Kinetic investigation of a solvent-free, chemoenzymatic reaction sequence towards enantioselective synthesis of a β-amino acid ester

Applied biocatalysis beyond just buffers - from aqueous to unconventional media. Options and guidelines

In nature, enzymes conventionally operate under aqueous conditions. Because of this, aqueous buffers are often the choice for reaction media when enzymes are applied in chemical synthesis. However, to meet the demands of an industrial application, due to the poor water solubility of many industrially relevant compounds, an aqueous reaction system will often not be able to provide sufficient substrate loadings. A switch to a non-aqueous solvent system can provide a solution, which is already common for lipases, but more challenging for biocatalysts from other enzyme classes. The choices in solvent types and systems, however, can be overwhelming. Furthermore, some engineering of the protein structure of biocatalyst formulation is required. In this review, a guide for those working with biocatalysts, who look for a way to increase their reaction productivity, is presented. Examples reported clearly show that bulk water is not necessarily required for biocatalytic reactions and that clever solvent systems design can support increased product concentrations thereby decreasing waste formation. Additionally, under these conditions, enzymes can also be combined in cascades with other, water-sensitive, chemical catalysts. Finally, we show that the application of non-aqueous solvents in biocatalysis can actually lead to more sustainable processes. At the hand of flowcharts, following simple questions, one can quickly find what solvent systems are viable.

One-Pot Lipase-Catalyzed Enantioselective Synthesis of (R)-(-)-N-Benzyl-3-(benzylamino)butanamide: The Effect of Solvent Polarity on Enantioselectivity

The use of the solvent engineering has been applied for controlling the resolution of lipase-catalyzed synthesis of β-aminoacids via Michael addition reactions. The strategy consisted of the thermodynamic control of products at equilibrium using the lipase CalB as a catalyst. The enzymatic chemo- and enantioselective synthesis of (R)-(−)-N-benzyl-3-(benzylamino)butanamide is reported, showing the influence of the solvent on the chemoselectivity of the aza-Michael addition and the subsequent kinetic resolution of the Michael adduct; both processes are catalyzed by CalB and both are influenced by the nature of the solvent medium. This approach allowed us to propose a novel one-pot strategy for the enzymatic synthesis of enantiomerically enriched β-aminoesters and β-aminoacids.

Thermodynamics of Bioreactions

Thermodynamic principles have been applied to enzyme-catalyzed reactions since the beginning of the 1930s in an attempt to understand metabolic pathways. Currently, thermodynamics is also applied to the design and analysis of biotechnological processes. The key thermodynamic quantity is the Gibbs energy of reaction, which must be negative for a reaction to occur spontaneously. However, the application of thermodynamic feasibility studies sometimes yields positive Gibbs energies of reaction even for reactions that are known to occur spontaneously, such as glycolysis. This article reviews the application of thermodynamics in enzyme-catalyzed reactions. It summarizes the basic thermodynamic relationships used for describing the Gibbs energy of reaction and also refers to the nonuniform application of these relationships in the literature. The review summarizes state-of-the-art approaches that describe the influence of temperature, pH, electrolytes, solvents, and concentrations of reacting agents on the Gibbs energy of reaction and, therefore, on the feasibility and yield of biological reactions.

Quantitative full time course analysis of nonlinear enzyme cycling kinetics

Enzyme inhibition due to the reversible binding of reaction products is common and underlies the origins of negative feedback inhibition in many metabolic and signaling pathways. Product inhibition generates non-linearity in steady-state time courses of enzyme activity, which limits the utility of well-established enzymology approaches developed under the assumption of irreversible product release. For more than a century, numerous attempts to find a mathematical solution for analysis of kinetic time courses with product inhibition have been put forth. However, no practical general method capable of extracting common enzymatic parameters from such non-linear time courses has been successfully developed. Here we present a simple and practical method of analysis capable of efficiently extracting steady-state enzyme kinetic parameters and product binding constants from non-linear kinetic time courses with product inhibition and/or substrate depletion. The method is general and applicable to all enzyme systems, independent of reaction schemes and pathways.

Synthesis and anti-TMV activity of dialkyl/dibenzyl 2-((6-substituted-benzo[d]thiazol-2-ylamino)(benzofuran-2-yl)methyl) malonates

Starting from benzofuran-2-methanal, 6-substituted benzothiazole-2-amines and malonic esters, sixteen title compounds were designed and synthesized seeking to introduce anti-TMV activity. The structures of the newly synthesized compounds were confirmed by ¹H-NMR, ¹³C-NMR, IR spectra, and MS (HREI) analysis. The bioassays identified some of these new compounds as having moderate to good anti-TMV activity. The compounds 5i and 5m have good antiviral activity against TMV with a curative rate of 52.23% and 54.41%, respectively, at a concentration of 0.5 mg/mL.

Molecular mechanism of deactivation of C. antarctica lipase B by methanol

The catalytic activity of Candida antarctica lipase B upon alcoholysis of a constant concentration of 15.2% vinyl acetate (vol/vol) and varying concentrations of methanol (0.7-60%) in toluene was determined experimentally by measuring the initial reaction velocity. The molecular mechanism of the deactivation of the enzyme by methanol was investigated by fitting the experimental data to a kinetic model and by molecular dynamics simulations of C. antarctica lipase B in toluene-methanol-water mixtures. The highest catalytic activity (280 U/mg) was observed at methanol concentrations as low as 0.7% methanol (vol/vol), followed by a sharp decrease at higher methanol concentrations. For methanol concentrations above 10% (vol/vol), catalytic activity was at 30% of the maximum activity. A variation of water activity in the range 0.02-0.09 had only minor effects. These experimental observations are described by a simple kinetic model using three assumptions: (1) a ping-pong bi-bi mechanism of the enzyme, (2) competitive inhibition by the substrate methanol, and (3) by describing enzyme kinetics by the thermodynamic activities of the substrates rather than by their concentrations. Two equilibrium constants of methanol (KM,MeOH=0.05 and Ki,MeOH=0.23) were derived by modeling methanol binding to the substrate binding site of the lipase in molecular dynamics simulations of protein-solvent systems at atomic resolution. Thus, the sharp maximum of catalytic activity of C. antarctica lipase B at 0.7% methanol is a direct consequence of the fact that methanol-toluene mixtures are far from ideal. Understanding the thermodynamics of solvent mixtures is prerequisite to a quantitative model of enzymatic activity in organic solvents.

A thermostable and organic-solvent tolerant esterase from Pseudomonas putida ECU1011: catalytic properties and performance in kinetic resolution of α-hydroxy acids

A novel esterase, rPPE01, from Pseudomonas putida ECU1011 was heterologously expressed in Escherichia coli and identified for enzymatic resolution of hydroxy acids via O-deacetylation. α-Acetoxy carboxylates were converted with approximately 50% yield and excellent enantioselectivity (E>200) at a substrate concentration of 100 mM. The half-lives of rPPE01 were 14 days at 50°C and 30 days at 30°C, indicating the enzyme has relatively high thermostability. Another remarkable advantage of rPPE01 is that both the activity and thermostability were enhanced significantly in the presence of hydrophobic alkanes and ethers. rPPE01 retained 159% of its initial activity after incubation with 50% (v/v) n-heptane at 30°C for 60 days. The attractive organic-solvent tolerance, good thermostability and high enantioselectivity towards α-acetoxy carboxylates endow rPPE01 with the potential of practical application for the production of enantiopure hydroxy acids.

Lipase-catalyzed (trans)esterification of 5-hydroxy- methylfurfural and separation from HMF esters using deep-eutectic solvents

5-Hydroxymethylfurfural (HMF) is a valuable biomass-derived building block. Among possible HMF valorization products, a broad range of HMF esters can be synthesized. These HMF esters have found some promising applications, such as monomers, fuels, additives, surfactants, and fungicides, and thus several catalytic approaches for HMF (trans)esterifications have been reported. The intrinsic reactivity of HMF is challenging, forcing the use of mild reaction conditions to avoid by-product formation. This paper explores the lipase-catalyzed (trans)esterification of HMF with different acyl donors (carboxylic acids and methyl- and ethyl esters) mostly in solvent-free conditions. The results demonstrate that lipases may be promising alternatives for the synthesis of HMF esters-with high productivities and reactions at high substrate loadings-provided that robust systems for lipase immobilization are applied to assure an adequate reusability of the enzymes. Once (trans)esterifications have been conducted, the separation of unreacted HMF and HMF esters is performed by using deep-eutectic solvents (DES) as separation agents. DES are able to dissolve hydrogen-bond donors (e.g., HMF), whereas non-hydrogen-bond donors (in this case HMF esters) form a second phase. By using this approach, high ester purities (>99 %) and efficiencies (up to >90 % HMF ester recovery) in separations were obtained by using choline chloride-based DES.

A chemo-enzymatic route to synthesize (S)-γ-valerolactone from levulinic acid

Levulinic acid is a feasible platform chemical derived from acid-catalyzed hydrolysis of lignocellulose. The conversion of this substrate to (S)-γ-valerolactone ((S)-GVL) was investigated in a chemo-enzymatic reaction sequence that benefits from mild reaction conditions and excellent enantiomeric excess of the desired (S)-GVL. For that purpose, levulinic acid was chemically esterified over the ion exchange resin Amberlyst 15 to yield ethyl levulinate (LaOEt). The keto ester was successfully reduced by (S)-specific carbonyl reductase from Candida parapsilosis (CPCR2) in a substrate-coupled cofactor regeneration system utilizing isopropanol as cosubstrate. In classical batch experiments, a maximum conversion of 95 % was achieved using a 20-fold excess of isopropanol. Continuous reduction of LaOEt was carried out for 24 h, and a productivity of more than 5 mg (S)-ethyl-4-hydroxypentanoate (4HPOEt) per μg CPCR2 was achieved. Afterwards (S)-4HPOEt (>99%ee) was substituted to lipase-catalyzed lactonization using immobilized lipase B from Candida antarctica to yield (S)-GVL in 90 % overall yield and >99%ee.