Chiral Polyol Synthesis Catalyzed by a Thermostable Transketolase Immobilized on Layered Double Hydroxides in Ionic liquids

Co-Immobilization of D-Amino Acid Oxidase, Catalase, and Transketolase for One-Pot, Two-Step Synthesis of L-Erythrulose

Here, we present an immobilized enzyme cascade in a basket-type reactor allowing a one-pot, two-step enzymatic synthesis of L-erythrulose from D-serine and glycolaldehyde. Three enzymes, D-amino acid oxidase from Rhodotorula gracilis (DAAORg), catalase from bovine liver (CAT), and transketolase from Geobacillus stearothermophilus (TKgst) were covalently immobilized on silica monolithic pellets, characterized by an open structure of interconnected macropores and a specific surface area of up to 300 m2/g. Three strategies were considered: (i) separate immobilization of enzymes on silica supports ([DAAO][CAT][TK]), (ii) co-immobilization of two of the three enzymes followed by the third ([DAAO+CAT][TK]), and (iii) co-immobilization of all three enzymes ([DAAO+CAT+TK]). The highest L-erythrulose concentrations were observed for the co-immobilization protocols (ii) and (iii) (30.7 mM and 29.1 mM, respectively). The reusability study showed that the best combination was [DAAO + CAT][TK], which led to the same level of L-erythrulose formation after two reuse cycles. The described process paves the way for the effective synthesis of a wide range of α-hydroxyketones from D-serine and suitable aldehydes.

High-Throughput Solid-Phase Assay for Substrate Profiling and Directed Evolution of Transketolase

Thiamine diphosphate-dependent enzymes, and specifically transketolases, form one of the most important families of biocatalytic tools for enantioselective carbon-carbon bond formation yielding various hydroxyketones of biological interest. To enable substrate profiling of transketolases for acceptance of different donors and acceptors, a simple, direct colorimetric assay based on pH reaction variation was developed to establish a high-throughput solid-phase assay. This assay reduces the screening effort in the directed evolution of transketolases, as only active variants are selected for further analysis. Transketolase activity is detected as bicarbonate anions released from the α-ketoacid donor substrate, which causes the pH to rise. A pH indicator, bromothymol blue, which changes color from yellow to blue in alkaline conditions, was used to directly detect, with the naked eye, clones expressing active transketolase variants, obviating enzyme extraction.

Innovative Electrochemical Screening Allows Transketolase Inhibitors to Be Identified

Transketolases (TKs) are ubiquitous thiamine pyrophosphate (TPP)-dependent enzymes of the nonoxidative branch of the pentose phosphate pathway. They are considered as interesting therapeutic targets in numerous diseases and infections (e.g., cancer, tuberculosis, malaria), for which it is important to find specific and efficient inhibitors. Current TK assays require important amounts of enzyme, are time-consuming, and are not specific. Here, we report a new high throughput electrochemical assay based on the oxidative trapping of the TK-TPP intermediate. After electrode characterization, the enzyme loading, electrochemical protocol, and substrate concentration were optimized. Finally, 96 electrochemical assays could be performed in parallel in only 7 min, which allows a rapid screening of TK inhibitors. Then, 1360 molecules of an in-house chemical library were screened and one early lead compound was identified to inhibit TK from E. coli with an IC₅₀ of 63 μM and an inhibition constant ( K_I) of 3.4 μM. The electrochemical assay was also used to propose an inhibition mechanism.

Horseradish peroxidase-nanoclay hybrid particles of high functional and colloidal stability

Highly stable dispersions of enzyme-clay nanohybrids of excellent horseradish peroxidase activity were developed. Layered double hydroxide nanoclay was synthesized and functionalized with heparin polyelectrolyte to immobilize the horseradish peroxidase enzyme. The formation of a saturated heparin layer on the platelets led to charge inversion of the positively charged bare nanoclay and to highly stable aqueous dispersions. Great affinity of the enzyme to the surface modified platelets resulted in strong horseradish peroxidase adsorption through electrostatic and hydrophobic interactions as well as hydrogen bonding network and prevented enzyme leakage from the obtained material. The enzyme kept its functional integrity upon immobilization and showed excellent activity in decomposition of hydrogen peroxide and oxidation of an aromatic compound in the test reactions. In addition, remarkable long term functional stability of the enzyme-nanoclay hybrid was observed making the developed colloidal system a promising antioxidant candidate in biomedical treatments and industrial processes.

Droplet millifluidics for kinetic study of transketolase

We present a continuous-flow reactor at the millifluidic scale coupled with an online, non-intrusive spectroscopic monitoring method for determining the kinetic parameters of an enzyme, transketolase (TK) used in biocatalysis for the synthesis of polyols by carboligation. The millifluidic system used is based on droplet flow, a well-established method for kinetic chemical data acquisition. The TK assay is based on the direct quantitative measurement of bicarbonate ions released during the transketolase-catalysed reaction in the presence of hydroxypyruvic acid as the donor, thanks to an irreversible reaction: bicarbonate ions react with phosphoenolpyruvate (PEP) in the presence of PEP carboxylase as the first auxiliary enzyme. The oxaloacetate formed is reduced to malate by NADH in the reaction catalysed by malate dehydrogenase as the second auxiliary enzyme. The extent of oxidation of NADH was measured by spectrophotometry at 340 nm. This system gives a direct, quantitative, generic method to evaluate the TK activity versus different substrates. We demonstrate the accuracy of this strategy to determine the enzymatic kinetic parameters and to study the substrate specificity of a thermostable TK from thermophilic microorganism Geobacillus stearothermophilus, offering promising prospects in biocatalysis. Millifluidic systems are useful in this regard as they can be used to rapidly evaluate the TK activity towards various substrates, and also different sets of conditions, identifying the optimal operating environment while minimizing resource consumption and ensuring high control over the operating conditions.