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.@en

In this case study, we compare the performance of an enzyme immobilised using two different methods: i) as carrier-free catalytically active inclusion bodies or ii) as carrier-attached immobilised enzyme. To make this comparison we used a trehalose transferase from Thermoproteus uzoniensis fused to the fluorescent thermostable protein mCherry. The fusion of mCherry to trehalose transferase allowed direct spectrophotometric quantification and visualisation of the enzyme in both native and denatured states. The catalytically active inclusion bodies outperformed the immobilised enzyme in their simplicity of biocatalyst production resulting in high enzyme productivity. Enzyme immobilised on carrier materials showed a higher catalytic activity and a more robust performance under batch process conditions.@en

The specific oxidation of 12α-OH group of hydroxysteroids is required for the preparation of cheno-and ursodeoxycholic acid (CDCA and UDCA, respectively). The C12 oxidation of hydroxysteroids into their 12-oxo derivatives can selectively be performed by employing 12α-hydroxysteroid dehydrogenases. These enzymes use NAD(P)+ as an electron acceptor, which has to be re-oxidized in a so-called “regeneration system”. Recently, the enzyme NAD(P)H oxidase (NOX) was applied for the regeneration of NAD+ in the enzymatic preparation of 12-oxo-CDCA from cholic acid (CA), which allows air to be used as an oxidant. However, the NOX system suffers from low activity and low stability. Moreover, the substrate loading is limited to 10 mM. In this study, the laccase/mediator system was investigated as a possible alternative to NOX, employing air as an oxidant. The laccase/mediator system shows higher productivity and scalability than the NOX system. This was proven with a preparative biotransformation of 20 g of CA into 12-oxo-CDCA (92% isolated yield) by employing a substrate loading of 120 mM (corresponding to 50 g/L). Additionally, the performance of the laccase/mediator system was compared with a classical ADH/acetone regeneration system and with other regeneration systems reported in literature.@en

Transketolase catalyzes asymmetric C−C bond formation of two highly polar compounds. Over the last 30 years, the reaction has unanimously been described in literature as irreversible because of the concomitant release of CO2 if using lithium hydroxypyruvate (LiHPA) as a substrate. Following the reaction over a longer period of time however, we have now found it to be initially kinetically controlled. Contrary to previous suggestions, for the non-natural conversion of synthetically more interesting apolar substrates, the complete change of active-site polarity is therefore not necessary. From docking studies it was revealed that water and hydrogen-bond networks are essential for substrate binding, thus allowing aliphatic aldehydes to be converted in the charged active site of transketolase.@en

Enzymes are nature's catalyst of choice for the highly selective and efficient coupling of carbohydrates. Enzymatic sugar coupling is a competitive technology for industrial glycosylation reactions, since chemical synthetic routes require extensive use of laborious protection group manipulations and often lack regio- and stereoselectivity. The application of Leloir glycosyltransferases has received considerable attention in recent years and offers excellent control over the reactivity and selectivity of glycosylation reactions with unprotected carbohydrates, paving the way for previously inaccessible synthetic routes. The development of nucleotide recycling cascades has allowed for the efficient production and reuse of nucleotide sugar donors in robust one-pot multi-enzyme glycosylation cascades. In this way, large glycans and glycoconjugates with complex stereochemistry can be constructed. With recent advances, LeLoir glycosyltransferases are close to being applied industrially in multi-enzyme, programmable cascade glycosylations.@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

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

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

The merger of enzyme immobilisation and flow chemistry has attracted the attention of the scientific community during recent years. Immobilisation enhances enzyme stability and enables recycling, flow chemistry allows process intensification. Their combination is desirable for the development of more efficient and environmentally friendly biocatalytic processes. In this feature article, we aim to point out important metrics for successful enzyme immobilisation and for reporting flow biocatalytic processes. Relevant examples of immobilised enzymes used in flow systems in organic, biphasic and aqueous systems are discussed. Finally, we describe recent developments to address the cofactor recycling hurdle.@en

Biocatalysis is one of the most promising technologies for the sustainable synthesis of molecules for pharmaceutical, biotechnological and industrial purposes. From the gram to the ton scale, biocatalysis is employed with success. This is underpinned by the fact that the global enzyme market is predicted to increase from $7 billion to $10 billion by 2024. This review concentrates on showing the strong benefits that biocatalysis and the use of enzymes can provide to synthetic chemistry. Several examples of successful implementations of enzymes are discussed highlighting not only high-value pharmaceutical processes but also low-cost bulk products. Thus, biocatalytic methods make the chemistry more environmentally friendly and product specific.@en

Nitrile reductases catalyse a two-step reduction of nitriles to amines. This requires the binding of two NADPH molecules during one catalytic cycle. For the nitrile reductase from E. coli (EcoNR) mass spectrometry studies of the catalytic mechanism were performed. EcoNR is dimeric and has no Rossman fold. It was demonstrated that during catalysis each active site binds one substrate molecule. NADPH binds independent of the substrate. The PreQ0 binding pocket of the active site is not involved in the binding of NADPH; this is in conflict with an earlier hypothesis. @en

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.@en

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.@en

MsAcT catalyzes the esterification of primary alcohols in water. When utilizing acid and alcohol as starting materials low yields dictated by thermodynamics were observed. However, with activated esters such as ethyl acetate and vinyl acetate very high yields of the desired ester can be achieved in combination with the appropriate alcohol. This study investigated both the intrinsic kinetic properties of MsAcT for the hydrolysis and transesterification of esters in water as well as the thermodynamics of the reaction. In comparison to the chemical or enzymatic ester synthesis using either toxic reagent, and harsh organic solvents, the MsAcT-catalyzed synthesis of esters of primary alcohols can be achieved efficiently in water without neutralization steps.@en

The enzymatic synthesis of esters and peptides is unfavoured in aqueous solvent systems due to competing hydrolysis. This can be overcome by using energy rich substrate analogues: elimination of a good leaving group temporarily establishes more favourable equilibrium conditions, allowing for (nearly) complete conversion. While kinetically controlled syntheses of esters and peptides in water are common knowledge in biocatalysis textbooks, the prevalence of kinetic control is less well known for other enzyme classes. Here, the general concepts of thermodynamic and kinetic control are illustrated at the example of the well-studied synthesis of β-lactam antibiotics and are shown to similarly also apply to other enzyme classes. Notably, the enzymatic synthesis of diastereomers shows the same characteristic energy profile as that of Diels-Alder reactions. This allows for the selective synthesis of different diastereomers under either thermodynamically or kinetically controlled conditions. Prospects and pitfalls of this notion are discussed at the example of the thermodynamic epimerisation of hydroxysteroids and recent examples of kinetically controlled aldol reactions. Kinetic reaction control can therefore not only be used to increase conversions towards a single product, but also to selectively afford different diastereomers. This review highlights the prevalence of both concepts within the field of biocatalysis.@en