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
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 [Salas02]. 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
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