Imine Reductases: Multifunctional Biocatalysts with Varying Active Sites and Catalytic Mechanisms

Abstract

The synthesis of chiral amines is significant in the pharmaceutical industry. Imine reductase (IRED), a promising biocatalyst that was previously known to only catalyze asymmetric imine reduction (IR), was revealed to achieve direct asymmetric reductive amination (RA) of ketones with excess amines, producing secondary and tertiary amines. Moreover, conjugate reduction (CR) and RA activity by a single IRED has been reported for the preparation of valuable amine diastereomers. IREDs catalyzing these different reactions share the same standard quaternary structure, but possess varying active sites, indicating correlations and differences between their catalytic mechanisms. In this review, we trace the catalytic mechanisms reported for IRED‐catalyzed IR, RA, and CR‐RA. Insights obtained from structural and protein engineering in understanding the IRED‐catalyzed asymmetric synthesis are also included. This review will help readers acquire comprehensive insights into the catalytic mechanism of IREDs and ultimately inspire the engineering of IREDs for industrial applications.

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

“…IREDs are NAD(P)Hdependent oxidoreductases that are capable of catalyzing reductive amination not only of cyclic imines, but also between ketone substrates and amine donors to produce primary, secondary and tertiary amines. 25,26,[94][95][96][97][98][99][100] Hereafter, we summarize the structures, mechanisms, evolutionary history and applications of IREDs.…”

Computational design of an imine reductase: mechanism-guided stereoselectivity reversion and interface stabilization

“…Several studies have proposed that protic residues, including conserved aspartate and tyrosine located near the NADPH cofactor, serve as proton donors and facilitate the transfer of protons necessary for the reduction of unsaturated imine bonds. 5,8 Furthermore, most IREDs possess either aspartate or tyrosine residues at equivalent positions in the active site, which frequently display opposite stereoselectivity. [6][7][8] This suggests that these residues not only serve as proton donors but also play a role in controlling stereoselectivity.…”

“…Aer screening IREDs stored in our laboratory, PmIR from Paenibacillus mucilaginosus emerged as one of the few enzymes capable of catalyzing the reduction of 2-DFPL to yield (R)-2-DFPD, which displayed the highest ee, exceeding 96%. Owing to the conserved binding position of NADPH in IREDs, 5,7,13 the PmIR-NADPH complex model was constructed by superimposing the PmIR crystal structure (PDB ID: 8KFK) on the NADPH-bound BcSIRED crystal structure (sequence identity 75.8%). To investigate the key residues of PmIR controlling the asymmetric reduction, 18 residues around Y187 within 10 Å, located at the subunit interface forming the binding pocket, were targeted for alanine scanning (Fig.…”

“…[1][2][3] Despite the diverse reactions catalyzed by IREDs, they all involve a pivotal step: the asymmetric reduction of the iminium cation intermediate. 4,5 This step is crucial for determining the stereoconguration of the nal amine product.…”

Enzymatic Oxy‐ and Amino‐Functionalization in Biocatalytic Cascade Synthesis: Recent Advances and Future Perspectives

“…Enzymes able to perform CÀ N bond formations are likewise vastly present in nature and are particularly relevant for selectively catalyzing the formation of chiral amines. Among them, TAs, [79,80] IREDs [19,81] as well as PALs [82] are wellknown and have been intensively studied for the synthesis of chiral amines. TAs are pyridoxal-5'-phosphate (PLP)-dependent enzymes, where the PLP group helps the transfer of the amino group from an amine donor to an acceptor, usually an aldehyde or a ketone.…”