Martina Sudar

Aldol addition of dihydroxyacetone to N-Cbz-3- aminopropanal catalyzed by two aldolases variants in microreactors

Aldol addition of dihydroxyacetone to N-Cbz-3-aminopropanal catalyzed by two d-fructose-6-phosphate aldolase variants, FSA A129S and FSA A129S/A165G, overexpressed in Escherichia coli was studied in microreactors. The presence of organic solvent was necessary due to poor solubility of N-Cbz-3-aminopropanal in water. Hence, three co-solvents were evaluated: ethyl acetate, acetonitrile and dimethylformamide (DMF). The influence of these solvents and their concentration on the enzyme activity was independently tested and it was found that all solvents significantly reduce the activity of FSA depending on their concentration. The reaction was carried out in three different microreactors; two without and one with micromixers. By increasing enzyme concentration, it was possible to achieve higher substrate conversion at lower residence time. Enzyme activity measured at the outlet flow of the microreactor at different residence time revealed that enzymes are more stable at lower residence times due to shorter time of exposure to organic solvent. The reaction in the batch reactor was compared with the results in microreactor with micromixers. Volume productivity was more than three fold higher in microreactor with micromixers than in the batch reactor for both aldolases. It was found to be 0.88Md(-1) and 0.80Md(-1) for FSA A129S and FSA A129S/A165G, respectively.

A new concept for production of (3S,4R)-6-[(benzyloxycarbonyl)amino]-5,6-dideoxyhex-2-ulose, a precursor of D-fagomine

A novel cascade reaction for the production of aldol adduct (3S,4R)-6-[(benzyloxycarbonyl)amino]-5,6-dideoxyhex-2-ulose was studied in this work. The strategy combines three enzymes in one pot: (i) horse liver alcohol dehydrogenase for the oxidation of N-Cbz-3-aminopropanol to the corresponding aldehyde, (ii) NADH oxidase for the regeneration of coenzyme NAD+ and (iii) D-fructose-6-phosphate aldolase from E. coli A129S variant for the aldol addition of dihydroxyacetone to N-Cbz-3-aminopropanal. On the basis of preliminary experiments, optimization of the initial reaction conditions was done using statistical methods, i.e. factorial design of experiments. 79% yield of aldol adduct was achieved in the batch reactor after optimization.

Mathematical model for aldol addition catalyzed by two d-fructose-6-phosphate aldolases variants overexpressed in E. coli

Two D-fructose-6-phosphate aldolase variants namely, single variant FSA A129S and double variant FSA A129S/A165G, were used as catalysts in the aldol addition of dihydroxyacetone (DHA) to N-Cbz-3-aminopropanal. Mathematical model for reaction catalyzed by both enzymes, consisting of kinetic and mass balance equations, was developed. Kinetic parameters were estimated from the experimental data gathered by using the initial reaction rate method. The model was validated in the batch and continuously operated ultrafiltration membrane reactor (UFMR). The same type of kinetic model could be applied for both enzymes. The operational stability of the aldolases was assessed by measuring enzyme activity during the experiments. FSA A129S/A165G had better operational stability in the batch reactor (half-life time 26.7 h) in comparison to FSA A129S (half-life time 5.78 h). Both variants were unstable in the continuously operated UFMR in which half-life times were 1.99 and 3.64 h for FSA A129S and FSA A129S/A165G, respectively.

Mathematical model of the MenD-catalyzed 1,4-addition (Stetter reaction) of α-ketoglutaric acid to acrylonitrile

The Stetter reaction, a conjugate umpolung reaction, is well known for cyanide-catalyzed transformations of mostly aromatic aldehydes. Enzymatic Stetter reactions, however, have been largely unexplored, especially with respect to preparative transformations. We have investigated the kinetics of the MenD-catalyzed 1,4-addition of α-ketoglutaric acid to acrylonitrile which has shown that acrylonitrile, while an interesting candidate, is a poor substrate for MenD due to low affinity of the enzyme for this substrate. The kinetic model of the reaction was simplified to double substrate Michaelis-Menten kinetics where the reaction rate linearly depends on acrylonitrile concentration. Experiments at different initial concentrations of acrylonitrile under batch, repetitive batch, and fed-batch reactor conditions were carried out to validate the developed mathematical model. Thiamine diphosphate dependent MenD proved to be quite a robust enzyme; nevertheless, enzyme operational stability decay occurs in the reactor. The spontaneous reactivity of acrylonitrile towards polymerization was also taken into account during mathematical modeling. Almost quantitative conversion of acrylonitrile was achieved in all batch reactor experiments, while the yield of the desired product was dependent on initial acrylonitrile concentration (i.e., the concentration of the stabilizer additive). Using the optimized reactor parameters, it was possible to synthesize the product, 6-cyano-4-oxohexanoic acid, in a concentration of 250 mM. The highest concentration of product was achieved in a repetitive batch reactor experiment. A fed-batch reactor experiment also delivered promising results, especially regarding the short reaction time needed to achieve a 200 mM concentration of product. Hence, the enzymatic Stetter reaction with a highly reactive acceptor substrate can be performed on a preparative scale, which should enable similar transformations with acrylate, methacrylate, and methyl vinyl ketone.

Operational stability of glucoamylase in continuously operated ultrafiltration membrane reactor – experimental methods and mathematical model

Operational stability of glucoamylase was studied at 45, 60 and 70 °C in the reaction of maltose hydrolysis. Experiments were carried out in the continuously operated ultrafiltration membrane reactor (UFMR) at constant residence time of 176.5 minutes. The rate of enzyme operational stability decay increased with temperature. This could be quantitatively observed from the measurements of volume activity during the experiments which were used to estimate enzyme operational stability decay rate constants. Results have shown that stationary conditions in UFMR can be maintained if sufficiently high enzyme concentration is used in the reactor, regardless of the enzyme operational stability decay that occurs. This was shown by the experiment carried out at 45 °C where it was proved that the enzyme operational decay occurs even though maltose conversion was at maximum during the entire experiment. Thus, the operational stability decay can be masked.