Engineering an (R)-selective transaminase for asymmetric synthesis of (R)-3-aminobutanol

(R)-selective transaminases show promise as catalysts for the asymmetric synthesis of chiral amines, which are building blocks of various small molecule drugs. However, their application is limited by poor substrate acceptance and low catalytic efficiency. Here, a potential (R)-selective transaminase from Fodinicurvata sediminis (FsTA) was identified through a substrate truncating strategy, and used as starting point for enzyme engineering toward catalysis of 4-hydroxy-2-butanone, a substrate that poses challenges in catalysis. Molecular docking and dynamics simulations revealed Y90 as the key residue responsible for poor substrate binding. Starting from the variant (Y90F, mut1) with initial activity, FsTA was systematically modified to improve substrate-binding through active site reshaping and consensus sequence strategy, yielding three variants (H30R, V152K, and Y156F) with improved activity. A quadruple mutation variant H30R/Y90F/V152K/Y156F (mut4) was also found to show a 7.95-fold greater catalytic efficiency (kcat/KM) than the initial variant mut1. Furthermore, mut4 also enhanced the thermostability of enzyme significantly, with the Tm value increasing by 10 °C. This variant also exhibited significantly improved activity toward a series of ketones that are either not accepted or poorly accepted by the wild-type. This study provides a basis for the rational design of an active to creating variants that can accommodate novel substrates.

Protein engineering of transaminase facilitating enzyme cascade reaction for the biosynthesis of azasugars

Azasugars, such as 1-deoxynojirimycin (1-DNJ), exhibit unique physiological functions and hold promising applications in medicine and health fields. However, the biosynthesis of 1-DNJ is hindered by the low activity and thermostability of the transaminase. In this study, the transaminase from Mycobacterium vanbaalenii (MvTA) with activity toward d-fructose was engineered through semi-rational design and high-throughput screening method. The final mutant M9-1 demonstrated a remarkable 31.2-fold increase in specific activity and an impressive 200-fold improvement in thermostability compared to the wild-type enzyme. Molecular dynamics (MD) simulations revealed that the mutation sites of H69R and K145R in M9-1 played crucial roles in the binding of the amino acceptor and donor, leading to the stable conformation of substrates within the active pocket. An enzyme cascade reaction was developed using M9-1 and the dehydrogenase from Paenibacillus polymyxa (GutB1) for the production of mannojirimycin (MJ), which provided a new idea for the in vitro biosynthesis of 1-DNJ.

Engineering of Reductive Aminases for Asymmetric Synthesis of Enantiopure Rasagiline

Reductive aminases (RedAms) for the stereoselective amination of ketones represent an environmentally benign and economically viable alternative to transition metal–catalyzed asymmetric chemical synthesis. Here, we report two RedAms from Aspergillus calidoustus (AcRedAm) and bacteria (BaRedAm) with NADPH-dependent features. The enzymes can synthesize a set of secondary amines using a broad range of ketone and amine substrates with up to 97% conversion. To synthesize the pharmaceutical ingredient (R)-rasagiline, we engineered AcRedAm through rational design to obtain highly stereoselective mutants. The best mutant Q237A from AcRedAm could synthesize (R)-rasagiline with >99% enantiomeric excess with moderate conversion. The features of AcRedAm and BaRedAm highlight their potential for further study and expand the biocatalytic toolbox for industrial applications.