This study presents a three-step one pot enzymatic cascade for the synthesis of a δ-lactone. Utilising acetaldehyde, combining 2-deoxyribose-5-phosphate aldolase (DERA) with an alcohol dehydrogenase (ADH) and a cofactor regeneration system this δ-lactone is synthesised with the same stereochemistry as the statin side chain precursor. The initial stage in this cascade involves the double aldol reaction, catalysed by DERA to produce the chiral lactone precursor from the achiral substrate acetaldehyde. The main challenge at this stage is the instability of DERA in the presence of high acetaldehyde concentrations. Therefore, Lactobacillus brevis DERA with a high natural acetaldehyde tolerance was genetically engineered to further improve this property. LbDERA C42M E78K exhibited improved activity and stability (no activity loss over 2 h) compared to the wild type (20% activity loss). In the second stage of the cascade, the aldol product is selectively oxidised to the lactone. A commercially available ADH was identified to selectively catalyse this oxidation using NADP⁺ as electron acceptor. NADP⁺ regeneration was achieved using O₂ as substrate in two different ways: using either photo-activated flavin or NADPH oxidase (NOX). The lactone was successfully purified from the enzymatic cascades from a preparative scale reaction in 97% purity with an optical rotation [α]_D = +34.2° (c = 0.7), proving the feasibility in a multi-enzyme three-step one-pot cascade.

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Oleate hydratases open a biocatalytic access to hydroxy fatty acids by hydration of unsaturated fatty acids. Their practical applicability, however, is hampered by their low stability. In this study we report the immobilization of the oleate hydratase from Rhodococcus erythropolis PR4 on functionalized porous, spherical polymer beads. Different carrier materials promoting covalent, hydrophobic, ionic and his-tag affinity were screened and immobilization yields typically >95 % were observed. The highest activity recovery of 32 % was achieved by immobilization via ionic interaction with quaternary ammonium functionalized beads. Biochemical properties of the enzyme immobilized via ionic interaction remain unchanged upon immobilization. The immobilized enzyme was applied for synthesis of 10-hydroxystearic acid remaining stable under process conditions. Conversion of up to 100 mM oleic acid gave 10-hydroxystearic acid achieving a TON of up to 19,000. Successful recycling of the biocatalyst for up to ten cycles further demonstrate its potential for the synthesis of 10-hydroxystearic acid.@en

Metal cofactors are essential for catalysis and enable countless conversions in nature. Interestingly, the metal cofactor is not always static but mobile with movements of more than 4 Å. These movements of the metal can have different functions. In the case of the xylose isomerase and medium-chain dehydrogenases, it clearly serves a catalytic purpose. The metal cofactor moves during substrate activation and even during the catalytic turnover. On the other hand, in class II aldolases, the enzymes display resting states and active states depending on the movement of the catalytic metal cofactor. This movement is caused by substrate docking, causing the metal cofactor to take the position essential for catalysis. As these metal movements are found in structurally and mechanistically unrelated enzymes, it has to be expected that this metal movement is more common than currently perceived.@en

Contemporary Biocatalysis heavily relies on enzyme engineering as natural enzymes frequently lack the requisite attributes for effective organic synthesis. The inherent limitations in stability, catalytic activity, and selectivity of wild-type enzymes often hinder their suitability for chemical synthesis. Over the past 25 years, there has been an unprecedented advancement in protein engineering tools, empowering enzymologists to customise enzymes to precisely meet the demands of organic synthesis. In this discussion, we delineate some of the most crucial techniques in enzyme engineering and their significance in facilitating chemical synthesis.

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Acyltransferase from Mycobacterium smegmatis is a versatile enzyme, which catalyzes the transesterification of esters in aqueous media due to a kinetic preference of the synthesis reaction over the thermodynamically favored hydrolysis reaction. In the active octamer, the active site is deeply buried and connected to the protein surface by long and hydrophobic substrate access channels. The role of the access channel in controlling catalytic activity and substrate specificity was investigated by molecular dynamics simulations and Markov-state models, and the thermodynamics and kinetics of binding of acyl donors, acceptors, and water were compared. Despite the hydrophobic nature of the substrate access channel, water is present in the channel and competes with the acyl acceptors for access to the active site. The binding free energy profiles in the access channel and the flux of butyl and benzyl alcohol and vinyl acetate were analyzed in the concentration range between 10 and 500 mM and compared to water. The flux showed a maximum at an alcohol concentration of 50–100 mM, in agreement with experimental observations. At the maximum, the flux of alcohol approaches 50% of the flux of water, which explains the high transesterification rate as compared to hydrolysis. The molecular origin of this effect is due to the accumulation of alcohol molecules along the access channel. Extensive molecular dynamics simulations and analysis of trajectories by a Markov-state model provided insights into the role of the access channel in activity and specificity by controlling access and binding of competing substrates.@en

This is a review on the feasibility of monolithic porous supports in biocatalysis carried out in a continuous flow system. It discusses factors affecting the efficiency and stability of enzyme immobilisation, kinetic parameters of enzyme processes carried out inside a monolith, biocatalysis in single and two-phase systems, and cascade reactions including cofactor regeneration. It also covers materials engineering (monolith types) and issues related to the flow of reactants through the monolith (chemical engineering). Emphasis is placed on the fact that the application of (bio)catalysis improves selectivity and atom economy, thus lowering the E factor. However, biocatalysts need to be employed in a reactor, which can aid further improvement towards green chemistry goals. The application of enzymes in flow chemistry has been shown to lead to higher space time yields (STYs) compared to batch reactions. In particular, with monolithic reactors a drastic decrease in volume and thus solvent can be achieved. By immobilising very high densities of enzymes directly on the monolith, reaction times dwindle, improving STYs. The small reaction volumes enable excellent heat transfer, helping to save energy. The underlying principles of monolithic flow reactors and their application in mono- and bi-phasic biocatalytic systems will be examined.@en

Granulicella tundricola hydroxynitrile lyase (GtHNL) is a manganese dependent cupin that catalyzes the enantioselective synthesis of cyanohydrins. The analysis of its active site shows high similarity with the active site of cupin Tm1459 from Thermotoga maritima, an enzyme that catalyzes the oxidative cleavage of styrene derivatives. GtHNL (GtHNL-WT) was found to catalyze the oxidative cleavage of α-methyl styrene, too. The conversion of α-methyl styrene yielded 23.6 ± 0.8% of acetophenone after 20 h. On the other hand, Tm1459 was not able to catalyze the synthesis of cyanohydrins efficiently. A low yield of rac-mandelonitrile was obtained from benzaldehyde and HCN using either Tm1459-WT or Tm1459-C106L, a variant more active in oxidative catalysis. On the basis of the molecular analysis of GtHNL and Tm1459 active sites, the variants GtHNL-H96A, GtHNL-H96F, and GtHNL-A40H/V42T/H96A/Q110H were produced and evaluated for improved catalytic activity toward oxidative cleavage of styrenes. The amino acid substitution H96A liberates an additional manganese coordination position and enlarges the GtHNL-WT active site cavity. Similarly, the amino acid substitution H96F liberates a coordination site as described for the GtHNL-H96A variant but without enlarging the active site space. All variants were able to catalyze the oxidative cleavage of styrene derivatives. The best results were observed using GtHNL-H96A as catalyst. It displayed a higher yield of acetophenone (42%) as compared to GtHNL-A40H/V42T/H96A/Q110H (12%) and GtHNL-H96F (11%) after 20 h of reaction time. No oxidation of Mn(II) to Mn(III) could be detected by electron paramagnetic resonance (EPR), whereas evidence for a radical mechanism is presented. Control reactions using 0.1 and 0.5 mM of MnCl₂ in the absence of enzyme showed no significant oxidation reaction.

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S-Adenosyl-l-methionine (SAM)-dependent methyltransferases (MTs) are highly chemoselective enzymes grouped in C-, N-, O-, S- and halide MTs, depending on the (hetero) atom that acts as the methyl group acceptor. So far, OMTs present the largest group, including many well investigated candidates. The catechol OMT from mammals such as from Rattus norvegicus (RnCOMT) is involved in the metabolism of neurotransmitters like dopamine. It is known to methylate the hydroxyl of the catechol ring in the 3 position. There are also reports showing that the regioselectivity of different COMTs can vary leading to different products with methyl groups in the 3 and or 4 positions. Nevertheless, there was only O-methylation reported for COMTs. Another related MT, the caffeate OMT involved in the lignin biosynthesis of plants has also been reported as a chemoselective enzyme. In nature, S-methylation is a rare phenomenon with different methyl donors being involved in the methyl transfer onto sulfur atoms. Several SAM-dependent MTs are identified as S-methyltransferases (SMTs), these are involved in salvaging pathways and xenobiotic metabolism of cells. Here, we report a new function of three OMTs; RnCOMT, a COMT from Myxococcus xanthus (MxSafC), and a CaOMT from Prunus persica (PpCaOMT) with acceptance towards different aromatic thiol substrates with up to full conversion.

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Effectiveness of catalytic processes using heterogeneous biocatalysts depends not only on the activity of the enzyme, but also on the efficiency of the used reactor. In this paper, we present a novel design of a basket reactor with a stationary catalyst bed (StatBioChem). The developed design was compared to a commercially available rotating bed reactor (SpinChem®). The biocatalysts used were invertase and acyltransferase from Mycobacterium smegmatis (MsAcT) immobilised on macroporous silica supports. The obtained values of initial reaction rate, both in the reaction of saccharose hydrolysis and 2,2-dimethyl-1,3-propanediol (NPG) transesterification, were twice higher for the StatBioChem reactor. A similar relationship was also observed regarding the process efficiency expressed as STY.

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Granulicella tundricola hydroxynitrile lyase (GtHNL) catalyses the synthesis of chiral (R)‐ cyanohydrins and (R)‐β‐nitro alcohols. The triple variant GtHNL‐A40H/V42T/Q110H (GtHNL‐3V) was immobilised on Celite R‐633 and used in monophasic MTBE saturated with 100 mM KPi buffer pH 7 for the synthesis of (R)‐2‐nitro‐1‐phenylethanol (NPE) in batch and continuous flow systems. Nitromethane was used as a nucleophile. A total of 82% of (R)‐NPE and excellent enantioselectivity (>99%) were achieved in the batch system after 24 hours of reaction time. GtHNL‐3V on Celite R‐ 633 was successfully recycled five times. During more recycling steps a significant decrease in yield was observed while the enantioselectivity remained excellent over eight cycles. The use of a flow system enabled the continuous synthesis of (R)‐NPE. A total of 15% formation of (R)‐NPE was reached using a flow rate of 0.1 mL min⁻¹; unfortunately, the enzyme was not stable, and the yield decreased to 4% after 4 hours on stream. A similar yield was observed during 15 hours at a rate of 0.01 mL min⁻¹. Surprisingly the use of a continuous flow system did not facilitate the process intensification. In fact, the batch system displayed a space‐time‐yield (STY/mgenzyme) of 0.10 g L⁻¹ h⁻¹ mgenzyme⁻¹ whereas the flow system displayed 0.02 and 0.003 g L⁻¹ h⁻¹ mgenzyme⁻¹ at 0.1 and 0.01 mL min⁻¹, respectively. In general, the addition of 1 M nitromethane potentially changed the polarity of the reaction mixture affecting the stability of Celite‐GtHNL‐3V. The nature of the batch system maintained the reaction conditions better than the flow system. The higher yield and productivity observed for the batch system show that it is a superior system for the synthesis of (R)‐NPE compared with the flow approach.

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In nature 2-deoxy-D-ribose-5-phosphate aldolase (DERA) catalyses the reversible formation of 2-deoxyribose 5-phosphate from D-glyceraldehyde 3-phosphate and acetaldehyde. In addition, this enzyme can use acetaldehyde as the sole substrate, resulting in a tandem aldol reaction, yielding 2,4,6-trideoxy-D-erythro-hexapyranose, which spontaneously cyclizes. This reaction is very useful for the synthesis of the side chain of statin-type drugs used to decrease cholesterol levels in blood. One of the main challenges in the use of DERA in industrial processes, where high substrate loads are needed to achieve the desired productivity, is its inactivation by high acetaldehyde concentration. In this work, the utility of different variants of Pectobacterium atrosepticum DERA (PaDERA) as whole cell biocatalysts to synthesize 2-deoxyribose 5-phosphate and 2,4,6-trideoxy-D-erythro-hexapyranose was analysed. Under optimized conditions, E. coli BL21 (PaDERA C-His AA C49M) whole cells yields 99 % of both products. Furthermore, this enzyme is able to tolerate 500 mM acetaldehyde in a whole-cell experiment which makes it suitable for industrial applications.

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Enantiomerically pure cyanohydrins are of great importance in the chemical and pharmaceutical industries. Their synthesis is possible through the use of highly selective hydroxynitrile lyases. In this work, an R-selective hydroxynitrile lyase (AtHNL) from Arabidopsis thaliana was immobilized inside a silica microreactor with a tortuous and hierarchical pore structure. After immobilization, the enzyme activity was tested for benzaldehyde 1a, and its analogs 4-fluorobenzaldehyde 1b, 4-methoxybenzaldehyde 1c and 4-(trifluoromethyl)benzaldehyde 1d. With their different degrees of reactivity they also display a different susceptibility to the racemic chemical background reaction. It was shown that the use of a flow microreactor suppressed the background reaction even for the most susceptible substrate 1d. Furthermore, the use of a flow microreactor enabled high substrate conversion (90-95%) while maintaining a high enantiomeric excess (90-98%) with residence times of 3 to 30 min. The productivity, which depended on substrate reactivity and flow rate, was evaluated by space-time-yield (STY) and reached a value from 60 g L⁻¹ h⁻¹ to 1290 g L⁻¹ h⁻¹. Additionally it was demonstrated that the stability of the immobilized enzyme depends on the flow rates used and thus on the shear forces acting inside the microreactor and interfacial effects associated with them.

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In this review the current state-of-the-art of S-adenosylmethionine (SAM)-dependent methyltransferases and SAM are evaluated. Their structural classification and diversity is introduced and key mechanistic aspects presented which are then detailed further. Then, catalytic SAM as a target for drugs, and approaches to utilise SAM as a cofactor in synthesis are introduced with different supply and regeneration approaches evaluated. The use of SAM analogues are also described. Finally O-, N-, C- and S-MTs, their synthetic applications and potential for compound diversification is given.

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Biocatalysis has an enormous impact on chemical synthesis. The waves in which biocatalysis has developed, and in doing so changed our perception of what organic chemistry is, were reviewed 20 and 10 years ago. Here we review the consequences of these waves of development. Nowadays, hydrolases are widely used on an industrial scale for the benign synthesis of commodity and bulk chemicals and are fully developed. In addition, further enzyme classes are gaining ever increasing interest. Particularly, enzymes catalysing selective C-C-bond formation reactions and enzymes catalysing selective oxidation and reduction reactions are solving long-standing synthetic challenges in organic chemistry. Combined efforts from molecular biology, systems biology, organic chemistry and chemical engineering will establish a whole new toolbox for chemistry. Recent developments are critically reviewed.

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A sequential-type as well as a tandem-type chemoenzymatic flow cascade combining an organocatalytic aldol reaction and a biocatalytic reduction to form stereoselectively a 1,3-diol with two stereogenic centers were developed. Initially, a comprehensive screening of 24 alcohol dehydrogenases was carried out and the identified candidates were applied in different multi-step flow cascades. All four stereoisomers of the desired 1,3-diol product are accessible via a sequential flow approach with product formation-related conversions of up to 76 % over two steps, isolated yields of up to 64 % and enantiomeric excess of >99 % in all cases. In addition, a tandem-type flow process, performing both reaction steps simultaneously, was established leading to 51 % conversion with >99 % ee and 8 : 1 d.r. and representing a combination of the fields of asymmetric chemocatalysis, biocatalysis and flow chemistry.

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Here, we present a two-step continuous flow enzymatic synthesis process in monolithic microreactors using basic sugars as substrates. In the first step UDP-glucose pyrophosphorylase (TaGalU) catalyses the synthesis of uridine-diphosphate-glucose (UDP-Glc) using uridine triphosphate (UTP) and glucose-1-phosphate (Glc-1-P). This is followed by the trehalose transferase (mCherry-TuTreT) catalysed reaction of UDP-Glc and Glc, to obtain trehalose. First, procedures for immobilisation of both enzymes on functionalised silica supports were studied and it was found that covalent bonding by amino groups using a glutaraldehyde linker gives highly active biocatalysts. Due to a drastic difference in temperature range of activity and stability of the immobilised enzymes a bi-reactor cascade was rationally the best solution. Depending on the applied flow rate and hence reaction (residence) time (1.5–10 min) the space-time-yield values varied, respectively, from 1.9 to 14.4 and 8.3 to 49.6 g_product·L^-1·h⁻¹·mg_protein^-1, for UDP-glucose pyrophosphorylase and trehalose transferase catalysed reactions. Prolonged (100 h) continuous flow operation showed that the system is operationally stable, but owing to neutral pH, it is prone to microbiological infections. They can be eliminated applying an antibacterial/antifungal therapy or preventive actions by storing and washing the reactors with a NaN₃ solution. The presented process paves the way for the continuous in flow synthesis of natural and non-natural trehalose analogues and disaccharides.

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Regulation of enzyme activity is vital for living organisms. In metalloenzymes, far-reaching rearrangements of the protein scaffold are generally required to tune the metal cofactor's properties by allosteric regulation. Here structural analysis of hydroxyketoacid aldolase from Sphingomonas wittichii RW1 (SwHKA) revealed a dynamic movement of the metal cofactor between two coordination spheres without protein scaffold rearrangements. In its resting state configuration (M²⁺_R), the metal constitutes an integral part of the dimer interface within the overall hexameric assembly, but sterical constraints do not allow for substrate binding. Conversely, a second coordination sphere constitutes the catalytically active state (M²⁺_A) at 2.4 Å distance. Bidentate coordination of a ketoacid substrate to M²⁺_A affords the overall lowest energy complex, which drives the transition from M²⁺_R to M²⁺_A. While not described earlier, this type of regulation may be widespread and largely overlooked due to low occupancy of some of its states in protein crystal structures.

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Oleate hydratase catalyses the addition of water to the CC double bond of oleic acid to produce (R)-10-hydroxystearic acid. The enzyme requires an FAD cofactor that functions to optimise the active site structure. A wide range of unsaturated fatty acids can be hydrated at the C10 and in some cases the C13 position. The substrate scope can be expanded using ‘decoy’ small carboxylic acids to convert small chain alkenes to secondary alcohols, albeit at low conversion rates. Systematic protein engineering and directed evolution to widen the substrate scope and increase the conversion rate is possible, supported by new high throughput screening assays that have been developed. Multi-enzyme cascades allow the formation of a wide range of products including keto-fatty acids, secondary alcohols, secondary amines and α,ω-dicarboxylic acids. Key points: • Phylogenetically distinct oleate hydratases may exhibit mechanistic differences. • Protein engineering to improve productivity and substrate scope is possible. • Multi-enzymatic cascades greatly widen the product portfolio.

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Diastereomers are characterised by an intrinsic energy difference, and thermodynamics dictate their distribution within a dynamic equilibrium. The characteristic mechanistic reversibility and non-ideal stereoselectivity of catalysts therefore simultaneously promote both synthesis and epimerization of products during the formation of diastereomers. This feature can even result in the thermodynamic inversion of a chiral centre against the catalyst's stereoselectivity. Here, we provide a comprehensive experimental and theoretical study of factors that govern thermodynamic epimerization in catalysis, using enzymes as example. Our analysis highlights, that the deduction of a catalyst's stereoselectivity based on the absolute configuration of the isolated product constitutes a potential pitfall. The selective formation of either the thermodynamic-, or the kinetic product is less determined by the catalyst, but rather by the reaction conditions. Next to low temperatures, a high maximal extent of conversion was identified to promote kinetically controlled conditions. For bimolecular reactions, conversions can be conveniently modulated via the use of one substrate in excess. Quantum mechanical calculations accurately predicted the diastereomeric excess under equilibrium conditions, which opens the prospect of a rational choice between thermodynamic and kinetic reaction control at an early stage of process design. Our findings are of critical importance for multi-step syntheses of stereocomplex molecules via catalytic cascade reactions or artificial metabolic pathways, as the final stereochemistry may be determined by the absolute configuration of the product that is overall lowest in energy.

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Although both the Inherent Safety Principles (ISPs) and the Safe-by-Design (SbD) approach revolve around the central value of safety, they have a slightly different focus in terms of developing add-on features or considering initial design choices. This paper examines the differences between these approaches and analyses which approach is more suitable for a specific type of research—fundamental or applied. By applying the ISPs and SbD to a case study focusing on miniaturized processes using Hydrogen Cyanide, we find that both approaches encounter internal value-conflicts and suffer from external barriers, or lock-ins, which hinder implementation of safety measures. By applying the Technology Readiness Levels (TRLs), we gain insight in the matureness of a technology (thereby distinguishing fundamental and applied research) and the extent of lock-ins being present. We conclude that the ISPs are better able to deal with lock-ins, which are more common in applied research stages, as this approach provides guidelines for add-on safety measures. Fundamental research is not subject to lock-ins yet, and therefore SbD would be a more suitable approach. Lastly, application of either approach should not be associated with a specific field of interest, but instead with associated known or uncertain risks.@en