Application of a Recombinant Escherichia coli Whole‐Cell Catalyst Expressing Hydroxynitrile Lyase and Nitrilase Activities in Ionic Liquids for the Production of ( S )‐Mandelic Acid and ( S )‐Mandeloamide

Nitrilase: a promising biocatalyst in industrial applications for green chemistry

Nitrilases are widely distributed in nature and are able to hydrolyze nitriles into their corresponding carboxylic acids and ammonia. In industry, nitrilases have been used as green biocatalysts for the production of high value-added products. To date, biocatalysts are considered to be important alternatives to chemical catalysts due to increasing environmental problems and resource scarcity. This review provides an overview of recent advances of nitrilases in aspects of distribution, enzyme screening, molecular structure and catalytic mechanism, protein engineering, and their potential applications in industry.

Ionic liquids in whole-cell biocatalysis: a compromise between toxicity and efficiency

Comparison of chemical catalysis by metal complexes, enzymatic catalysis and whole-cell biocatalysis shows well-addressed advantages of the latter approach. However, a critical limitation in the practical applications originates from the high sensitivity of microorganisms to the toxic effects of organic solvents. In the present review, we consider toxic solvent properties of ionic liquid/water systems towards the development of efficient applications in practical organic transformations.

Whole-Cell Biocatalysis in Ionic Liquids

The use of whole-cell biocatalysis in ionic liquid (IL)-containing systems has attracted increasing attention in recent years. Compared to bioreactions catalyzed by isolated enzymes, the major advantage of using whole cells in biocatalytic processes is that the cells provide a natural intracellular environment for the enzymes to function with in situ cofactor regeneration. To date, the applications of whole-cell biocatalysis in IL-containing systems have focused on the production of valuable compounds, mainly through reduction, oxidation, hydrolysis, and transesterification reactions. The interaction mechanisms between the ILs and biocatalysts in whole-cell biocatalysis offer the possibility to effectively integrate ILs with biotransformation. This chapter discusses these interaction mechanisms between ILs and whole-cell catalysts. In addition, examples of whole-cell catalyzed reactions with ILs will also be discussed. Graphical Abstract.

Cascade catalysis – strategies and challenges en route to preparative synthetic biology

In this feature article recent progress and future perspectives of cascade catalysis combining bio/bio or bio/chemo catalysts are presented.

Applications and mechanisms of ionic liquids in whole-cell biotransformation

Ionic liquids (ILs), entirely composed of cations and anions, are liquid solvents at room temperature. They are interesting due to their low vapor pressure, high polarity and thermostability, and also for the possibility to fine-tune their physicochemical properties through modification of the chemical structures of their cations or anions. In recent years, ILs have been widely used in biotechnological fields involving whole-cell biotransformations of biodiesel or biomass, and organic compound synthesis with cells. Research studies in these fields have increased from the past decades and compared to the typical solvents, ILs are the most promising alternative solvents for cell biotransformations. However, there are increasing limitations and new challenges in whole-cell biotransformations with ILs. There is little understanding of the mechanisms of ILs' interactions with cells, and much remains to be clarified. Further investigations are required to overcome the drawbacks of their applications and to broaden their application spectrum. This work mainly reviews the applications of ILs in whole-cell biotransformations, and the possible mechanisms of ILs in microbial cell biotransformation are proposed and discussed.

Immobilisation of hydroxynitrile lyases

Hydroxynitrile lyases are a versatile group of enzymes that are applied both in the laboratory and on an industrial scale. What makes them particularly interesting is that to date five structurally unrelated categories of hydroxynitrile lyases have been discovered. Given their great importance they have often been immobilised utilising many different methodologies. Therefore the hydroxynitrile lyases are ideally suited to compare different immobilisation methods and their dependence on the structural features of the enzyme in question, since the activity is the same in all cases. This review examines all the different immobilisation methods applied to hydroxynitrile lyases and draws conclusions on the effect of the approach.

Enantioselective synthesis of α-oxy amides via Umpolung amide synthesis

α-Oxy amides are prepared through enantioselective synthesis using a sequence beginning with a Henry addition of bromonitromethane to aldehydes and finishing with Umpolung Amide Synthesis (UmAS). Key to high enantioselection is the finding that ortho-iodo benzoic acid salts of the chiral copper(II) bis(oxazoline) catalyst deliver both diastereomers of the Henry adduct with high enantiomeric excess, homochiral at the oxygen-bearing carbon. Overall, this approach to α-oxy amides provides an innovative complement to alternatives that focus almost entirely on the enantioselective synthesis of α-oxy carboxylic acids.

Efficient anti-Prelog enantioselective reduction of acetyltrimethylsilane to (R)-1-trimethylsilylethanol by immobilized Candida parapsilosis CCTCC M203011 cells in ionic liquid-based biphasic systems

BACKGROUND

Biocatalytic asymmetric reductions with whole cells can offer high enantioselectivity, environmentally benign processes and energy-effective operations and thus are of great interest. The application of whole cell-mediated bioreduction is often restricted if substrate and product have low water solubility and/or high toxicity to the biocatalyst. Many studies have shown that a biphasic system is often useful in this instance. Hence, we developed efficient biphasic reaction systems with biocompatible water-immiscible ionic liquids (ILs), to improve the biocatalytic anti-Prelog enantioselective reduction of acetyltrimethylsilane (ATMS) to (R)-1-trimethylsilylethanol {(R)-1-TMSE}, which is key synthon for a large number of silicon-containing drugs, using immobilized Candida parapsilosis CCTCC M203011 cells as the biocatalyst.

RESULTS

It was found that the substrate ATMS and the product 1-TMSE exerted pronounced toxicity to immobilized Candida parapsilosis CCTCC M203011 cells. The biocompatible water-immiscible ILs can be applied as a substrate reservoir and in situ extractant for the product, thus greatly enhancing the efficiency of the biocatalytic process and the operational stability of the cells as compared to the IL-free aqueous system. Various ILs exerted significant but different effects on the bioreduction and the performances of biocatalysts were closely related to the kinds and combination of cation and anion of ILs. Among all the water-immiscible ILs investigated, the best results were observed in 1-butyl-3-methylimidazolium hexafluorophosphate (C(4)mim·PF(6))/buffer biphasic system. Furthermore, it was shown that the optimum substrate concentration, volume ratio of buffer to IL, buffer pH, reaction temperature and shaking rate for the bioreduction were 120 mM, 8/1 (v/v), 6.0, 30°C and 180 r/min, respectively. Under these optimized conditions, the initial reaction rate, the maximum yield and the product e.e. were 8.1 μmol/min g(cwm), 98.6% and >99%, respectively. The efficient whole-cell biocatalytic process was shown to be feasible on a 450-mL scale. Moreover, the immobilized cells remained around 87% of their initial activity even after being used repeatedly for 8 batches in the C(4)mim·PF(6)/buffer biphasic system, exhibiting excellent operational stability.

CONCLUSIONS

For the first time, we have successfully utilized immobilized Candida parapsilosis CCTCC M203011 cells, for efficiently catalyzing anti-Prelog enantioselective reduction of ATMS to enantiopure (R)-1-TMSE in the C(4)mim·PF(6)/buffer biphasic system. The substantially improved biocatalytic process appears to be effective and competitive on a preparative scale.

Efficient asymmetric reduction of 4-(trimethylsilyl)-3-butyn-2-one by Candida parapsilosis cells in an ionic liquid-containing system

Hydrophilic ionic liquids (ILs) were employed as green solvents to construct an IL-containing co-solvent system for improving the asymmetric reduction of 4-(trimethylsilyl)-3-butyn-2-one by immobilized Candida parapsilosis cells. Among 14 hydrophilic ILs examined, 1-(2'-hydroxyl)ethyl-3-methylimidazolium nitrate (C(2)OHMIM·NO(3)) was considered as the most suitable IL for the bioreduction with the fastest initial reaction rate, the highest yield and the highest product e.e., which may be due to the good biocompatibility with the cells. For a better understanding of the bioreduction performed in the C(2)OHMIM·NO(3)-containing co-solvent system, the effects of several crucial variables were systematically investigated. The optimal C(2)OHMIM·NO(3) content, substrate concentration, buffer pH, co-substrate concentration and temperature were 10% (v/v), 3.0 mmol/L, 5.0, 98.1 mmol/L and 30°C, respectively. Under the optimal conditions, the initial reaction rate, the maximum yield and the product e.e. were 17.3 µmol/h g(cell), 95.2% and >99.9%, respectively, which are much better than the corresponding results previously reported. Moreover, the immobilized cells remained more than 83% of their initial activity even after being used repeatedly for 10 batches in the C(2)OHMIM·NO(3)-containing system, exhibiting excellent operational stability.