Enzymatic synthesis of nylon precursors by 4-aminobutyrate aminotransferase and 6-oxohexanoate dehydrogenase

Abstract

6-Aminocaproic acid and adipic acid are the key value-added chemical precursors in the pharmaceutical, solvent and polyamide industry, including nylon-6, and nylon-6, 6. An enzymatic interconversion of the two precursors can provide a convenient and eco-friendly biosynthetic route to each of the precursors and thus, require analysis of the reaction process. Herein, an in vitro enzymatic method was employed to convert the two precursors while most studies so far have focused on the whole cell bioconversion to investigate the process. 4-Aminobutyrate aminotransferase was utilized to mediate the reactions between 6-aminocaproic acid and the intermediate 6-oxohexanoic acid with the aid of pyridoxal 5’-phosphate and amine donor/acceptor. 6-Oxohexanoate dehydrogenase was utilized for the reaction from 6-oxohexanoic acid to adipic acid with NADP⁺. A range of reaction conditions were investigated including the type of amine donor, pH conditions, the concentrations of enzyme and amine donor/acceptor. The optimum condition resulted in 78% yield for the reaction from 6-oxohexanoic acid to 6-aminocaproic acid. The yield for the one-pot, two-step enzymatic cascade from 6-aminocaproic acid via 6-oxohexanoate intermediate to adipic acid was 88%, which was higher than the yield for each individual step in the cascade, with 31% and 32%, for the first and second step, respectively. Furthermore, structural analysis on the active site of the 4-aminobutyrate aminotransferase docked with a range of amine donors implicates the optimal donor is glutamate in accordance with the experimental data and suggests enzyme engineering possibilities for more readily available donors to facilitate the industrial application of the process.