Nitrile Reductases: A Forthcoming Wave in Biocatalysis?

Biocatalytic Reduction Reactions from a Chemist's Perspective

Reductions play a key role in organic synthesis, producing chiral products with new functionalities. Enzymes can catalyse such reactions with exquisite stereo-, regio- and chemoselectivity, leading the way to alternative shorter classical synthetic routes towards not only high-added-value compounds but also bulk chemicals. In this review we describe the synthetic state-of-the-art and potential of enzymes that catalyse reductions, ranging from carbonyl, enone and aromatic reductions to reductive aminations.

Broadening the Scope of Biocatalysis in Sustainable Organic Synthesis

This Review is aimed at synthetic organic chemists who may be familiar with organometallic catalysis but have no experience with biocatalysis, and seeks to provide an answer to the perennial question: if it is so attractive, why wasn't it extensively used in the past? The development of biocatalysis in industrial organic synthesis is traced from the middle of the last century. Advances in molecular biology in the last two decades, in particular genome sequencing, gene synthesis and directed evolution of proteins, have enabled remarkable improvements in scope and substantially reduced biocatalyst development times and cost contributions. Additionally, improvements in biocatalyst recovery and reuse have been facilitated by developments in enzyme immobilization technologies. Biocatalysis has become eminently competitive with chemocatalysis and the biocatalytic production of important pharmaceutical intermediates, such as enantiopure alcohols and amines, has become mainstream organic synthesis. The synthetic space of biocatalysis has significantly expanded and is currently being extended even further to include new-to-nature biocatalytic reactions.

Interplay of nucleophilic catalysis with proton transfer in the nitrile reductase QueF from Escherichia coli

Proton relay through an active-site network of hydrogen bonds promotes enzymatic nitrile reduction to amine via a covalent thioimidate enzyme intermediate.

Generation of novel family of reductases from PCR based library for the synthesis of chiral alcohols and amines

Biocatalysis has shown tremendous potential in the synthesis of drugs and drug intermediates in the last decade. Screening of novel biocatalysts from the natural genome space is the growing trend to replenish the harsh chemical synthetic routes, commonly used in the pharmaceutical and chemical industry. Here, we report a novel ketoreductase (KERD) and a nitrile reductase isolated from the PCR based library generated from the genome of Rhodococcus ruber and Bacillus subtilis, respectively. Both the proteins are hypothetical in nature as there is no putative homology found in the database, although both the enzymes have significant activity towards the synthesis of chiral alcohols and amines. Enzyme activity over a wide range of substrates (aromatic and aliphatic) for both the novel catalysts was observed. From the unique gene sequence to activity over a broad range of substrate and >99% conversion at higher concentrations (100 mM and above) entitles both the hypothetical enzymes as novel. The novel KERD has shown >99% selectivity for the synthesis of (S)-phenylethanol which makes it a potential candidate for industrial catalysis. The novel nitrile reductase has also shown promising activity for the synthesis of (R)-2-phenylethanolamine, which is a difficult moiety to synthesize chemically. In this report, starting from a homology based library, two highly potent whole cell biocatalysts are obtained.

Evidence of a sequestered imine intermediate during reduction of nitrile to amine by the nitrile reductase QueF from Escherichia coli

In the biosynthesis of the tRNA-inserted nucleoside queuosine, the nitrile reductase QueF catalyzes conversion of 7-cyano-7-deazaguanine (preQ₀) to 7-aminomethyl-7-deazaguanine (preQ₁), a biologically unique four-electron reduction of a nitrile to an amine. The QueF mechanism involves a covalent thioimide adduct between the enzyme and preQ₀ that undergoes reduction to preQ₁ in two NADPH-dependent steps, presumably via an imine intermediate. Protecting a labile imine from interception by water is fundamental to QueF catalysis for proper enzyme function. In the QueF from Escherichia coli, the conserved Glu⁸⁹ and Phe²²⁸ residues together with a mobile structural element composing the catalytic Cys¹⁹⁰ form a substrate-binding pocket that secludes the bound preQ₀ completely from solvent. We show here that residue substitutions (E89A, E89L, and F228A) targeted at opening up the binding pocket weakened preQ₀ binding at the preadduct stage by up to +10 kJ/mol and profoundly affected catalysis. Unlike wildtype enzyme, the QueF variants, including L191A and I192A, were no longer selective for preQ₁ formation. The E89A, E89L, and F228A variants performed primarily (≥90%) a two-electron reduction of preQ₀, releasing hydrolyzed imine (7-formyl-7-deazaguanine) as the product. The preQ₀ reduction by L191A and I192A gave preQ₁ and 7-formyl-7-deazaguanine at a 4:1 and 1:1 ratio, respectively. The proportion of 7-formyl-7-deazaguanine in total product increased with increasing substrate concentration, suggesting a role for preQ₀ in a competitor-induced release of the imine intermediate. Collectively, these results provide direct evidence for the intermediacy of an imine in the QueF-catalyzed reaction. They reveal determinants of QueF structure required for imine sequestration and hence for a complete nitrile-to-amine conversion by this class of enzymes.

Enantioselective imine reduction catalyzed by imine reductases and artificial metalloenzymes

Adding value to organic synthesis. Novel imine reductases enable the enantioselective reduction of imines to afford optically active amines. Likewise, novel bioinspired artificial metalloenzymes can perform the same reaction as well. Emerging proof-of-concepts are herein discussed.

Nitrile reductase as a biocatalyst: opportunities and challenges

The review highlights the recent progress and challenges in developing a family of nitrile reductases as biocatalysts for nitrile-to-amine transformation.

Expression and characterization of the nitrile reductase queF from E. coli

The expression and characterization of a nitrile reductase from Escherichia coli K-12 (EcoNR), a newly discovered enzyme class, is described. This enzyme has a potential application for an alternative nitrile reduction pathway. The enzyme activity towards its natural substrate, preQ(0), was demonstrated and optimal working conditions were found to be at 37°C and at pH 7 with Tris buffer.