Production of nonnatural straight-chain amino acid 6-aminocaproate via an artificial iterative carbon-chain-extension cycle

Abstract

Bioplastics produced from microbial source are promising green alternatives to traditional petrochemical-derived plastics. Nonnatural straight-chain amino acids, especially 5-aminovalerate, 6-aminocaproate and 7-aminoheptanoate are potential monomers for the synthesis of polymeric bioplastics as their primary amine and carboxylic acid are ideal functional groups for polymerization. Previous pathways for 5-aminovalerate and 6-aminocaproate biosynthesis in microorganisms are derived from L-lysine catabolism and citric acid cycle, respectively. Here, we show the construction of an artificial iterative carbon-chain-extension cycle in Escherichia coli for simultaneous production of a series of nonnatural amino acids with varying chain length. Overexpression of L-lysine α-oxidase in E. coli yields 2-keto-6-aminocaproate as a non-native substrate for the artificial iterative carbon-chain-extension cycle. The chain-extended α-ketoacid is subsequently decarboxylated and oxidized by an α-ketoacid decarboxylase and an aldehyde dehydrogenase, respectively, to yield the nonnatural straight-chain amino acid products. The engineered system demonstrated simultaneous in vitro production of 99.16 mg/L of 5-aminovalerate, 46.96 mg/L of 6-aminocaproate and 4.78 mg/L of 7-aminoheptanoate after 8 hours of enzyme catalysis starting from 2-keto-6-aminocaproate as the substrate. Furthermore, simultaneous production of 2.15 g/L of 5-aminovalerate, 24.12 mg/L of 6-aminocaproate and 4.74 mg/L of 7-aminoheptanoate was achieved in engineered E. coli. This work illustrates a promising metabolic-engineering strategy to access other medium-chain organic acids with -NH₂,-SCH₃, -SOCH₃, -SH, -COOH, -COH, or -OH functional groups through carbon-chain-elongation chemistry.