Occurrence and the possible physiological role of 2-enoate reductases

A molecular simulation study on amine-functionalized silica/polysulfone mixed matrix membrane for mixed gas separation

Polysulfone (PSF) based mixed matrix membranes (MMMs) are one of the most broadly studied polymeric materials used for CO₂/CH₄ separation. The performance of existing PSF membranes encounters a bottleneck for widespread expansion in industrial applications due to the trade-off amongst permeability and selectivity. Membrane performance has been postulated to be enhanced via functionalization of filler at different weight percentages. Nonetheless, the preparation of functionalized MMMs without defects and its empirical study that exhibits improved CO₂/CH₄ separation performance is challenging at an experimental scale that needs prior knowledge of the compatibility between the filler and polymer. Molecular simulation approaches can be used to explore the effect of functionalization on MMM's gas transport properties at an atomic level without the challenges in the experimental study, however, they have received less scrutiny to date. In addition, most of the research has focused on pure gas studies while mixed gas transport properties that reflect real separation in functionalized silica/PSF MMMs are scarcely available. In this work, a molecular simulation computational framework has been developed to investigate the structural, physical properties and gas transport behavior of amine-functionalized silica/PSF-based MMMs. The effect of varying weight percentages (i.e., 15-30 wt.%) of amine-functionalized silica and gas concentrations (i.e., 30% CH₄/CO₂, 50% CH₄/CO₂, and 70% CH₄/CO₂) on physical and gas transport characteristics in amine-functionalized silica/PSF MMMs at 308.15 K and 1 atm has been investigated. Functionalization of silica nanoparticles was found to increase the diffusion and solubility coefficients, leading to an increase in the percentage enhancement of permeability and selectivity for amine-functionalized silica/PSF MMM by 566% and 56%, respectively, compared to silica/PSF-based MMMs at optimal weight percentage of 20 wt.%. The model's permeability differed by 7.1% under mixed gas conditions. The findings of this study could help to improve real CO₂/CH₄ separation in the future design and concept of functionalized MMMs using molecular simulation and empirical modeling strategies.

A gut bacterial pathway metabolizes aromatic amino acids into nine circulating metabolites

The human gut microbiota produces dozens of metabolites that accumulate in the bloodstream, where they can have systemic effects on the host. Although these small molecules commonly reach concentrations similar to those achieved by pharmaceutical agents, remarkably little is known about the microbial metabolic pathways that produce them. Here we use a combination of genetics and metabolic profiling to characterize a pathway from the gut symbiont Clostridium sporogenes that generates aromatic amino acid metabolites. Our results reveal that this pathway produces twelve compounds, nine of which are known to accumulate in host serum. All three aromatic amino acids (tryptophan, phenylalanine and tyrosine) serve as substrates for the pathway, and it involves branching and alternative reductases for specific intermediates. By genetically manipulating C. sporogenes, we modulate serum levels of these metabolites in gnotobiotic mice, and show that in turn this affects intestinal permeability and systemic immunity. This work has the potential to provide the basis of a systematic effort to engineer the molecular output of the gut bacterial community.

Recombinant expression and characterisation of the oxygen-sensitive 2-enoate reductase from Clostridium sporogenes

‘Ene’-reductases have attracted significant attention for the preparation of chemical intermediates and biologically active products. To date, research has been focussed primarily on Old Yellow Enzyme-like proteins, due to their ease of handling, whereas 2-enoate reductases from clostridia have received much less attention, because of their oxygen sensitivity and a lack of suitable expression systems. A hypothetical 2-enoate reductase gene, fldZ, was identified in Clostridium sporogenes DSM 795. The encoded protein shares a high degree of homology to clostridial FMN- and FAD-dependent 2-enoate reductases, including the cinnamic acid reductase proposed to be involved in amino acid metabolism in proteolytic clostridia. The gene was cloned and overexpressed in Escherichia coli. Successful expression depended on the use of strictly anaerobic conditions for both growth and enzyme preparation, since FldZ was oxygen-sensitive. The enzyme reduced aromatic enoates, such as cinnamic acid or p-coumaric acid, but not short chain unsaturated aliphatic acids. The β,β-disubstituted nitroalkene, (E)-1-nitro-2-phenylpropene, was reduced to enantiopure (R)-1-nitro-2-phenylpropane with a yield of 90 %. By contrast, the α,β-disubstituted nitroalkene, (E)-2-nitro-1-phenylpropene, was reduced with a moderate yield of 56 % and poor enantioselectivity (16 % ee for (S)-2-nitro-1-phenylpropane). The availability of an expression system for this recombinant clostridial 2-enoate reductase will facilitate future characterisation of this unusual class of ‘ene’-reductases, and expand the biocatalytic toolbox available for enantioselective hydrogenation of carbon-carbon double bonds.

Biocatalytic Amide Reduction Using Clostridium sporogenes

Washed cells of Clostridium sporogenes reduced benzamide (up to 20 mM: ) to benzylamine in yields up to 73% using H2 as electron donor with less than 10 g biocatalyst/l over 24 h. Product formation exhibited complex kinetics, with a lag before benzylamine production began. Very little substrate was hydrolysed since the maximum yield of benzoic acid was only 9% of the substrate added. Boiled cells were inactivated thus confirming that amide reduction was enzyme-catalysed.

Enoate reductases of Clostridia. Cloning, sequencing, and expression

The enr genes specifying enoate reductases of Clostridium tyrobutyricum and Clostridium thermoaceticum were cloned and sequenced. Sequence comparison shows that enoate reductases are similar to a family of flavoproteins comprising 2,4-dienoyl-coenzyme A reductase from Escherichia coli and old yellow enzyme from yeast. The C. thermoaceticum enr gene product was expressed in recombinant Escherichia coli cells growing under anaerobic conditions. The recombinant enzyme was purified and characterized.

Anaerobic oxidation of p-cresol mediated by a partially purified methylhydroxylase from a denitrifying bacterium

Anoxic cell extracts of a denitrifying bacterial isolate (PC-07) were shown to oxidize p-cresol to p-hydroxybenzoate. Oxidation of the substrate was independent of molecular oxygen and required nitrate as the natural terminal electron acceptor. Two enzyme activities were implicated in the pathway utilized by PC-07. A p-cresol methylhydroxylase mediated the oxidation of p-cresol to p-hydroxybenzaldehyde, which was further oxidized to p-hydroxybenzoate by an NAD+-dependent dehydrogenase. The PC-07 methylhydroxylase was partially purified by anion-exchange chromatography. The protein appeared to be a multifunctional flavocytochrome, which first oxidized p-cresol to p-hydroxybenzyl alcohol, which was then oxidized to p-hydroxybenzaldehyde. The identity of the aldehyde was confirmed by mass spectroscopy. The PC-07 methylhydroxylase had a limited substrate range and required an alkyl-substituted phenolic ring with a hydroxyl group in the para position. From the available evidence, p-cresol, a naturally occurring phenol, exhibited the greatest affinity to the enzyme and therefore may be its natural substrate.

On the occurrence of enoate reductase and 2-oxo-carboxylate reductase in clostridia and some observations on the amino acid fermentation by Peptostreptococcus anaerobius

Enoate reductase present in Clostridium kluyveri and Clostridium spec. La 1 could be detected in three strains of C. tyrobutyricum and ten clostridia belonging to the groups of proteolytic and saccharolytic or proteolytic species, respectively. In C. pasteurianum, C. butyricum and C. propionicum enoate reductase could not be found even after growth on (E)-2-butenoate. A 2-oxo-carboxylate reductase was present in rather low activities in the non-proteolytic clostridia which produce enoate reductase. High activities (up to 10 U/mg protein) of 2-oxo-carboxylate reductase were found in six of ten proteolytic clostridia. The substrate specificities of the enoate reductase and the 2-oxo-carboxylate reductases from the proteolytic clostridia were determined with different alpha, beta-unsaturated carboxylates (enoates) and 2-oxo-carboxylates, respectively. Enoates as well as 2-oxo-carboxylates are intermediates of the pathway by which amino acids are degraded. An explanation is offered for the long known but not understood fact that in the Stickland reaction isoleucine always acts as an electron donor and leucine and phenylalanine can be electron acceptors as well as donors. Peptostreptococcus anaerobius converting some amino acids to the same products as C. sporogenes did this also with the intermediates which were found for the reductive deamination of amino acids in C. sporogenes, however, in crude extracts reduction of enoates occurred only in an activated form.

Stimulation of the anaerobic growth of Salmonella typhimurium by reduction of L-carnitine, carnitine derivatives and structure-related trimethylammonium compounds

In view of the development of a L-carnitine deficiency, the metabolism of L-carnitine and structure-related trimethylammonium compounds was studied in Salmonella typhimurium LT2 by means of thin-layer chromatography (TLC). L-Carnitine, crotonobetaine and acetyl-L-carnitine stimulated the anaerobic growth in a complex medium significantly. The stimulation depended on the formation of gamma-butyrobetaine. The reduction of L-carnitine proceeded in two steps: (1) Dehydration of the L-carnitine to crotonobetaine, (2) hydrogenation of crotonobetaine to gamma-butyrobetaine. The reduction of crotonobetaine was responsible for the growth stimulation. Terminal electron acceptors of the anaerobic respiration such as nitrate and trimethylamine N-oxide, but not fumarate, suppressed the catabolism of L-carnitine completely. Glucose fermentation, too, inhibited the reduction of L-carnitine but optimal growth with a high carnitine catabolism was achieved by D-ribose. The esters of carnitine with medium- and long-chain fatty acids inhibited the growth considerably because of their detergent properties.