MetaCyc Pathway: acyl-ACP thioesterase pathway

Pathway diagram: acyl-ACP thioesterase pathway

This view shows enzymes only for those organisms listed below, in the list of taxa known to possess the pathway. If an enzyme name is shown in bold, there is experimental evidence for this enzymatic activity.

Superclasses: Biosynthesis Fatty Acid and Lipid Biosynthesis

Some taxa known to possess this pathway include ? : Arabidopsis thaliana col

Expected Taxonomic Range: Archaea , Bacteria , Eukaryota

Acyl-ACP thioesterase releases free fatty acids from Acyl-ACPs, synthesized from de novo fatty acid biosynthesis (see fatty acid elongation -- saturated). The reaction terminates fatty acid biosynthesis. In plants, fatty acid biosynthesis occurs in the plastid and thus requires plastid-localized acyl-ACP thioesterases. The main products of acyl-ACP thioesterase are oleate (C18:0) and to a lesser extent palmitate (C16:0) in the vegetative tissues of all plants. The released free fatty acids are re-esterified to coenzyme A in the plastid envelope and exported out of plastid.

There are two isoforms of acyl-ACP thioesterase, FatA and FatB. Substrate specificity of these isoforms determines the chain length and level of saturated fatty acids in plants. The highest activity of FatA is with C18:1-ACP. FatA has very low activities towards other acyl-ACPs when compared with C18:1-ACP. FatB has highest activity with C16:0-ACP. It also has significant high activity with C18:1-ACP, followed by C18:0-ACP and C16:1-ACP. Kinetics studies of FatA and FatB indicate that their substrate specificities with different acyl-ACPs came from the Kcat values, rather than from Km. Km values of the two isoforms with different substrates are similar, in the micromolar order. Domain swapping of FatA and FatB indicates the N-terminus of the isoforms determines their substrate specificities [Salas02a]. For those plants which predominantly accumulate medium-chain length saturated fatty acids in seeds, they evolved with specialized FatB and/or FatA thioesterases [Voelker01]. For example, laurate (12:0) is the predominant seed oil in coconut. Correspondingly, the medium-chain specific acyl-ACP thioesterase activity was detected in coconut seeds.

Unification Links: AraCyc:PWY-5142


Salas02a: Salas JJ, Ohlrogge JB (2002). "Characterization of substrate specificity of plant FatA and FatB acyl-ACP thioesterases." Arch Biochem Biophys 403(1);25-34. PMID: 12061798

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."

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Please cite the following article in publications resulting from the use of MetaCyc: Caspi et al, Nucleic Acids Research 42:D459-D471 2014
Page generated by SRI International Pathway Tools version 19.0 on Mon Jul 6, 2015, biocyc11.