Highly efficient Michael-type addition of acetaldehyde to β-nitrostyrenes by whole resting cells of Escherichia coli expressing 4-oxalocrotonate tautomerase

Enhancing polyethylene degradation: a novel bioprocess approach using Acinetobacter nosocomialis pseudo-resting cells

Despite the discovery of several bacteria capable of interacting with polymers, the activity of the natural bacterial isolates is limited. Furthermore, there is a lack of knowledge regarding the development of bioprocesses for polyethylene (PE) degradation. Here, we report a bioprocess using pseudo-resting cells for efficient degradation of PE. The bacterial strain Acinetobacter nosocomialis was isolated from PE-containing landfills and characterized using low-density PE (LDPE) surface oxidation when incubated with LDPE. We optimized culture conditions to generate catalytic pseudo-resting cells of A. nosocomialis that are capable of degrading LDPE films in a bioreactor. After 28 days of bioreactor operation using pseudo-resting cells of A. nosocomialis, we observed the formation of holes on the PE film (39 holes per 217 cm2, a maximum diameter of 1440 μm). This study highlights the potential of bacteria as biocatalysts for the development of PE degradation processes. KEY POINTS: • New bioprocess has been proposed to degrade polyethylene (PE). • Process with pseudo-resting cells results in the formation of holes in PE film. • We demonstrated PE degradation using A. nosocomialis as a biocatalyst.

Formaldehyde reacts with N-terminal proline residues to give bicyclic aminals

Formaldehyde (HCHO) is a potent electrophile that is toxic above threshold levels, but which is also produced in the nuclei of eukaryotic cells by demethylases. We report studies with the four canonical human histones revealing that histone H2B reacts with HCHO, including as generated by a histone demethylase, to give a stable product. NMR studies show that HCHO reacts with the N-terminal proline and associated amide of H2B to give a 5,5-bicyclic aminal that is relatively stable to competition with HCHO scavengers. While the roles of histone modification by this reaction require further investigation, we demonstrated the potential of N-terminal aminal formation to modulate protein function by conducting biochemical and cellular studies on the effects of HCHO on catalysis by 4-oxalocrotonate tautomerase, which employs a nucleophilic N-terminal proline. The results suggest that reactions of N-terminal residues with HCHO and other aldehydes have potential to alter protein function.

Enabling protein-hosted organocatalytic transformations

In this review, the development of organocatalytic artificial enzymes will be discussed. This area of protein engineering research has underlying importance, as it enhances the biocompatibility of organocatalysis for applications in chemical and synthetic biology research whilst expanding the catalytic repertoire of enzymes. The approaches towards the preparation of organocatalytic artificial enzymes, techniques used to improve their performance (selectivity and reactivity) as well as examples of their applications are presented. Challenges and opportunities are also discussed.

High-efficiency and low-cost production of cadaverine from a permeabilized-cell bioconversion by a Lysine-induced engineered Escherichia coli

Cadaverine is the monomer of bio-based nylons polyamide 5.4, 5.6 and 5.10. In this study, a litre-scale integrated strategy was developed for high-efficiency and low-cost production of cadaverine using an engineered Escherichia coli. Firstly, the engineered strain BL21-P_cad-CadA induced by cheap l-lysine-HCl instead of IPTG was constructed. Then the permeabilized cells were served as the biocatalyst for the production of cadaverine, because the enhanced permeability facilitated the mass transfer of the substrate and the release of products. After the replacement of industrial materials and the solution of the scale-up permeabilization process, cadaverine concentration reached 205 g/L with the yield of 92.1% after 20 h in a 2 L bioconversion system, achieving the level of industrial production. Furthermore, the costs of industrial materials for 2 L integrated strategy ($2.78) was only 1/11 of the lab reagents ($30.88). Therefore, the proposed strategy is a promising candidate for the industrial process of cadaverine.

Substituting the catalytic proline of 4-oxalocrotonate tautomerase with non-canonical analogues reveals a finely tuned catalytic system

The enzyme 4-oxalocrotonate tautomerase shows remarkable catalytic versatility due to the secondary amine of its N-terminal proline moiety. In this work, we incorporated a range of proline analogues into the enzyme and examined the effects on structure and activity. While the structure of the enzyme remained unperturbed, its promiscuous Michael-type activity was severely affected. This finding demonstrates how atomic changes in a biocatalytic system can abolish its activity. Our work provides a toolbox for successful generation of enzyme variants with non-canonical catalytic proline analogues.

Metabolic engineering of cofactor flavin adenine dinucleotide (FAD) synthesis and regeneration in Escherichia coli for production of α-keto acids

Cofactor flavin adenine dinucleotide (FAD) plays a vital role in many FAD-dependent enzymatic reactions; therefore, how to efficiently accelerate FAD synthesis and regeneration is an important topic in biocatalysis and metabolic engineering. In this study, a system involving the synthesis pathway and regeneration of FAD was engineered in Escherichia coli to improve α-keto acid production-from the corresponding l-amino acids-catalyzed by FAD-dependent l-amino acid deaminase (l-AAD). First, key genes, ribH, ribC, and ribF, were overexpressed and fine-tuned for FAD synthesis. In the resulting E. coli strain PHCF7, strong overexpression of pma, ribC, and ribF and moderate overexpression of ribH yielded a 90% increase in phenylpyruvic acid (PPA) titer: 19.4 ± 1.1 g · L^-1 . Next, formate dehydrogenase (FDH) and NADH oxidase (NOX) were overexpressed to strengthen the regeneration rate of cofactors FADH₂ /FAD using FDH for FADH₂ /FAD regeneration and NOX for NAD⁺ /NADH regeneration. The resulting E. coli strain PHCF7-FDH-NOX yielded the highest PPA production: 31.4 ± 1.1 g · L^-1 . Finally, this whole-cell system was adapted to production of other α-keto acids including α-ketoglutaric acid, α-ketoisocaproate, and keto-γ-methylthiobutyric acid to demonstrate the broad utility of strengthening of FAD synthesis and FADH₂ /FAD regeneration for production of α-keto acids. Notably, the strategy reported herein may be generally applicable to other flavin-dependent biocatalysis reactions and metabolic pathway optimizations. Biotechnol. Bioeng. 2017;114: 1928-1936. © 2017 Wiley Periodicals, Inc.

Building Bridges: Biocatalytic C-C-Bond Formation toward Multifunctional Products

Carbon-carbon bond formation is the key reaction for organic synthesis to construct the carbon framework of organic molecules. The review gives a selection of biocatalytic C-C-bond-forming reactions which have been investigated during the last 5 years and which have already been proven to be applicable for organic synthesis. In most cases, the reactions lead to products functionalized at the site of C-C-bond formation (e.g., α-hydroxy ketones, aminoalcohols, diols, 1,4-diketones, etc.) or allow to decorate aromatic and heteroaromatic molecules. Furthermore, examples for cyclization of (non)natural precursors leading to saturated carbocycles are given as well as the stereoselective cyclopropanation of olefins affording cyclopropanes. Although many tools are already available, recent research also makes it clear that nature provides an even broader set of enzymes to perform specific C-C coupling reactions. The possibilities are without limit; however, a big library of variants for different types of reactions is required to have the specific enzyme for a desired specific (stereoselective) reaction at hand.

Enzyme-catalyzed domino reaction: efficient construction of spirocyclic oxindole skeleton using porcine pepsin

Pepsin from porcine gastric mucosa was used as a catalyst in the domino Knoevenagel/Michael/Michael reaction for the synthesis of spirooxindoles.

Chemomimetic biocatalysis: exploiting the synthetic potential of cofactor-dependent enzymes to create new catalysts

Despite the astonishing breadth of enzymes in nature, no enzymes are known for many of the valuable catalytic transformations discovered by chemists. Recent work in enzyme design and evolution, however, gives us good reason to think that this will change. We describe a chemomimetic biocatalysis approach that draws from small-molecule catalysis and synthetic chemistry, enzymology, and molecular evolution to discover or create enzymes with non-natural reactivities. We illustrate how cofactor-dependent enzymes can be exploited to promote reactions first established with related chemical catalysts. The cofactors can be biological, or they can be non-biological to further expand catalytic possibilities. The ability of enzymes to amplify and precisely control the reactivity of their cofactors together with the ability to optimize non-natural reactivity by directed evolution promises to yield exceptional catalysts for challenging transformations that have no biological counterparts.

Recent developments in enzyme promiscuity for carbon-carbon bond-forming reactions

Numerous enzymes have been found to catalyze additional and completely different types of reactions relative to the natural activity they evolved for. This phenomenon, called catalytic promiscuity, has proven to be a fruitful guide for the development of novel biocatalysts for organic synthesis purposes. As such, enzymes have been identified with promiscuous catalytic activity for, one or more, eminent types of carbon-carbon bond-forming reactions like aldol couplings, Michael(-type) additions, Mannich reactions, Henry reactions, and Knoevenagel condensations. This review focuses on enzymes that promiscuously catalyze these reaction types and exhibit high enantioselectivities (in case chiral products are obtained).

Hydrolase-catalyzed asymmetric carbon–carbon bond formation in organic synthesis

This article reviews the hydrolase-catalyzed asymmetric carbon–carbon bond-forming reactions for the preparation of enantiomerically enriched compounds in organic synthesis.

Chemoselective biocatalytic reduction of conjugated nitroalkenes: new application for an Escherichia coli BL21(DE3) expression strain

Chemoselective reduction of activated carbon-carbon double bond in conjugated nitroalkenes was achieved using Escherichia coli BL21(DE3) whole cells. Nine different substrates have been used furnishing the reduced products in moderate to good yields. 1-Nitro-4-phenyl-1,3-butadiene and (2-nitro-1-propenyl)benzene were successfully biotransformed with corresponding product yields of 54% and 45% respectively. Using this simple and environmentally friendly system 2-(2-nitropropyl)pyridine and 2-(2-nitropropyl)naphthalene were synthesized and characterized for the first time. High substrate conversion efficiency was coupled with low enantioselectivity, however 29% enantiomeric excess was detected in the case of 2-(2-nitropropyl)pyridine. It was shown that electronic properties of the aromatic ring, which affected polarity of the double bond, were not highly influential factors in the reduction process, but the presence of the nitro functionality was essential for the reaction to proceed. 1-Phenyl-4-nitro-1,3-butadiene could not be biotransformed by whole cells of Pseudomonas putida KT2440 or Bacillus subtilis 168 while it was successfully reduced by E. coli DH5α but with lower efficiency in comparison to E. coli BL21(DE3). Knockout mutant affected in nemA gene coding for N-ethylmaleimide reductase (BL21ΔnemA) could still catalyze bioreductions suggesting multiple active reductases within E. coli BL21(DE3) biocatalyst. The described biocatalytic reduction of substituted nitroalkenes provides an efficient route for the preparation of the corresponding nitroalkanes and introduces the new application of the strain traditionally utilized for recombinant protein expression.

Synthesis of γ-nitroaldehydes containing quaternary carbon in the α-position using a 4-oxalocrotonate tautomerase whole-cell biocatalyst

Synthetically valuable quaternary carbon containing γ-nitroaldehydes were obtained from branched chain aldehydes and a range of α,β-unsaturated nitroalkenes by a whole-cell biocatalytic reaction using 4-oxalocrotonate tautomerase as catalyst.