Ionozymes for Efficient Synthesis of Cadaverine: Offering a Sustainable Way for Bio-nylon 5X Production

Abstract

Bio-based cadaverine, a crucial monomer for the production of bio-nylon 5X, can be synthesized during the bioconversion of l-lysine HCl relying on lysine decarboxylases. The rationally designed lysine decarboxylase ΔLdcEt3 has exhibited outstanding alkaline stability in pH 8.0 (half-life: 362 h); however, its catalytic activity still needs to be improved to meet the requirements of industrial cadaverine production. A novel ionozyme strategy can create a preferable reaction microenvironment, affect the intermediate formation, and/or improve the enzymatic stability and activity. Therefore, ionozymes ΔLdcEt3-[Emim]Cl and ΔLdcEt3-[Ch][Ser] have been successfully developed for efficient cadaverine production in this study. The results showed that the catalytic activities of ΔLdcEt3-[Emim]Cl and ΔLdcEt3-[Ch][Ser] improved 124.2 and 116.2%, respectively. Meanwhile, the catalytic efficiencies (kcat/Km) of ΔLdcEt3-[Emim]Cl and ΔLdcEt3-[Ch][Ser] were also increased by 1.5- and 1.2-fold, respectively. Particle size, circular dichroism, and Raman spectrum analyses showed that [Emim]Cl and [Ch][Ser] could affect short-range attractions related to the aggregation state and change the secondary structure. Protein surface analysis demonstrated that the addition of ionic liquids changed the hydrophobicity of ΔLdcEt3. In addition, isothermal titration calorimetry and molecular docking revealed that [Emim]Cl and [Ch][Ser] could promote the protein–ligand complexation during the enthalpy-driven l-lysine and ΔLdcEt3 binding process, which was confirmed by molecular dynamics. Therefore, ionozymes ΔLdcEt3-[Emim]Cl/[Ch][Ser] provide a novel possibility for high-level cadaverine production.