Reductive amination is one of the most important methods for the synthesis of chiral amines. Here we report the discovery of an NADP(H)-dependent reductive aminase from Aspergillus oryzae (AspRedAm, Uniprot code Q2TW47) that can catalyse the reductive coupling of a broad set of carbonyl compounds with a variety of primary and secondary amines with up to >98% conversion and with up to >98% enantiomeric excess. In cases where both carbonyl and amine show high reactivity, it is possible to employ a 1:1 ratio of the substrates, forming amine products with up to 94% conversion. Steady-state kinetic studies establish that the enzyme is capable of catalysing imine formation as well as reduction. Crystal structures of AspRedAm in complex with NADP(H) and also with both NADP(H) and the pharmaceutical ingredient (R)-rasagiline are reported. We also demonstrate preparative scale reductive aminations with wild-type and Q240A variant biocatalysts displaying total turnover numbers of up to 32,000 and space time yields up to 3.73 g l d.
Enzymes are increasingly being used as biocatalysts in the generation of products that have until now been derived using traditional chemical processes. Such products range from pharmaceutical and agrochemical building blocks to fine and bulk chemicals and, more recently, components of biofuels. For a biocatalyst to be effective in an industrial process, it must be subjected to improvement and optimization, and in this respect the directed evolution of enzymes has emerged as a powerful enabling technology. Directed evolution involves repeated rounds of (i) random gene library generation, (ii) expression of genes in a suitable host and (iii) screening of libraries of variant enzymes for the property of interest. Both in vitro screening-based methods and in vivo selection-based methods have been applied to the evolution of enzyme function and properties. Significant developments have occurred recently, particularly with respect to library design, screening methodology, applications in synthetic transformations and strategies for the generation of new enzyme function.
Aliphatic hydrocarbons such as fatty alcohols and petroleum-derived alkanes have numerous applications in the chemical industry. In recent years, the renewable synthesis of aliphatic hydrocarbons has been made possible by engineering microbes to overaccumulate fatty acids. However, to generate end products with the desired physicochemical properties (e.g., fatty aldehydes, alkanes, and alcohols), further conversion of the fatty acid is necessary. A carboxylic acid reductase (CAR) from Mycobacterium marinum was found to convert a wide range of aliphatic fatty acids (C 6 –C 18 ) into corresponding aldehydes. Together with the broad-substrate specificity of an aldehyde reductase or an aldehyde decarbonylase, the catalytic conversion of fatty acids to fatty alcohols (C 8 –C 16 ) or fatty alkanes (C 7 –C 15 ) was reconstituted in vitro. This concept was applied in vivo , in combination with a chain-length-specific thioesterase, to engineer Escherichia coli BL21(DE3) strains that were capable of synthesizing fatty alcohols and alkanes. A fatty alcohol titer exceeding 350 mg·L −1 was obtained in minimal media supplemented with glucose. Moreover, by combining the CAR-dependent pathway with an exogenous fatty acid-generating lipase, natural oils (coconut oil, palm oil, and algal oil bodies) were enzymatically converted into fatty alcohols across a broad chain-length range (C 8 –C 18 ). Together with complementing enzymes, the broad substrate specificity and kinetic characteristics of CAR opens the road for direct and tailored enzyme-catalyzed conversion of lipids into user-ready chemical commodities.
Abstractα-Chiral amines are key intermediates for the synthesis of a plethora of chemical compounds on industrial scale. Here we present a biocatalytic hydrogen-borrowing amination of primary and secondary alcohols that allows for the efficient and environmentally benign production of enantiopure amines. The method relies on the combination of an alcohol dehydrogenase (ADHs from Aromatoleum sp., Lactobacillus sp. and Bacillus sp.) enzyme operating in tandem with an amine dehydrogenase (AmDHs engineered from Bacillus sp.) to aminate a structurally diverse range of aromatic and aliphatic alcohols (up to 96% conversion and 99% enantiomeric excess). Furthermore, primary alcohols are aminated with high conversion (up to 99%). This redox selfsufficient network possesses high atom efficiency, sourcing nitrogen from ammonium and generating water as the sole by-product.Amines are amongst the most frequently used chemical intermediates for the production of APIs (active pharmaceutical ingredients), fine chemicals, agrochemicals, polymers, dyestuffs, pigments, emulsifiers and plasticizing agents (1). However, the requisite amines are scarce in nature and their industrial production mainly relies upon the metal-catalysed hydrogenation of enamides (i.e. obtained from related ketone precursors), a process requiring transition metal complexes, which are expensive and increasingly unsustainable (2). Moreover, the asymmetric synthesis of amines from ketone precursors requires protection and deprotection steps that generate copious amounts of waste. As a consequence, various chemical processes for the direct conversion of alcohols into amines have been developed during the last decade. The intrinsic advantage of the direct amination of an Europe PMC Funders Author ManuscriptsEurope PMC Funders Author Manuscripts alcohol is that the reagent and the product are in the same oxidation state and therefore, theoretically, additional redox equivalents are not required.However, many of these methods have low efficiency and high environmental impact (e.g. Mitsunobu reaction) (3). The amination of simple alcohols such as methanol and ethanol, via heterogeneous catalysis, requires harsh conditions (>200 °C) and more structurally diverse alcohols are either converted with extremely low chemoselectivity or not converted at all (4). Furthermore, most of the work in this field involves non-chiral substrates whereas 40% of the commercial optically active drugs are chiral amines (2). Increasingly, biocatalytic methods are applied for the production of optically active amines, e.g. the lipase catalysed resolution of racemic mixtures of amines or the ω-transaminase process with a most recent example employing an engineered enzyme applied to the industrial manufacture of the diabetes medication Januvia® (sitagliptin) (5,6,7).Multi-step chemical reactions in one pot avoid the need for isolation of intermediates and purification steps. This approach leads to economic as well as environmental benefits since time-consuming intermediate work-ups are not...
Enzymatic catalysis and homogeneous catalysis offer complementary means to address synthetic challenges, both in chemistry and in biology. Despite its attractiveness, the implementation of concurrent cascade reactions that combine an organometallic catalyst with an enzyme has proven challenging because of the mutual inactivation of both catalysts. To address this, we show that incorporation of a d(6)-piano stool complex within a host protein affords an artificial transfer hydrogenase (ATHase) that is fully compatible with and complementary to natural enzymes, thus enabling efficient concurrent tandem catalysis. To illustrate the generality of the approach, the ATHase was combined with various NADH-, FAD- and haem-dependent enzymes, resulting in orthogonal redox cascades. Up to three enzymes were integrated in the cascade and combined with the ATHase with a view to achieving (i) a double stereoselective amine deracemization, (ii) a horseradish peroxidase-coupled readout of the transfer hydrogenase activity towards its genetic optimization, (iii) the formation of L-pipecolic acid from L-lysine and (iv) regeneration of NADH to promote a monooxygenase-catalysed oxyfunctionalization reaction.
Transaminases have attracted considerable interest in their use as biocatalysts for the synthesis of compounds containing chiral amine units, which are widespread within the pharmaceutical, agrochemical, and fine chemical industry. Recent developments in enzyme- and process-engineering have expedited their use in asymmetric synthesis; however, industrial applications are still hindered by a number of factors, including equilibrium thermodynamics, product inhibition, and poor substrate tolerance. Detailed and comprehensive approaches to each of these challenges have been reported during the last two decades; the most representative enzyme discovery and screening strategies, protein and equilibrium engineering, and immobilization techniques are reviewed herein. Furthermore, we present a detailed look into the applications of transaminases for the synthesis of a variety of amine-containing compounds and the integration of transaminases into multienzymatic systems that allow access to a variety of highly complex products for the end user.
The development of cost-effective and sustainable catalytic methods for the production of enantiomerically pure chiral amines is a key challenge facing the pharmaceutical and fine chemical industries. This challenge is highlighted by the estimate that 40-45% of drug candidates contain a chiral amine, fueling a demand for broadly applicable synthetic methods that deliver target structures in high yield and enantiomeric excess. Herein we describe the development and application of a "toolbox" of monoamine oxidase variants from Aspergillus niger (MAO-N) which display remarkable substrate scope and tolerance for sterically demanding motifs, including a new variant, which exhibits high activity and enantioselectivity toward substrates containing the aminodiphenylmethane (benzhydrylamine) template. By combining rational structure-guided engineering with high-throughput screening, it has been possible to expand the substrate scope of MAO-N to accommodate amine substrates containing bulky aryl substituents. These engineered MAO-N biocatalysts have been applied in deracemization reactions for the efficient asymmetric synthesis of the generic active pharmaceutical ingredients Solifenacin and Levocetirizine as well as the natural products (R)-coniine, (R)-eleagnine, and (R)-leptaflorine. We also report a novel MAO-N mediated asymmetric oxidative Pictet-Spengler approach to the synthesis of (R)-harmicine.
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