Acyl-ACP thioesterase enzymes, which cleave fatty acyl thioester bonds to release free fatty acids, contribute to much of the fatty acid diversity in plants. In Arabidopsis thaliana, a family of four single hot-dog fold domain, plastid-localized acyl-lipid thioesterases (AtALT1-4) generate medium-chain (C6-C14) fatty and β–keto fatty acids as secondary metabolites. These volatile products may serve to attract insect pollinators or deter predatory insects. Homologs of AtALT1-4 are present in all plant taxa, but are nearly all uncharacterized. Despite high sequence identity, AtALT1-4 generate different lipid products, suggesting that ALT homologs in other plants also have highly varied activities. We investigated the catalytic diversity of ALT-like thioesterases by screening the substrate specificities of 15 ALT homologs from monocots, eudicots, a lycophyte, a green microalga, and the ancient gymnosperm Gingko biloba, via expression in Escherichia coli. Overall, these enzymes had highly varied substrate preferences compared to one another and to AtALT1-4, and could be classified into four catalytic groups comprising members from diverse taxa. Group 1 ALTs primarily generated 14:1 β-keto fatty acids, Group 2 ALTs produced 6–10 carbon fatty/β-keto fatty acids, Group 3 ALTs predominantly produced 12–14 carbon fatty acids, and Group 4 ALTs mainly generated 16 carbon fatty acids. Enzymes in each group differed significantly in the quantities of lipids and types of minor products they generated in E. coli. Medium-chain fatty acids are used to manufacture insecticides, pharmaceuticals, and biofuels, and ALT-like proteins are ideal candidates for metabolic engineering to produce specific fatty acids in significant quantities.

Additional Metadata
Keywords Acyl carrier protein, Enzyme family, Fatty acid, Methylketone, Substrate specificity, Thioesterase
Persistent URL dx.doi.org/10.1016/j.plaphy.2018.03.013
Journal Plant Physiology and Biochemistry
Citation
Kalinger, R.S. (Rebecca S.), Pulsifer, I.P. (Ian P.), & Rowland, O. (2018). Elucidating the substrate specificities of acyl-lipid thioesterases from diverse plant taxa. Plant Physiology and Biochemistry, 127, 104–118. doi:10.1016/j.plaphy.2018.03.013