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. 2022 Jan 27;12:814138. doi: 10.3389/fpls.2021.814138

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Copyright © 2022 Bizzio, Tieman and Munoz.

This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

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Possible biosynthesis routes to volatile branched-chain acyl esters. (A) Mitochondrial BCAT- and BCKDH-mediated catabolism of free branched-chain amino acids to branched-chain acyl-CoAs followed by esterification with alcohols by AAT enzymes. (B) De novo chloroplast synthesis of branched-chain α-ketoacids followed by BCKDH-mediated catabolism to branched-chain acyl-CoAs and subsequent AAT-mediated esterification with alcohols. (C) CMS-initiated synthesis of α-keto-β-methylpentanoate followed by BCKDH-mediated catabolism to 2-methylbutyl acyl-CoA and subsequent AAT-mediated esterification with alcohols to form 2-methylbutanoate esters. (D) Branched-chain aldehyde synthesis from free branched-chain amino acids via SDC-L, followed by conversion of branched-chain aldehydes to branched-chain carboxylic acids through ALDH enzymes, followed by activation to branched-chain acyl-CoAs by CCL enzymes and subsequent condensation with alcohols via AAT to generate branched-chain acyl esters. The first, second, and fourth routes are capable of generating acyl esters with all three branched-chain structures, while the third route can only generate 2-methylbutanoate esters. The four pathways are illustrated yielding 2-methylbutanoate esters from appropriate precursors for ease of comparison. BCAT, branched-chain amino acid aminotransferase; BCKDH, branched-chain α-ketoacid dehydrogenase complex; AAT, alcohol acyltransferase; BCKA, branched-chain α-ketoacids; SDC-L, serine-decarboxylase like enzyme; ALDH, aldehyde dehydrogenase; CCL, carboxyl-CoA ligase.