Enzymatic synthesis of benzylisoquinoline alkaloids using a parallel cascade strategy and tyrosinase variants

Benzylisoquinoline alkaloid derived pharmaceuticals are widely applied in modern medicines. Recent studies on the microbial production of benzylisoquinolines have highlighted key biological syntheses towards these natural products. Routes to non-natural benzylisoquinolines have been less explored, particularly halogenated compounds which are more challenging. Here, we show the use of a tyrosinase, tyrosine decarboxylase, transaminase, and norcoclaurine synthase which are combined in a parallel cascade design, in order to generate halogenated benzylisoquinoline alkaloids in high enantiomeric excess. Notably, mutagenesis studies are applied to generate tyrosinase mutants, which enhance the acceptance of halogenated tyrosines for use in the biocatalytic cascades developed.

Pharmaceuticals derived from benzylisoquinoline alkaloids find application in modern medicine. Here, the authors report the design of a parallel enzymatic cascade to generate halogenated benzylisoquinoline alkaloids, that included engineering of tyrosinase mutants to enhance substrate capabilities.

Direct Conversion of Hydrazones to Amines using Transaminases

Transaminase enzymes (TAms) have been widely used for the amination of aldehydes and ketones, often resulting in optically pure products. In this work, transaminases were directly reacted with hydrazones in a novel approach to form amine products. Several substrates were investigated, including those with furan and phenyl moieties. It was determined that the amine yields increased when an additional electrophile was added to the reaction mixture, suggesting that they can sequester the hydrazine released in the reaction. Pyridoxal 5'-phosphate (PLP), a cofactor for transaminases, and polyethylene glycol (PEG)-aldehydes were both found to increase the yield of amine formed. Notably, the amination of (S)-(-)-1-amino-2-(methoxymethyl)pyrrolidine (SAMP) hydrazones gave promising results as a method to form chiral β-substituted amines in good yield.

Chemoenzymatic Cascades toward Methylated Tetrahydroprotoberberine and Protoberberine Alkaloids

Tetrahydroprotoberberine and protoberberine alkaloids are a group of biologically active natural products with complex molecular scaffolds. Isolation from plants is challenging and stereoselective synthetic routes, particularly of methylated compounds are limited, reducing the potential use of these compounds. In this work, we describe chemoenzymatic cascades toward various 13-methyl-tetrahydroprotoberberbine scaffolds using a stereoselective Pictet-Spenglerase, regioselective catechol O-methyltransferases and selective chemical Pictet-Spengler reactions. All reactions could be performed sequentially, without the workup or purification of any synthetic intermediates. Moreover, the naturally occurring alkaloids have the (+)-configuration and importantly here, a strategy to the (-)-isomers was developed. A methyl group at C-8 was also introduced with some stereocontrol, influenced by the stereochemistry at C-13. Furthermore, a single step reaction was found to convert tetrahydroprotoberberine alkaloids into the analogous protoberberine scaffold, avoiding the use of harsh oxidizing conditions or a selective oxidase. This work provides facile, selective routes toward novel analogues of bioactive alkaloids.

Multienzyme One-Pot Cascades Incorporating Methyltransferases for the Strategic Diversification of Tetrahydroisoquinoline Alkaloids

The tetrahydroisoquinoline (THIQ) ring system is present in a large variety of structurally diverse natural products exhibiting a wide range of biological activities. Routes to mimic the biosynthetic pathways to such alkaloids, by building cascade reactions in vitro, represents a successful strategy and can offer better stereoselectivities than traditional synthetic methods. S-Adenosylmethionine (SAM)-dependent methyltransferases are crucial in the biosynthesis and diversification of THIQs; however, their application is often limited in vitro by the high cost of SAM and low substrate scope. In this study, we describe the use of methyltransferases in vitro in multi-enzyme cascades, including for the generation of SAM in situ. Up to seven enzymes were used for the regioselective diversification of natural and non-natural THIQs on an enzymatic preparative scale. Regioselectivites of the methyltransferases were dependent on the group at C-1 and presence of fluorine in the THIQs. An interesting dual activity was also discovered for the catechol methyltransferases used, which were found to be able to regioselectively methylate two different catechols in a single molecule.

Multienzyme One-Pot Cascades Incorporating Methyltransferases for the Strategic Diversification of Tetrahydroisoquinoline Alkaloids

The tetrahydroisoquinoline (THIQ) ring system is present in a large variety of structurally diverse natural products exhibiting a wide range of biological activities. Routes to mimic the biosynthetic pathways to such alkaloids, by building cascade reactions in vitro, represents a successful strategy and can offer better stereoselectivities than traditional synthetic methods. S-Adenosylmethionine (SAM)-dependent methyltransferases are crucial in the biosynthesis and diversification of THIQs; however, their application is often limited in vitro by the high cost of SAM and low substrate scope. In this study, we describe the use of methyltransferases in vitro in multi-enzyme cascades, including for the generation of SAM in situ. Up to seven enzymes were used for the regioselective diversification of natural and non-natural THIQs on an enzymatic preparative scale. Regioselectivites of the methyltransferases were dependent on the group at C-1 and presence of fluorine in the THIQs. An interesting dual activity was also discovered for the catechol methyltransferases used, which were found to be able to regioselectively methylate two different catechols in a single molecule.

Characterisation of a hyperthermophilic transketolase from Thermotoga maritima DSM3109 as a biocatalyst for 7-keto-octuronic acid synthesis

Transketolase (TK) is a fundamentally important enzyme in industrial biocatalysis which carries out a stereospecific carbon-carbon bond formation, and is widely used in the synthesis of prochiral ketones. This study describes the biochemical and molecular characterisation of a novel and unusual hyperthermophilic TK from Thermotoga maritima DSM3109 (TKtmar). TKtmar has a low protein sequence homology compared to the already described TKs, with key amino acid residues in the active site highly conserved. TKtmar has a very high optimum temperature (>90 °C) and shows pronounced stability at high temperature (e.g. t1/2 99 and 9.3 h at 50 and 80 °C, respectively) and in presence of organic solvents commonly used in industry (DMSO, acetonitrile and methanol). Substrate screening showed activity towards several monosaccharides and aliphatic aldehydes. In addition, for the first time, TK specificity towards uronic acids was achieved with TKtmar catalysing the efficient conversion of d-galacturonic acid and lithium hydroxypyruvate into 7-keto-octuronic acid, a very rare C8 uronic acid, in high yields (98%, 49 mM).

Single step syntheses of (1S)-aryl-tetrahydroisoquinolines by norcoclaurine synthases

The 1-aryl-tetrahydroisoquinoline (1-aryl-THIQ) moiety is found in many biologically active molecules. Single enantiomer chemical syntheses are challenging and although some biocatalytic routes have been reported, the substrate scope is limited to certain structural motifs. The enzyme norcoclaurine synthase (NCS), involved in plant alkaloid biosynthesis, has been shown to perform stereoselective Pictet-Spengler reactions between dopamine and several carbonyl substrates. Here, benzaldehydes are explored as substrates and found to be accepted by both wild-type and mutant constructs of NCS. In particular, the variant M97V gives a range of (1 S)-aryl-THIQs in high yields (48-99%) and e.e.s (79-95%). A co-crystallised structure of the M97V variant with an active site reaction intermediate analogue is also obtained with the ligand in a pre-cyclisation conformation, consistent with (1 S)-THIQs formation. Selected THIQs are then used with catechol O-methyltransferases with exceptional regioselectivity. This work demonstrates valuable biocatalytic approaches to a range of (1 S)-THIQs.

Aminopolyols from Carbohydrates: Amination of Sugars and Sugar-Derived Tetrahydrofurans with Transaminases

Carbohydrates are the major component of biomass and have unique potential as a sustainable source of building blocks for chemicals, materials, and biofuels because of their low cost, ready availability, and stereochemical diversity. With a view to upgrading carbohydrates to access valuable nitrogen-containing sugar-like compounds such as aminopolyols, biocatalytic aminations using transaminase enzymes (TAms) have been investigated as a sustainable alternative to traditional synthetic strategies. Demonstrated here is the reaction of TAms with sugar-derived tetrahydrofuran (THF) aldehydes, obtained from the regioselective dehydration of biomass-derived sugars, to provide access to cyclic aminodiols in high yields. In a preliminary study we have also established the direct transamination of sugars to give acyclic aminopolyols. Notably, the reaction of the ketose d-fructose proceeds with complete stereoselectivity to yield valuable aminosugars in high purity.

Data on a thermostable enzymatic one-pot reaction for the production of a high-value compound from l-arabinose

The dataset presented in this article is related to the research article entitled "One-pot, two-step transaminase and transketolase synthesis of l-gluco-heptulose from l-arabinose" (Bawn et al., 2018 in press) [1]. This article presents data on initial experiments that were carried out to investigate new thermostable transketolase (TK) activities with l-arabinose. Transaminase (TAm) sequences from an in-house library of thermophilic strains were analyzed to compare homologies to characterized TAms with desired activity. DNA and amino acid sequences are presented for all the enzymes investigated. Calibration curves for products of the TK and TAm reactions are also presented along with chromatographic analysis of the various one-pot reactions.

Enzymatic synthesis of chiral amino-alcohols by coupling transketolase and transaminase-catalyzed reactions in a cascading continuous-flow microreactor system

Rapid biocatalytic process development and intensification continues to be challenging with currently available methods. Chiral amino‐alcohols are of particular interest as they represent key industrial synthons for the production of complex molecules and optically pure pharmaceuticals. (2S,3R)‐2‐amino‐1,3,4‐butanetriol (ABT), a building block for the synthesis of protease inhibitors and detoxifying agents, can be synthesized from simple, non‐chiral starting materials, by coupling a transketolase‐ and a transaminase‐catalyzed reaction. However, until today, full conversion has not been shown and, typically, long reaction times are reported, making process modifications and improvement challenging. In this contribution, we present a novel microreactor‐based approach based on free enzymes, and we report for the first time full conversion of ABT in a coupled enzyme cascade for both batch and continuous‐flow systems. Using the compartmentalization of the reactions afforded by the microreactor cascade, we overcame inhibitory effects, increased the activity per unit volume, and optimized individual reaction conditions. The transketolase‐catalyzed reaction was completed in under 10 min with a volumetric activity of 3.25 U ml⁻¹. Following optimization of the transaminase‐catalyzed reaction, a volumetric activity of 10.8 U ml⁻¹ was attained which led to full conversion of the coupled reaction in 2 hr. The presented approach illustrates how continuous‐flow microreactors can be applied for the design and optimization of biocatalytic processes.

An integrated biorefinery concept for conversion of sugar beet pulp into value-added chemicals and pharmaceutical intermediates

Over 8 million tonnes of sugar beet are grown annually in the UK. Sugar beet pulp (SBP) is the main by-product of sugar beet processing which is currently dried and sold as a low value animal feed. SBP is a rich source of carbohydrates, mainly in the form of cellulose and pectin, including d-glucose (Glu), l-arabinose (Ara) and d-galacturonic acid (GalAc). This work describes the technical feasibility of an integrated biorefinery concept for the fractionation of SBP and conversion of these monosaccharides into value-added products. SBP fractionation is initially carried out by steam explosion under mild conditions to yield soluble pectin and insoluble cellulose fractions. The cellulose is readily hydrolysed by cellulases to release Glu that can then be fermented by a commercial yeast strain to produce bioethanol at a high yield. The pectin fraction can be either fully hydrolysed, using physico-chemical methods, or selectively hydrolysed, using cloned arabinases and galacturonases, to yield Ara-rich and GalAc-rich streams. These monomers can be separated using either Centrifugal Partition Chromatography (CPC) or ultrafiltration into streams suitable for subsequent enzymatic upgrading. Building on our previous experience with transketolase (TK) and transaminase (TAm) enzymes, the conversion of Ara and GalAc into higher value products was explored. In particular the conversion of Ara into l-gluco-heptulose (GluHep), that has potential therapeutic applications in hypoglycaemia and cancer, using a mutant TK is described. Preliminary studies with TAm also suggest GluHep can be selectively aminated to the corresponding chiral aminopolyol. The current work is addressing the upgrading of the remaining SBP monomer, GalAc, and the modelling of the biorefinery concept to enable economic and Life Cycle Analysis (LCA).