If an enzyme name is shown in bold, there is experimental evidence for this enzymatic activity.
Locations of Mapped 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 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 stearoyl-[acp] has been detected in many species [Jaworski74a]. At least in plants, the elongation from palmitoyl-[acp] to 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 palmitoyl-[acp] is encoded by KASI) [Jaworski74, Voelker01].
Strearoyl-[acp] can be hydrolyzed by acyl-ACP thioesterase enzymes [Jones95]. However, before the fatty acids can be exported out of the chloroplast they must be activated to fatty acyl-CoAs by an outer envelope LACS9 activity. Once activated, they are exported to the endoplasmic reticulum [Schnurr04].
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 stearoyl-[acp] to generate 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.
Pathway Evidence Glyph:
This organism is in the expected taxonomic range for this pathway.
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 is unique to this pathway in MetaCyc
Jaworski74: 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
Jaworski74a: 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
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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