A Reductive Aminase Switches to Imine Reductase Mode for a Bulky Amine Substrate

Recent advances in biocatalytic C-N bond-forming reactions

Molecules containing C-N bonds are of paramount importance in a diverse array of organic-based materials, natural products, pharmaceutical compounds, and agricultural chemicals. Biocatalytic C-N bond-forming reactions represent powerful strategies for producing these valuable targets, and their significance in the field of synthetic chemistry has steadily increased over the past decade. In this review, we provide a concise overview of recent advancements in the development of C-N bond-forming enzymes, with a particular emphasis on the inherent chemistry involved in these enzymatic processes. Overall, these enzymatic systems have proven their potential in addressing long-standing challenges in traditional small-molecule catalysis.

An imine reductase that captures reactive intermediates in the biosynthesis of the indolocarbazole reductasporine

Imine reductases (IREDs) and reductive aminases have been used in the synthesis of chiral amine products for drug manufacturing; however, little is known about their biological contexts. Here we employ structural studies and site-directed mutagenesis to interrogate the mechanism of the IRED RedE from the biosynthetic pathway to the indolocarbazole natural product reductasporine. Cocrystal structures with the substrate-mimic arcyriaflavin A reveal an extended active site cleft capable of binding two indolocarbazole molecules. Site-directed mutagenesis of a conserved aspartate in the primary binding site reveals a new role for this residue in anchoring the substrate above the NADPH cofactor. Variants targeting the secondary binding site greatly reduce catalytic efficiency, while accumulating oxidized side-products. As indolocarbazole biosynthetic intermediates are susceptible to spontaneous oxidation, we propose the secondary site acts to protect against autooxidation, and the primary site drives catalysis through precise substrate orientation and desolvation effects. The structure of RedE with its extended active site can be the starting point as a new scaffold for engineering IREDs and reductive aminases to intercept large substrates relevant to industrial applications.

Biocatalytic reductive aminations with NAD(P)H-dependent enzymes: enzyme discovery, engineering and synthetic applications

Chiral amines are pivotal building blocks for the pharmaceutical industry. Asymmetric reductive amination is one of the most efficient and atom economic methodologies for the synthesis of optically active amines. Among the various strategies available, NAD(P)H-dependent amine dehydrogenases (AmDHs) and imine reductases (IREDs) are robust enzymes that are available from various sources and capable of utilizing a broad range of substrates with high activities and stereoselectivities. AmDHs and IREDs operate via similar mechanisms, both involving a carbinolamine intermediate followed by hydride transfer from the co-factor. In addition, both groups catalyze the formation of primary and secondary amines utilizing both organic and inorganic amine donors. In this review, we discuss advances in developing AmDHs and IREDs as biocatalysts and focus on evolutionary history, substrate scope and applications of the enzymes to provide an outlook on emerging industrial biotechnologies of chiral amine production.

Biocatalytic Construction of Chiral Pyrrolidines and Indolines via Intramolecular C(sp3)-H Amination

Nature harnesses exquisite enzymatic cascades to construct N-heterocycles and further uses these building blocks to assemble the molecules of life. Here we report an enzymatic platform to construct important chiral N-heterocyclic products, pyrrolidines and indolines, via abiological intramolecular C(sp3)-H amination of organic azides. Directed evolution of cytochrome P411 (a P450 enzyme with serine as the heme-ligating residue) yielded variant P411-PYS-5149, capable of catalyzing the insertion of alkyl nitrene into C(sp3)-H bonds to build pyrrolidine derivatives with good enantioselectivity and catalytic efficiency. Further evolution of activity on aryl azide substrates yielded variant P411-INS-5151 that catalyzes intramolecular C(sp3)-H amination to afford chiral indolines. In addition, we show that these enzymatic aminations can be coupled with a P411-based carbene transferase or a tryptophan synthase to generate an α-amino lactone or a noncanonical amino acid, respectively, underscoring the power of new-to-nature biocatalysis in complexity-building chemical synthesis.

Biocatalysis in Drug Design: Engineered Reductive Aminases (RedAms) Are Used to Access Chiral Building Blocks with Multiple Stereocenters

Novel building blocks are in constant demand during the search for innovative bioactive small molecule therapeutics by enabling the construction of structure-activity-property-toxicology relationships. Complex chiral molecules containing multiple stereocenters are an important component in compound library expansion but can be difficult to access by traditional organic synthesis. Herein, we report a biocatalytic process to access a specific diastereomer of a chiral amine building block used in drug discovery. A reductive aminase (RedAm) was engineered following a structure-guided mutagenesis strategy to produce the desired isomer. The engineered RedAm (IR-09 W204R) was able to generate the (S,S,S)-isomer 3 in 45% conversion and 95% ee from the racemic ketone 2. Subsequent palladium-catalyzed deallylation of 3 yielded the target primary amine 4 in a 73% yield. This engineered biocatalyst was used at preparative scale and represents a potential starting point for further engineering and process development.

Structure of the imine reductase from Ajellomyces dermatitidis in three crystal forms

The NADPH-dependent imine reductase from Ajellomyces dermatitidis (AdRedAm) catalyzes the reductive amination of certain ketones with amine donors supplied in an equimolar ratio. The structure of AdRedAm has been determined in three forms. The first form, which belongs to space group P3121 and was refined to 2.01 Å resolution, features two molecules (one dimer) in the asymmetric unit in complex with the redox-inactive cofactor NADPH4. The second form, which belongs to space group C21 and was refined to 1.73 Å resolution, has nine molecules (four and a half dimers) in the asymmetric unit, each complexed with NADP+. The third form, which belongs to space group P3121 and was refined to 1.52 Å resolution, has one molecule (one half-dimer) in the asymmetric unit. This structure was again complexed with NADP+ and also with the substrate 2,2-difluoroacetophenone. The different data sets permit the analysis of AdRedAm in different conformational states and also reveal the molecular basis of stereoselectivity in the transformation of fluorinated acetophenone substrates by the enzyme.