Enzymes of known structure