Study on the kinetic characteristics of the asymmetric production of R-(−)-mandelic acid with immobilized Saccharomyces cerevisiae FD11b

Optimization of the medium composition and product extraction for R-mandelic acid using recombinant Escherichia coli expressing Alcaligenes sp. nitrilase

The aim of this study is to investigate the effects of the components of the medium on nitrilase expression in recombinant Escherichia coli ( E. coli), which is applied in the hydrolysis of racemic mandelonitrile (MN) to R-mandelic acid (MA). In addition, the separation of R-MA from the reaction mixture is studied. The glycerol medium is screened, and compositions are optimized in single-factor experiments. The nitrilase activity is 1.96-fold higher than before using the optimal medium containing peptone 15 g/L, yeast extract 12 g/L, NaCl 10 g/L, glycerol 15 g/L, (NH4)2SO4 5 g/L, KH2PO4 10 mM, K2HPO4·3H2O 10 mM, and MgSO4·7H2O 15 mM. During the acid-base extraction of R-MA, the optimal alkaline pH, the alkaline extractant ratio, the optimal acidic pH, the acidic extractant ratio and the amount of activated carbon are 10.0, 1:0.5 (once), 2.50, 1:2 (twice) and 1%, respectively. This study provides a basis for the industrial production of R-MA.

Biodegradation Kinetic Studies of Phenol and p-Cresol in a Batch and Continuous Stirred-Tank Bioreactor with Pseudomonas putida ATCC 17484 Cells

The biodegradation of phenol, p-cresol, and phenol plus p-cresol mixtures was evaluated using Pseudomonas putida ATCC 17484 in aerobic batch reactors. Shake-flask experiments were performed separately using growth medium with initial nominal concentrations of phenol (50–600 mg L−1) and p-cresol (50–600 mg L−1) as well as phenol (50–600 mg L−1) plus p-cresol (50–600 mg L−1). The complete degradation of phenol and p-cresol was achieved within 48 h and 48–56 h, respectively, for all initial concentrations of phenol and p-cresol. The maximum cell growth rate using phenol (μmax,P = 0.45 h−1) was much faster than that using p-cresol (μmax,C = 0.185−1 h). The larger Ki value for phenol (310.5 mg L−1) revealed that the P. putida cells had a higher resistance to phenol inhibition compared with p-cresol (243.56 mg L−1). A mixture of phenol and p-cresol in batch experiments resulted in the complete removal of phenol within 52–56 h for initial phenol concentrations of 50–500 mg L−1. The time needed to remove p-cresol completely was 48–56 h for initial p-cresol concentrations of 50–500 mg L−1. In the continuous-flow immobilized cells reactor, the degradation efficiency for phenol and p-cresol was 97.6 and 89.1%, respectively, at a stable condition.

Phenol Degradation Kinetics by Free and Immobilized Pseudomonas putida BCRC 14365 in Batch and Continuous-Flow Bioreactors

Phenol degradation by Pseudomonas putida BCRC 14365 was investigated at 30 °C and a pH of 5.0–9.0 in the batch tests. Experimental results for both free and immobilized cells demonstrated that a maximum phenol degradation rate occurred at an initial pH of 7. The peak value of phenol degradation rates by the free and immobilized cells were 2.84 and 2.64 mg/L-h, respectively. Considering the culture at 20 °C, there was a lag period of approximately 44 h prior to the start of the phenol degradation for both free and immobilized cells. At the temperatures ranging from 25 to 40 °C, the immobilized cells had a higher rate of phenol degradation compared to the free cells. Moreover, the removal efficiencies of phenol degradation at the final stage were 59.3–92% and 87.5–92%, for the free and immobilized cells, respectively. The optimal temperature was 30 °C for free and immobilized cells. In the batch experiments with various initial phenol concentrations of 68.3–563.4 mg/L, the lag phase was practically negligible, and a logarithmic growth phase of a particular duration was observed from the beginning of the culture. The specific growth rate (μ) in the exponential growth phase was 0.085–0.192 h−1 at various initial phenol concentrations between 68.3 and 563.4 mg/L. Comparing experimental data with the Haldane kinetics, the biokinetic parameters, namely, maximum specific growth rate (μmax), the phenol half-saturation constant (Ks) and the phenol inhibition constant (KI), were determined to equal 0.31 h−1, 26.2 mg/L and 255.0 mg/L, respectively. The growth yield and decay coefficient of P. putida cells were 0.592 ± 4.995 × 10−3 mg cell/mg phenol and 5.70 × 10−2 ± 1.122 × 10−3 day−1, respectively. A completely mixed and continuous-flow bioreactor with immobilized cells was set up to conduct the verification of the kinetic model system. The removal efficiency for phenol in the continuous-flow bioreactor was approximately 97.7% at a steady-state condition. The experimental and simulated methodology used in this work can be applied, in the design of an immobilized cell process, by various industries for phenol-containing wastewater treatment.

Immobilization of recombinant Escherichia coli whole cells harboring xylose reductase and glucose dehydrogenase for xylitol production from xylose mother liquor

In this study, recombinant E. coli BL21(DE3)/pCDFDuet-1-XR-GDH harboring xylose reductase (XR) and glucose dehydrogenase (GDH) were immobilized and applied for the production of xylitol from xylose mother liquor (XML). Various immobilization methods were screened and the cross-linking approach with diatomite and polyetherimide as the raw materials and glutaraldehyde as the cross-linking agent was the optimal one, and the recovery activity reached of 80.3% after immobilization. The half-life of immobilized cells was 1.52 times to that of free cells. Batch experiments showed that the enzyme activity of immobilized cells remained 70.5% of the initial activity after 10 batches and the space-time yield of xylitol reached of 11.5 g/(L h). The production of xylitol from xylose mother liquor by immobilized E. coli cells containing xylose reductase and glucose dehydrogenase was reported for the first time, which paved a foundation for industrial production of xylitol from waste xylose mother liquor.

Progress in biocatalysis with immobilized viable whole cells: systems development, reaction engineering and applications

Viable microbial cells are important biocatalysts in the production of fine chemicals and biofuels, in environmental applications and also in emerging applications such as biosensors or medicine. Their increasing significance is driven mainly by the intensive development of high performance recombinant strains supplying multienzyme cascade reaction pathways, and by advances in preservation of the native state and stability of whole-cell biocatalysts throughout their application. In many cases, the stability and performance of whole-cell biocatalysts can be highly improved by controlled immobilization techniques. This review summarizes the current progress in the development of immobilized whole-cell biocatalysts, the immobilization methods as well as in the bioreaction engineering aspects and economical aspects of their biocatalytic applications.

Production of chiral compounds using immobilized cells as a source of biocatalysts

The importance of chiral compounds in all fields of technology and life sciences is shown. Small chiral molecules are mainly used as building blocks in the synthesis of more complex and functionalized compounds. Nature creates and imposes stereoselectivity by means of enzymes, which are highly efficient biocatalysts. The use of whole cells as a biocatalyst source is a promising strategy for avoiding some drawbacks associated with the use of pure enzymes, especially their high cost. The use of free cells is also challenging, since cell lysis can also occur under the reaction conditions. However, cell immobilization has been employed to increase the catalytic potential of enzymes by extending their lifetimes in organic solvents and non-natural environments. Besides, immobilized cells maintain their biocatalytic performance for several reaction cycles. Considering the above-mentioned arguments, several authors have synthesized different classes of chiral compounds such as alcohols, amines, carboxylic acids, amides, sulfides and lactones by means of immobilized cells. Our aim was to discuss the main aspects of the production of chiral compounds using immobilized cells as a source of biocatalysts, except under fermentation conditions.

Improvement of Alcaligenes faecalis nitrilase by gene site saturation mutagenesis and its application in stereospecific biosynthesis of (R)-(-)-mandelic acid

Nitrilases have recently received considerable attention as the biocatalysts for stereospecific production of carboxylic acids. To improve the activity, the nitrilase from Alcaligenes faecalis was selected for further modification by the gene site saturation mutagenesis method (GSSM), based on homology modeling and previous reports about mutations. After mutagenesis, the positive mutants were selected using a convenient two-step high-throughput screening method based on product formation and pH indicator combined with the HPLC method. After three rounds of GSSM, Mut3 (Gln196Ser/Ala284Ile) with the highest activity and ability of tolerance to the substrate was selected. As compared to the wild-type A. faecalis nitrilase, Mut3 showed 154% higher specific activity. Mut3 could retain 91.6% of its residual activity after incubation at pH 6.5 for 6 h. In a fed-batch reaction with 800 mM mandelonitrile as the substrate, the cumulative production of (R)-(-)-mandelic acid after 7.5 h of conversion reached 693 mM with an enantiomeric excess of 99%, and the space-time productivity of Mut3 was 21.50-fold higher than that of wild-type nitrilase. The Km, Vmax, and k(cat) of wild-type and Mut3 for mandelonitrile were 20.64 mM, 33.74 μmol mg(-1) min(-1), 24.45 s(-1), and 9.24 mM, 47.68 μmol mg(-1) min(-1), and 34.55 s(-1), respectively. A homology modeling and molecular docking study showed that the diameter of the catalytic tunnel of Mut3 became longer and that the tunnel volume was smaller. These structural changes are proposed to improve the hydrolytic activity and pH stability of Mut3. Mut3 has the potential for industrial applications in the upscale production of (R)-(-)-mandelic acid.

Stereo-selective conversion of mandelonitrile to (R)-(−)-mandelic acid using immobilized cells of recombinant Escherichia coli

Immobilized cells of a recombinant Escherichia coli expressing nitrilase from Pseudomonas putida were used to catalyze the hydrolysis of mandelonitrile (2-hydroxy-2-phenylacetonitrile) to (R)-(−)-mandelic acid. The cells had been immobilized by entrapment in an alginate matrix. Conditions for the hydrolysis reaction were optimized in shake flasks and in a packed bed reactor. In shake flasks the best conditions for the reaction were a temperature of 40 °C, pH 8, biocatalyst bead diameter of 4.3 mm, sodium alginate concentration in the gel matrix of 2 % (w/v, g/100 mL), a cell dry mass concentration in the bead matrix of 20 mg/mL, an initial substrate concentration of 50 mM and a reaction time of 60 min. Under these conditions, the conversion of mandelonitrile was nearly 95 %. In the packed bed reactor, a feed flow rate of 20 mL/h at a substrate concentration of 200 mM proved to be the best at 40 °C, pH 8, using 4.3 mm beads (2 % w/v sodium alginate in the gel matrix, 20 mg dry cell concentration per mL of gel matrix). This feed flow rate corresponded to a residence time of 0.975 h in the packed bed.