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Aquifex aeolicus VF5 Pathway: stearate biosynthesis II (bacteria and plants)

Pathway diagram: stearate biosynthesis II (bacteria and plants)

If an enzyme name is shown in bold, there is experimental evidence for this enzymatic activity.

Locations of Mapped Genes:

Schematic showing all replicons, marked with selected genes

Synonyms: stearic acid biosynthesis

Superclasses: Biosynthesis Fatty Acid and Lipid Biosynthesis Fatty Acid Biosynthesis Stearate Biosynthesis

Pathway Summary from MetaCyc:
Stearate is a saturated fatty acid that occurs in many animal and vegetable fats and oils. It is an important constituent of milk fats, lard, and cocoa and shea butters. Stearate was first described by Michel Eugene Chevreul in 1823, and its name comes from Greek for "hard fat", reflecting the fact that stearate forms a waxy solid [Sampath05].

De novo plant fatty acid synthesis occurs primarily in plastids and chloroplasts (it was shown that at least in Arabidopsis fatty acid synthesis also occurs in the mitochondria). The process involves a series of reactions catalyzed by fatty acid synthase (FAS) complexes that provoke the condensation of a malonyl-[acp] and an acyl-[acyl-carrier protein] (see fatty acid elongation -- saturated).

In animals the last [acp]-bound intermediate appears to be a palmitoyl-[acp], and elongation to stearate proceeds via CoA-bound intermediates in the ER (see stearate biosynthesis I (animals and fungi)). In bacteria and plants elongation to stearate appears to continue via [acp]-bound intermediates, and a stearoyl-[acp] has been detected in many species [Jaworski74]. At least in plants, the elongation from a palmitoyl-[acp] to a stearoyl-[acp] is catalyzed by a dedicated β-ketoacyl-[acp] synthase, encoded by KASII (The enzyme involved in de novo fatty acid synthesis system up to a palmitoyl-[acp] is encoded by KASI) [Jaworski74a, Voelker01].

Strearoyl-[acp] can be hydrolyzed by acyl-ACP thioesterase enzymes, in which case the free fatty acids are exported to the cytosol [Jones95].

However, a more common route involves desaturation of stearate to oleate. Plants contain acyl-ACP desaturase enzymes, which introduce a double bond in the 9 position of a stearoyl-[acp] to generate an oleoyl-[acp] (see oleate biosynthesis I (plants)). Stearate is the only fatty acid that is desaturated in its ACP-bound form. All other desaturation reactions occur only after the fatty acid has been attached to a glycerolipid backbone.

In cyanobacteria stearate is also often desaturated to oleate. However, unlike plants, in cyanobacteria this desaturation reaction occurs only after the incorporation of stearate into a lipid. Prior to incorporation into a lipid, the fatty acid is transferred to a coenzyme A molecule, resulting in formation of stearoyl-CoA.

Since the activity of the desaturase enzymes is often very high, oleate is often the main product of intraplastidial fatty acid synthesis [Harwood96].

Pathway Evidence Glyph:

Pathway evidence glyph

Key to pathway glyph edge colors: ?

  An enzyme catalyzing this reaction is present in this organism
  An enzyme catalyzing this reaction was identified in this organism by the Pathway Hole Filler
  No enzyme catalyzing this reaction has been identified in this organism
  The reaction and any enzyme that catalyzes it (if one has been identified) is unique to this pathway

Credits:
Created in MetaCyc 31-Jul-2008 by Caspi R , SRI International
Imported from MetaCyc 08-Aug-2014 by Subhraveti P , SRI International


References

Harwood96: Harwood JL (1996). "Recent advances in the biosynthesis of plant fatty acids." Biochim Biophys Acta 1301(1-2);7-56. PMID: 8652653

Jaworski74: Jaworski JG, Stumpf PK (1974). "Fat metabolism in higher plants. Properties of a soluble stearyl-acyl carrier protein desaturase from maturing Carthamus tinctorius." Arch Biochem Biophys 162(1);158-65. PMID: 4831331

Jaworski74a: Jaworski JG, Goldschmidt EE, Stumpf PK (1974). "Fat metabolism in higher plants. Properties of the palmityl acyl carrier protein: stearyl acyl carrier protein elongation system in maturing safflower seed extracts." Arch Biochem Biophys 163(2);769-76. PMID: 4153356

Jones95: Jones A, Davies HM, Voelker TA (1995). "Palmitoyl-acyl carrier protein (ACP) thioesterase and the evolutionary origin of plant acyl-ACP thioesterases." Plant Cell 7(3);359-71. PMID: 7734968

Sampath05: Sampath H, Ntambi JM (2005). "The fate and intermediary metabolism of stearic acid." Lipids 40(12);1187-91. PMID: 16477801

Voelker01: Voelker T, Kinney AJ (2001). "Variations in the biosynthesis of seed-storage lipids." Annu Rev Plant Physiol Plant Mol Biol 52;335-361. PMID: 11337402

Other References Related to Enzymes, Genes, Subpathways, and Substrates of this Pathway

Byers07: Byers DM, Gong H (2007). "Acyl carrier protein: structure-function relationships in a conserved multifunctional protein family." Biochem Cell Biol 85(6);649-62. PMID: 18059524

Latendresse13: Latendresse M. (2013). "Computing Gibbs Free Energy of Compounds and Reactions in MetaCyc."

Rubio06: Rubio S, Larson TR, Gonzalez-Guzman M, Alejandro S, Graham IA, Serrano R, Rodriguez PL (2006). "An Arabidopsis mutant impaired in coenzyme A biosynthesis is sugar dependent for seedling establishment." Plant Physiol 140(3);830-43. PMID: 16415216


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