Metabolic engineering combined with site‐directed saturated mutations of α‐keto acid decarboxylase for efficient production of 6‐aminocaproic acid and 1,6‐hexamethylenediamine

Abstract 6‐Aminocaproic acid (6ACA) and 1,6‐hexamethylenediamine (HMDA) are key precursors for nylon synthesis, and both are produced using petroleum‐based chemical processes. However, the utilization of bio‐based raw materials for biological production of monomers is crucial for nylon industry. In this study, we demonstrated that metabolic engineering of Escherichia coli and selected mutations of α‐keto acid decarboxylase successfully synthesized 6ACA and HMDA. An artificial iterative cycle from l ‐lysine to chain‐extended α‐ketoacids was introduced into Escherichia coli BL21 (DE3). Then, the extended α‐ketoacids were decarboxylated and oxidized for 6ACA production. Overexpression of catalase (KatE) combined with the site‐directed mutations of α‐isopropylmalate synthase (LeuA) contributed synergistic enhancement effect on synthesis of 6ACA, resulting in a 1.3‐fold increase in 6ACA titer. Selected mutations in α‐keto acid decarboxylase (KivD) improved its specificity and 170.00 ± 5.57 mg/L of 6ACA with a yield of 0.13 mol/mol (6ACA/ l ‐lysine hydrochloride) was achieved by shake flask cultivation of the engineered strain with the KivD# (F381Y/V461I). Meanwhile, the engineered E. coli could accumulate 84.67 ± 4.04 mg/L of HMDA with a yield of 0.08 mol/mol (HMDA/ l ‐lysine hydrochloride) by replacing aldehyde dehydrogenase with bi‐aminotransferases. This achievement marks a significant advancement in the biological synthesis of 6‐carbon compounds, since the biosynthetic pathways of HMDA are rarely identified.