Guidelines and Cost Analysis for Catalyst Production in Biocatalytic Processes

Stability engineering of the Geobacillus stearothermophilus alcohol dehydrogenase and application for the synthesis of a polyamide 12 precursor

The thermostable NAD(+)-dependent alcohol dehydrogenase from Geobacillus stearothermophilus (BsADH) was exploited with regard to the biocatalytic synthesis of ω-oxo lauric acid methyl ester (OLAMe), a key intermediate for biobased polyamide 12 production, from the corresponding long-chain alcohol. Recombinant BsADH was produced in Escherichia coli as a homogeneous tetrameric enzyme and showed high activity towards the industrially relevant substrate ω-hydroxy lauric acid methyl ester (HLAMe) with K M = 86 μM and 44 U mg(-1). The equilibrium constant for HLAMe oxidation to the aldehyde (OLAMe) with NAD(+) was determined as 2.16 × 10(-3) from the kinetic parameters of the BsADH-catalyzed forward and reverse reactions. Since BsADH displayed limited stability under oxidizing conditions, the predominant oxidation-prone residue Cys257 was mutated to Leu based on sequence homology with related enzymes and computational simulation. This substitution resulted in an improved BsADH variant exhibiting prolonged stability and an elevated inactivation temperature. Semi-preparative biocatalysis at 60 °C using the stabilized enzyme, employing butyraldehyde for in situ cofactor regeneration with only catalytic amounts of NAD(+), yielded up to 23 % conversion of HLAMe to OLAMe after 30 min. In contrast to other oxidoreductases, no overoxidation to the dodecanoic diacid monomethyl ester was detected. Thus, the mutated BsADH offers a promising biocatalyst for the selective oxidation of fatty alcohols to yield intermediates for industrial polymer production.

Multi-step biocatalytic strategies for chiral amino alcohol synthesis

Chiral amino alcohols are structural motifs present in sphingolipids, antibiotics, and antiviral glycosidase inhibitors. Their chemical synthesis presents several challenges in establishing at least two chiral centres. Here a de novo metabolic pathway using a transketolase enzyme coupled with a transaminase enzyme has been assembled. To synthesise this motif one of the strategies to obtain high conversions from the transaminase/transketolase cascade is the use of hydroxypyruvate (HPA) as a two-carbon donor for the transketolase reaction; although commercially available it is relatively expensive limiting application of the pathway on an industrial scale. Alternately, HPA can be synthesised but this introduces a further synthetic step. In this study two different biocatalytic strategies were developed for the synthesis of (2S,3R)-2-amino-1,3,4-butanetriol (ABT) without adding HPA into the reaction. Firstly, a sequential cascade of three enzymatic steps (two transaminases and one transketolase) for the synthesis of ABT from serine, pyruvate and glycolaldehyde as substrates. Secondly, a two-step recycling cascade where serine is used as donor to aminate erythrulose (catalysed by a transketolase) for the simultaneous synthesis of ABT and HPA. In order to test the novel pathways, three new transaminases are described, two ω-transaminases able to accept a broad range of amine acceptors with serine as amine donor; and an α-transaminase, which showed high affinity towards serine (KM: 18mM) using pyruvate as amine acceptor. After implementation of the above enzymes in the biocatalytic pathways proposed in this paper, the two-step recycling pathway was found to be the most promising for its integration with E. coli metabolism. It was more efficient (10-fold higher conversion), more sustainable and cost-effective (use of low cost natural substrates and only two enzymes), and the reaction could be performed in a one-pot system.

The taming of oxygen: biocatalytic oxyfunctionalisations

The scope and limitations of oxygenases as catalysts for preparative organic synthesis is discussed.

Purification and simultaneous immobilization of Arabidopsis thaliana hydroxynitrile lyase using a family 2 carbohydrate-binding module

Tedious, time- and labor-intensive protein purification and immobilization procedures still represent a major bottleneck limiting the widespread application of enzymes in synthetic chemistry and industry. We here exemplify a simple strategy for the direct site-specific immobilization of proteins from crude cell extracts by fusion of a family 2 carbohydrate-binding module (CBM) derived from the exoglucanase/xylanase Cex from Cellulomonas fimi to a target enzyme. By employing a tripartite fusion protein consisting of the CBM, a flavin-based fluorescent protein (FbFP), and the Arabidopsis thaliana hydroxynitrile lyase (AtHNL), binding to cellulosic carrier materials can easily be monitored via FbFP fluorescence. Adsorption properties (kinetics and quantities) were studied for commercially available Avicel PH-101 and regenerated amorphous cellulose (RAC) derived from Avicel. The resulting immobilizates showed similar activities as the wild-type enzyme but displayed increased stability in the weakly acidic pH range. Finally, Avicel, RAC and cellulose acetate (CA) preparations were used for the synthesis of (R)-mandelonitrile in micro-aqueous methyl tert-butyl ether (MTBE) demonstrating the applicability and stability of the immobilizates for biotransformations in both aqueous and organic reaction systems.

High-yield hydrogen production from biomass by in vitro metabolic engineering: Mixed sugars coutilization and kinetic modeling

The use of hydrogen (H2) as a fuel offers enhanced energy conversion efficiency and tremendous potential to decrease greenhouse gas emissions, but producing it in a distributed, carbon-neutral, low-cost manner requires new technologies. Herein we demonstrate the complete conversion of glucose and xylose from plant biomass to H2 and CO2 based on an in vitro synthetic enzymatic pathway. Glucose and xylose were simultaneously converted to H2 with a yield of two H2 per carbon, the maximum possible yield. Parameters of a nonlinear kinetic model were fitted with experimental data using a genetic algorithm, and a global sensitivity analysis was used to identify the enzymes that have the greatest impact on reaction rate and yield. After optimizing enzyme loadings using this model, volumetric H2 productivity was increased 3-fold to 32 mmol H2⋅L(-1)⋅h(-1). The productivity was further enhanced to 54 mmol H2⋅L(-1)⋅h(-1) by increasing reaction temperature, substrate, and enzyme concentrations--an increase of 67-fold compared with the initial studies using this method. The production of hydrogen from locally produced biomass is a promising means to achieve global green energy production.

Microscale technology and biocatalytic processes: opportunities and challenges for synthesis

Despite the expanding presence of microscale technology in chemical synthesis and energy production as well as in biomedical devices and analytical and diagnostic tools, its potential in biocatalytic processes for pharmaceutical and fine chemicals, as well as related industries, has not yet been fully exploited. The aim of this review is to shed light on the strategic advantages of this promising technology for the development and realization of biocatalytic processes and subsequent product recovery steps, demonstrated with examples from the literature. Constraints, opportunities, and the future outlook for the implementation of these key green engineering methods and the role of supporting tools such as mathematical models to establish sustainable production processes are discussed.

Guidelines for development and implementation of biocatalytic P450 processes

Biocatalytic reactions performed by cytochrome P450 monooxygenases are interesting in pharmaceutical research since they are involved in human drug metabolism. Furthermore, they are potentially interesting as biocatalysts for synthetic chemistry because of the exquisite selectivity of the chemistry they undertake. For example, selective hydroxylation can be undertaken on a highly functionalized molecule without the need for functional group protection. Recent progress in the discovery of novel P450s as well as protein engineering of these enzymes strongly encourages further development of their application, including use in synthetic processes. The biological characteristics of P450s (e.g., cofactor dependence) motivate the use of whole-cell systems for synthetic processes, and those processes implemented in industry are so far dominated by growing cells and native host systems. However, for an economically feasible process, the expression of P450 systems in a heterologous host with sufficient biocatalyst yield (g/g cdw) for non-growing systems or space-time yield (g/L/h) for growing systems remains a major challenge. This review summarizes the opportunities to improve P450 whole-cell processes and strategies in order to apply and implement them in industrial processes, both from a biological and process perspective. Indeed, a combined approach of host selection and cell engineering, integrated with process engineering, is suggested as the most effective route to implementation.

Characterization, performance, and applications of a yeast surface display-based biocatalyst

Yeast surface display (YSD) of two lipases. Measured expression level and copy number. Synthetic and hydrolytic activity comparable to commercial lipase. Cost analysis of YSD system vs. commercial formulations.

Specific oxyfunctionalisations catalysed by peroxygenases: opportunities, challenges and solutions

Peroxygenases are promising oxyfunctionalisation catalysts for organic synthesis.

Expanding the toolbox of organic chemists: directed evolution of P450 monooxygenases as catalysts in regio- and stereoselective oxidative hydroxylation

Cytochrome P450 enzymes (CYPs) have been used for more than six decades as catalysts for the CH-activating oxidative hydroxylation of organic compounds with formation of added-value products.

Highly efficient enzymatic biodiesel production promoted by particle-induced emulsification

BACKGROUND

At present, the conversion of oils to biodiesel is predominantly carried out using chemical catalysts. However, the corresponding lipase-catalysed process has important advantages, which include mild reaction conditions and the possibility of using cheap, low quality feedstocks with a high free fatty acid content. Further increases in the efficiency of the enzymatic process are desired to make it even more attractive and suitable for large-scale applications.

RESULTS

Herein, we present a simple and efficient two-phase lipase-catalysed system for the preparation of biodiesel in which different parameters (biocatalyst composition, ethanol concentration and the presence of additives) were optimised in order to obtain the maximum productivity starting from triolein with a high free oleic acid content. In the two-phase system, the enzyme tolerated high-ethanol concentrations, which made it possible to reach high conversions. The addition of silica particles increased the reaction rate substantially. It was suggested that such particles can catalyse acyl migration as a step to the full conversion to glycerol and biodiesel. However, in the system studied here, the effect of the particles was shown to be due to the formation of smaller and more uniform emulsion droplets leading to better mass transfer between the two phases. Particles of widely different size had positive effects, and the highest rate was obtained with silica particles derivatised with phenyl groups. The optimal conditions were applied to the solvent-free ethanolysis of rapeseed oil, and a yield of 96% was reached in 5 h. Under the mild conditions used, chemical catalysts were inefficient.

CONCLUSIONS

Triacylglycerol oils with a high free fatty acid content can be efficiently converted to ethyl esters using Thermomyces lanuginosus lipase as the catalyst in an aqueous/organic two-phase system. Fast mass transfer can be achieved using silica particles, which helped to decrease the size of the emulsion droplets and thus led to a more efficient process. The high-ethanol concentration tolerated by the lipase in this system made it possible to reach almost quantitative yields.

Engineered baker's yeast as whole-cell biocatalyst for one-pot stereo-selective conversion of amines to alcohols

Background

One-pot multi-step biocatalysis is advantageous over step-by-step synthesis as it reduces the number of process operation units, leading to significant process intensification. Whole-cell biocatalysis with metabolically active cells is especially valuable since all enzymes can be co-expressed in the cell whose metabolism can be exploited for supply of co-substrates and co-factors.

Results

In this study, a heterologous enzymatic system consisting of ω-transaminase and ketone reductase was introduced in Saccharomyces cerevisiae, and evaluated for one-pot stereo-selective conversion of amines to alcohols. The system was applied for simultaneous kinetic resolution of racemic 1-phenylethylamine to (R)-1-phenylethylamine and reduction of the ketone intermediate to (R)-1-phenylethanol. Glucose was used as sole co-substrate for both the supply of amine acceptor and the regeneration of NADPH in the reduction step.

Conclusions

The whole-cell biocatalyst was shown to sustain transaminase-reductase-catalyzed enantioselective conversion of amines to alcohols with glucose as co-substrate. The transamination catalyzed by recombinant vanillin aminotransferase from Capsicum chinense proved to be the rate-limiting step as a three-fold increase in transaminase gene copy number led to a two-fold increased conversion. The (R)-selective NADPH-dependent alcohol dehydrogenase from Lactobacillus kefir proved to be efficient in catalyzing the reduction of the acetophenone generated in the transamination reaction.

Enantioselective, continuous (R)- and (S)-2-butanol synthesis: achieving high space-time yields with recombinant E. coli cells in a micro-aqueous, solvent-free reaction system

The stereoselective production of (R)- or (S)-2-butanol is highly challenging. A potent synthesis strategy is the biocatalytic asymmetric reduction of 2-butanone applying alcohol dehydrogenases. However, due to a time-dependent racemisation process, high stereoselectivity is only obtained at incomplete conversion after short reaction times. Here, we present a solution to this problem: by using a continuous process, high biocatalytic selectivity can be achieved while racemisation is suppressed successfully. Furthermore, high conversion was achieved by applying recombinant, lyophilised E. coli cells hosting Lactobacillus brevis alcohol dehydrogenase in a micro-aqueous solvent-free continuous reaction system. The optimisation of residence time (τ) and 2-butanone concentration boosted both conversion (>99%) and enantiomeric excess (ee) of (R)-2-butanol (>96%). When a residence time of only τ=3.1 min was applied, productivity was extraordinary with a space-time yield of 2278±29g/(L×d), thus exceeding the highest values reported to date by a factor of more than eight. The use of E. coli cells overexpressing an ADH of complementary stereoselectivity yielded a synthesis strategy for (S)-2-butanol with an excellent ee (>98%). Although conversion was only moderate (up to 46%), excellent space-time yields of up to 461g/(L×d) were achieved. The investigated concept represents a synthesis strategy that can also be applied to other biocatalytic processes where racemisation poses a challenge.

Recent achievements in developing the biocatalytic toolbox for chiral amine synthesis

Novel enzyme activities and chemoenzymatic reaction concepts have considerably expanded the biocatalytic toolbox for chiral amine synthesis. Creating new activities or extending the scope of existing enzymes by protein engineering is a common trend in biocatalysis and in chiral amine synthesis specifically. For instance, an amine dehydrogenase that allows for the direct asymmetric amination of ketones with ammonia was created by mutagenesis of an l-amino acid dehydrogenase. Another trend in chiral amine chemistry is the development of strategies allowing for the synthesis of secondary amines. For example the smart choice of substrates for amine transaminases provided access to secondary amines by chemoenzymatic reactions. Furthermore novel biocatalysts for the synthesis of secondary amines such as imine reductases and Pictet-Spenglerases have been identified and applied. Recent examples showed that the biocatalytic amine synthesis is emerging from simple model reactions towards industrial scale preparation of pharmaceutical relevant substances, for instance, as shown in the synthesis of a Janus kinase 2 inhibitor using an amine transaminase. A comparison of important process parameters such as turnover number and space-time yield demonstrates that biocatalytic strategies for asymmetric reductive amination are maturing and can already compete with established chemical methods.

Combining phospholipases and a liquid lipase for one-step biodiesel production using crude oils

BACKGROUND

Enzymatic biodiesel is becoming an increasingly popular topic in bioenergy literature because of its potential to overcome the problems posed by chemical processes. However, the high cost of the enzymatic process still remains the main drawback for its industrial application, mostly because of the high price of refined oils. Unfortunately, low cost substrates, such as crude soybean oil, often release a product that hardly accomplishes the final required biodiesel specifications and need an additional pretreatment for gums removal. In order to reduce costs and to make the enzymatic process more efficient, we developed an innovative system for enzymatic biodiesel production involving a combination of a lipase and two phospholipases. This allows performing the enzymatic degumming and transesterification in a single step, using crude soybean oil as feedstock, and converting part of the phospholipids into biodiesel. Since the two processes have never been studied together, an accurate analysis of the different reaction components and conditions was carried out.

RESULTS

Crude soybean oil, used as low cost feedstock, is characterized by a high content of phospholipids (900 ppm of phosphorus). However, after the combined activity of different phospholipases and liquid lipase Callera Trans L, a complete transformation into fatty acid methyl esters (FAMEs >95%) and a good reduction of phosphorus (P <5 ppm) was achieved. The combination of enzymes allowed avoidance of the acid treatment required for gums removal, the consequent caustic neutralization, and the high temperature commonly used in degumming systems, making the overall process more eco-friendly and with higher yield. Once the conditions were established, the process was also tested with different vegetable oils with variable phosphorus contents.

CONCLUSIONS

Use of liquid lipase Callera Trans L in biodiesel production can provide numerous and sustainable benefits. Besides reducing the costs derived from enzyme immobilization, the lipase can be used in combination with other enzymes such as phospholipases for gums removal, thus allowing the use of much cheaper, non-refined oils. The possibility to perform degumming and transesterification in a single tank involves a great efficiency increase in the new era of enzymatic biodiesel production at industrial scale.

Exploiting cell metabolism for biocatalytic whole-cell transamination by recombinant Saccharomyces cerevisiae

The potential of Saccharomyces cerevisiae for biocatalytic whole-cell transamination was investigated using the kinetic resolution of racemic 1-phenylethylamine (1-PEA) to (R)-1-PEA as a model reaction. As native yeast do not possess any ω-transaminase activity for the reaction, a recombinant yeast biocatalyst was constructed by overexpressing the gene coding for vanillin aminotransferase from Capsicum chinense. The yeast-based biocatalyst could use glucose as the sole co-substrate for the supply of amine acceptor via cell metabolism. In addition, the biocatalyst was functional without addition of the co-factor pyridoxal-5′-phosphate (PLP), which can be explained by a high inherent cellular capacity to sustain PLP-dependent reactions in living cells. In contrast, external PLP supplementation was required when cell viability was low, as it was the case when using pyruvate as a co-substrate. Overall, the results indicate a potential for engineered S. cerevisiae as a biocatalyst for whole-cell transamination and with glucose as the only co-substrate for the supply of amine acceptor and PLP.

Whole-cell biocatalysis for selective and productive C-O functional group introduction and modification

During the last decades, biocatalysis became of increasing importance for chemical and pharmaceutical industries. Regarding regio- and stereospecificity, enzymes have shown to be superior compared to traditional chemical synthesis approaches, especially in C-O functional group chemistry. Catalysts established on a process level are diverse and can be classified along a functional continuum starting with single-step biotransformations using isolated enzymes or microbial strains towards fermentative processes with recombinant microorganisms containing artificial synthetic pathways. The complex organization of respective enzymes combined with aspects such as cofactor dependency and low stability in isolated form often favors the use of whole cells over that of isolated enzymes. Based on an inventory of the large spectrum of biocatalytic C-O functional group chemistry, this review focuses on highlighting the potentials, limitations, and solutions offered by the application of self-regenerating microbial cells as biocatalysts. Different cellular functionalities are discussed in the light of their (possible) contribution to catalyst efficiency. The combined achievements in the areas of protein, genetic, metabolic, and reaction engineering enable the development of whole-cell biocatalysts as powerful tools in organic synthesis.

Microbial enzymes: tools for biotechnological processes

Microbial enzymes are of great importance in the development of industrial bioprocesses. Current applications are focused on many different markets including pulp and paper, leather, detergents and textiles, pharmaceuticals, chemical, food and beverages, biofuels, animal feed and personal care, among others. Today there is a need for new, improved or/and more versatile enzymes in order to develop more novel, sustainable and economically competitive production processes. Microbial diversity and modern molecular techniques, such as metagenomics and genomics, are being used to discover new microbial enzymes whose catalytic properties can be improved/modified by different strategies based on rational, semi-rational and random directed evolution. Most industrial enzymes are recombinant forms produced in bacteria and fungi.

Economic and environmental assessment of propionic acid production by fermentation using different renewable raw materials

Production of propionic acid by fermentation of glycerol as a renewable resource has been suggested as a means for developing an environmentally-friendly route for this commodity chemical. However, in order to quantify the environmental benefits, life cycle assessment of the production, including raw materials, fermentation, upstream and downstream processing is required. The economic viability of the process also needs to be analysed to make sure that any environmental savings can be realised. In this study an environmental and economic assessment from cradle-to-gate has been conducted. The study highlights the need for a highly efficient bioprocess in terms of product titre (more than 100g/L and productivity more than 2g/(L · h)) in order to be sustainable. The importance of the raw materials and energy production for operating the process to minimize emissions of greenhouse gases is also shown.

Biocatalysis in organic chemistry and biotechnology: past, present, and future

Enzymes as catalysts in synthetic organic chemistry gained importance in the latter half of the 20th century, but nevertheless suffered from two major limitations. First, many enzymes were not accessible in large enough quantities for practical applications. The advent of recombinant DNA technology changed this dramatically in the late 1970s. Second, many enzymes showed a narrow substrate scope, often poor stereo- and/or regioselectivity and/or insufficient stability under operating conditions. With the development of directed evolution beginning in the 1990s and continuing to the present day, all of these problems can be addressed and generally solved. The present Perspective focuses on these and other developments which have popularized enzymes as part of the toolkit of synthetic organic chemists and biotechnologists. Included is a discussion of the scope and limitation of cascade reactions using enzyme mixtures in vitro and of metabolic engineering of pathways in cells as factories for the production of simple compounds such as biofuels and complex natural products. Future trends and problems are also highlighted, as is the discussion concerning biocatalysis versus nonbiological catalysis in synthetic organic chemistry. This Perspective does not constitute a comprehensive review, and therefore the author apologizes to those researchers whose work is not specifically treated here.

Enzymatic biodiesel production: an overview of potential feedstocks and process development

The increased global demand for biofuels has prompted the search for alternatives to edible oils for biodiesel production. Given the abundance and cost, waste and nonedible oils have been investigated as potential feedstocks. A recent research interest is the conversion of such feedstocks into biodiesel via enzymatic processes, which have considerable advantages over conventional alkali-catalyzed processes. To expand the viability of enzymatic biodiesel production, considerable effort has been directed toward process development in terms of biodiesel productivity, application to wide ranges of contents of water and fatty acids, adding value to glycerol byproducts, and bioreactor design. A cost evaluation suggested that, with the current enzyme prices, the cost of catalysts alone is not competitive against that of alkalis. However, it can also be expected that further process optimization will lead to a reduced cost in enzyme preparation as well as in downstream processes.

Enzymatic transformation of nonfood biomass to starch

The global demand for food could double in another 40 y owing to growth in the population and food consumption per capita. To meet the world's future food and sustainability needs for biofuels and renewable materials, the production of starch-rich cereals and cellulose-rich bioenergy plants must grow substantially while minimizing agriculture's environmental footprint and conserving biodiversity. Here we demonstrate one-pot enzymatic conversion of pretreated biomass to starch through a nonnatural synthetic enzymatic pathway composed of endoglucanase, cellobiohydrolyase, cellobiose phosphorylase, and alpha-glucan phosphorylase originating from bacterial, fungal, and plant sources. A special polypeptide cap in potato alpha-glucan phosphorylase was essential to push a partially hydrolyzed intermediate of cellulose forward to the synthesis of amylose. Up to 30% of the anhydroglucose units in cellulose were converted to starch; the remaining cellulose was hydrolyzed to glucose suitable for ethanol production by yeast in the same bioreactor. Next-generation biorefineries based on simultaneous enzymatic biotransformation and microbial fermentation could address the food, biofuels, and environment trilemma.