Developing and enhancing promiscuous activity for NAD(P)H-dependent flavin reductase via elimination of cofactor

Promiscuous enzymes have shown their synthetic abilities in generating various organic compounds with high selectively and efficiency under mild conditions. Therefore, the design and development of conditions to raise promiscuity to the enzymes have been under the spotlight in recent years. Flavin reductase, that reduces flavins by using NADH as a cofactor, has not been studied in promiscuous reactions. In the present study, it was aimed to develop a catalytic promiscuous activity in the recombinant E.coli flavin reductase by removing its cofactor. The flavin reductase demonstrated a promiscuous activity for Knoevenagel condensation and Michael addition reactions individually. The cofactor-independent promiscuous activity of the flavin reductase was further enhanced by altering the reaction conditions to proceed a Knoevenagel-Michael addition cascade for tetraketone synthesis. Yet, the presence of the cofactor blocked the promiscuous Knoevenagel condensation, Michael addition, and therefore the cascade reaction, demonstrating that the removal of NADH was pivotal in inducing the promiscuous activity. Furthermore, molecular docking and MD simulations were performed to obtain more structural and mechanistic details of the transformation. The computational studies identified the most likely catalytic sites of the flavin reductase in the reaction. Additionally, a truncated variant of the enzyme that lacked 28 residues from the C-terminus displayed comparable activity to the wild-type enzyme, indicating the robustness of the enzyme in performing the cascade reaction. In brief, the cofactor-elimination method presented in this work could be considered as a straightforward and economical approach for inducing enzyme promiscuity in promoting organic transformations.

Continuous Flow Bioamination of Ketones in Organic Solvents at Controlled Water Activity using Immobilized ω-Transaminases

Compared with biocatalysis in aqueous media, the use of enzymes in neat organic solvents enables increased solubility of hydrophobic substrates and can lead to more favorable thermodynamic equilibria, avoidance of possible hydrolytic side reactions and easier product recovery. ω-Transaminases from Arthrobacter sp. (AsR-ωTA) and Chromobacterium violaceum (Cv-ωTA) were immobilized on controlled porosity glass metal-ion affinity beads (EziG) and applied in neat organic solvents for the amination of 1-phenoxypropan-2-one with 2-propylamine. The reaction system was investigated in terms of type of carrier material, organic solvents and reaction temperature. Optimal conditions were found with more hydrophobic carrier materials and toluene as reaction solvent. The system's water activity (aw) was controlled via salt hydrate pairs during both the biocatalyst immobilization step and the progress of the reaction in different non-polar solvents. Notably, the two immobilized ωTAs displayed different optimal values of aw, namely 0.7 for EziG3-AsR-ωTA and 0.2 for EziG3-Cv-ωTA. In general, high catalytic activity was observed in various organic solvents even when a high substrate concentration (450-550 mM) and only one equivalent of 2-propylamine were applied. Under batch conditions, a chemical turnover (TTN) above 13000 was obtained over four subsequent reaction cycles with the same batch of EziG-immobilized ωTA. Finally, the applicability of the immobilized biocatalyst in neat organic solvents was further demonstrated in a continuous flow packed-bed reactor. The flow reactor showed excellent performance without observable loss of enzymatic catalytic activity over several days of operation. In general, ca. 70% conversion was obtained in 72 hours using a 1.82 mL flow reactor and toluene as flow solvent, thus affording a space-time yield of 1.99 g L-1 h-1. Conversion reached above 90% when the reaction was run up to 120 hours.

Biotic and Abiotic Synthesis of Renewable Aliphatic Polyesters from Short Building Blocks Obtained from Biotechnology

Biobased polymers have seen their attractiveness increase in recent decades thanks to the significant development of biorefineries to allow access to a wide variety of biobased building blocks. Polyesters are one of the best examples of the development of biobased polymers because most of them now have their monomers produced from renewable resources and are biodegradable. Currently, these polyesters are mainly produced by using traditional chemical catalysts and harsh conditions, but recently greener pathways with nontoxic enzymes as biocatalysts and mild conditions have shown great potential. Bacterial polyesters, such as poly(hydroxyalkanoate)s (PHA), are the best example of the biotic production of high molar mass polymers. PHAs display a wide variety of macromolecular architectures, which allow a large range of applications. The present contribution aims to provide an overview of recent progress in studies on biobased polyesters, especially those made from short building blocks, synthesized through step-growth polymerization. In addition, some important technical aspects of their syntheses through biotic or abiotic pathways have been detailed.