Integration of whole-cell reaction and product isolation: Highly hydrophobic solvents promote in situ substrate supply and simplify extractive product isolation

Whole-cell bioconversion using non-Leloir transglycosylation reactions: a review

Microbial biocatalysts are evolving technological tools for glycosylation research in food, feed and pharmaceuticals. Advances in bioengineered Leloir and non-Leloir carbohydrate-active enzymes allow for whole-cell biocatalysts to curtail production costs of purified enzymes while enhancing glucan synthesis through continued enzyme expression. Unlike sugar nucleotide-dependent Leloir glycosyltransferases, non-Leloir enzymes require inexpensive sugar donors and can be designed to match the high value, yield and selectivity of the former. This review addresses the current state of bacterial cell-based production of glucans and glycoconjugates via transglycosylation, and describes how alterations made to microbial hosts to surpass purified enzymes as the preferred mode of catalysis are steadily being acquired through genetic engineering, rational design and process optimization. A comprehensive exploration of relevant literature has been summarized to describe whole-cell biocatalysis in non-Leloir glycosylation reactions with various donors and acceptors, and the characterization, application and latest developments in the optimization of their use.

Ensuring the Sustainability of Biocatalysis

Biocatalysis offers many attractive features for the synthetic chemist. In many cases, the high selectivity and ability to tailor specific enzyme features via protein engineering already make it the catalyst of choice. From the perspective of sustainability, several features such as catalysis under mild conditions and use of a renewable and biodegradable catalyst also look attractive. Nevertheless, to be sustainable at a larger scale it will be essential to develop processes operating at far higher concentrations of product, and which make better use of the enzyme via improved stability. In this Concept, it is argued that a particular emphasis on these specific metrics is of particular importance for the future implementation of biocatalysis in industry, at a level that fulfills its true potential.

Performance Evaluation of Bio-Based Fractions Derived from Bacillus spp. for Potential In Situ Soil Stabilisation

Current and future research focuses on the use of renewable technologies and materials to stabilise weak soils, of varying degrees, for road construction applications. Soil stabilisation is a method of strengthening a natural soil to meet this purpose. Our interest is in the use of bio-based components, derived from microbial growth processes, that contribute to the needed desirable strength characteristics for in situ soil stabilisation. This investigation focuses on novel Bacillus-based stabilisers obtained from the vegetative and spore growth stage. In this study, eighteen bio-based components were derived from a Bacillus licheniformis fermentation and extracted into various aqueous and non-aqueous fractions for strength property assessment. The strength properties of the treated soils (i.e., dolerite and weathered granite soil) were assessed via previously developed lab-scale equipment to rapidly pre-select the best performing fractions, (i.e., compression stress, erosion, abrasion, and water absorption tests). The effect of one superior performing prototype (a) was validated at large-scale, using standard erosion and abrasion tests (i.e., whole broth at 1.8% stabiliser concentrations), and showed resistance to abrasion (3.37 ± 0.03%) (p value ≤ 0.0001) and resistance to erosion (33.20 ± 0.15%) (p value ≤ 0.001). The elemental composition and microstructure of the bio-stabilised soil was determined using energy dispersive X-ray spectroscopy and scanning electron microscopy, respectively. This evaluation formed part of the selection of the best performing Bacillus derived fractions and achieved a proof of concept for the next phase of product prototype development. This study demonstrated a novel bio-mediated approach to the overall criteria for evaluation and selection of candidate product prototype/s, for stabilisation of two varying soils, and for potential application in road construction works.

Reductive enzymatic dynamic kinetic resolution affording 115 g/L (S)-2-phenylpropanol

Background

Published biocatalytic routes for accessing enantiopure 2-phenylpropanol using oxidoreductases afforded maximal product titers of only 80 mM. Enzyme deactivation was identified as the major limitation and was attributed to adduct formation of the aldehyde substrate with amino acid residues of the reductase.

Results

A single point mutant of Candida tenuis xylose reductase (CtXR D51A) with very high catalytic efficiency (43·10³ s⁻¹ M⁻¹) for (S)-2-phenylpropanal was found. The enzyme showed high enantioselectivity for the (S)-enantiomer but was deactivated by 0.5 mM substrate within 2 h. A whole-cell biocatalyst expressing the engineered reductase and a yeast formate dehydrogenase for NADH-recycling provided substantial stabilization of the reductase. The relatively slow in situ racemization of 2-phenylpropanal and the still limited biocatalyst stability required a subtle adjustment of the substrate-to-catalyst ratio. A value of 3.4 g_substrate/g_{cell-dry-weight} was selected as a suitable compromise between product ee and the conversion ratio. A catalyst loading of 40 g_{cell-dry-weight} was used to convert 1 M racemic 2-phenylpropanal into 843 mM (115 g/L) (S)-phenylpropanol with 93.1% ee.

Conclusion

The current industrial production of profenols mainly relies on hydrolases. The bioreduction route established here represents an alternative method for the production of profenols that is competitive with hydrolase-catalyzed kinetic resolutions.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12896-021-00715-5.

Advances in deep eutectic solvents and water: applications in metal- and biocatalyzed processes, in the synthesis of APIs, and other biologically active compounds

This review highlights recent advances in metal- and biocatalyzed transformations, in the synthesis of APIs and other biologically active compounds, when employing deep eutectic solvents and water as environmentally responsible solvents.

Pushing the limits: Cyclodextrin-based intensification of bioreductions

The asymmetric reduction of ketones is a frequently used synthesis route towards chiral alcohols. Amongst available chemo- and biocatalysts the latter stand out in terms of product enantiopurity. Their application is, however, restricted by low reaction output, often rooted in limited enzyme stability under operational conditions. Here, addition of 2-hydroxypropyl-β-cyclodextrin to bioreductions of o-chloroacetophenone enabled product concentrations of up to 29 % w/v at full conversion and 99.97 % e.e. The catalyst was an E. coli strain co-expressing NADH-dependent Candida tenuis xylose reductase and a yeast formate dehydrogenase for coenzyme recycling. Analysis of the lyophilized biocatalyst showed that E. coli cells were leaky with catalytic activity found as free-floating enzymes and associated with the biomass. The biocatalyst was stabilized and activated in the reaction mixture by 2-hydroxypropyl-β-cyclodextrin. Substitution of the wild-type xylose reductase by a D51A mutant further improved bioreductions. In previous optimization strategies, hexane was added as second phase to protect the labile catalyst from adverse effects of hydrophobic substrate and product. The addition of 2-hydroxypropyl-β-cyclodextrin (11 % w/v) instead of hexane (20 % v/v) increased the yield on biocatalyst 6.3-fold. A literature survey suggests that bioreduction enhancement by addition of cyclodextrins is not restricted to specific enzyme classes, catalyst forms or substrates.

Accelerating the implementation of biocatalysis in industry

Despite enormous progress in protein engineering, complemented by bioprocess engineering, the revolution awaiting the application of biocatalysis in the fine chemical industry has still not been fully realized. In order to achieve that, further research is required on several topics, including (1) rapid methods for protein engineering using machine learning, (2) mathematical modelling of multi-enzyme cascade processes, (3) process standardization, (4) continuous process technology, (5) methods to identify improvements required to achieve industrial implementation, (6) downstream processing, (7) enzyme stability modelling and prediction, as well as (8) new reactor technology. In this brief mini-review, the status of each of these topics will be briefly discussed.

Reactive Extraction of Lactic Acid, Formic Acid and Acetic Acid from Aqueous Solutions with Tri-n-octylamine/1-Octanol/n-Undecane

The present work develops the basics for the isolation of lactic acid, acetic acid and formic acid from a single as well as a mixed feed stream, as is present, for example, in fermentation broth for lactic acid production. Modelling of the phase equilibria data is performed using the law of mass action and shows that the acids are extracted according to their pka value, where formic acid is preferably extracted in comparison to lactic and acetic acid. Back-extraction was performed by 1 M NaHCO3 solution and shows the same tendency regarding the pka value. Based on lactic acid, the solvent phase composition, consisting of tri-n-octylamine/1-octanol/n-undecane, was optimized in terms of the distribution coefficient. The data clearly indicate that, compared to physical extraction, mass transfer can be massively enhanced by reactive extraction. With increasing tri-n-octylamine and 1-octanol concentration, the equilibrium constant increases. However, even when mass transfer increases, tri-n-octylamine concentrations above 40 wt%, lead to third phase formation, which needs to be prevented for technical application. The presented data are the basis for the transfer to liquid membrane permeation, which enables the handling of emulsion tending systems.