Cloning, expression, and purification of choline dehydrogenase from the moderate halophile Halomonas elongata

Expanded phylogeny of extremely halophilic archaea shows multiple independent adaptations to hypersaline environments

Extremely halophilic archaea (Haloarchaea, Nanohaloarchaeota, Methanonatronarchaeia and Halarchaeoplasmatales) thrive in saturating salt concentrations where they must maintain osmotic equilibrium with their environment. The evolutionary history of adaptations enabling salt tolerance remains poorly understood, in particular because the phylogeny of several lineages is conflicting. Here we present a resolved phylogeny of extremely halophilic archaea obtained using improved taxon sampling and state-of-the-art phylogenetic approaches designed to cope with the strong compositional biases of their proteomes. We describe two uncultured lineages, Afararchaeaceae and Asbonarchaeaceae, which break the long branches at the base of Haloarchaea and Nanohaloarchaeota, respectively. We obtained 13 metagenome-assembled genomes (MAGs) of these archaea from metagenomes of hypersaline aquatic systems of the Danakil Depression (Ethiopia). Our phylogenomic analyses including these taxa show that at least four independent adaptations to extreme halophily occurred during archaeal evolution. Gene-tree/species-tree reconciliation suggests that gene duplication and horizontal gene transfer played an important role in this process, for example, by spreading key genes (such as those encoding potassium transporters) across extremely halophilic lineages.

Choline Dehydrogenase Contributes to Salt Tolerance in Dunaliella through Betaine Synthesis

In Dunaliella tertiolecta, a microalga renowned for its extraordinary tolerance to high salinity levels up to 4.5 M NaCl, the mechanisms underlying its stress response have largely remained a mystery. In a groundbreaking discovery, this study identifies a choline dehydrogenase enzyme, termed DtCHDH, capable of converting choline to betaine aldehyde. Remarkably, this is the first identification of such an enzyme not just in D. tertiolecta but across the entire Chlorophyta. A 3D model of DtCHDH was constructed, and molecular docking with choline was performed, revealing a potential binding site for the substrate. The enzyme was heterologously expressed in E. coli Rosetta (DE3) and subsequently purified, achieving enzyme activity of 672.2 U/mg. To elucidate the role of DtCHDH in the salt tolerance of D. tertiolecta, RNAi was employed to knock down DtCHDH gene expression. The results indicated that the Ri-12 strain exhibited compromised growth under both high and low salt conditions, along with consistent levels of DtCHDH gene expression and betaine content. Additionally, fatty acid analysis indicated that DtCHDH might also be a FAPs enzyme, catalyzing reactions with decarboxylase activity. This study not only illuminates the role of choline metabolism in D. tertiolecta's adaptation to high salinity but also identifies a novel target for enhancing the NaCl tolerance of microalgae in biotechnological applications.

Halomonas elongata: a microbial source of highly stable enzymes for applied biotechnology

Extremophilic microorganisms, which are resistant to extreme levels of temperature, salinity, pH, etc., have become popular tools for biotechnological applications. Due to their availability and cost-efficacy, enzymes from extremophiles are getting the attention of researchers and industries in the field of biocatalysis to catalyze diverse chemical reactions in a selective and sustainable manner. In this mini-review, we discuss the advantages of Halomonas elongata as moderate halophilic bacteria to provide suitable enzymes for biotechnology. While enzymes from H. elongata are more resistant to the presence of salt compared to their mesophilic counterparts, they are also easier to produce in heterologous hosts compared with more extremophilic microorganisms. Herein, a set of different enzymes (hydrolases, transferases, and oxidoreductases) from H. elongata are showcased, highlighting their interesting properties as more efficient and sustainable biocatalysts. With this, we aim to improve the visibility of halotolerant enzymes and their uncommon properties to integrate biocatalysis in industrial set-ups. KEYPOINTS: • Production and use of halotolerant enzymes can be easier than strong halophilic ones. • Enzymes from halotolerant organisms are robust catalysts under harsh conditions. • Halomonas elongata has shown a broad enzyme toolbox with biotechnology applications.

Non-active Site Residue in Loop L4 Alters Substrate Capture and Product Release in d-Arginine Dehydrogenase

Numerous studies demonstrate that enzymes undergo multiple conformational changes during catalysis. The malleability of enzymes forms the basis for allosteric regulation: residues located far from the active site can exert long-range dynamical effects on the active site residues to modulate catalysis. The structure of Pseudomonas aeruginosa d-arginine dehydrogenase (PaDADH) shows four loops (L1, L2, L3, and L4) that span the substrate and the FAD-binding domains. Loop L4 comprises residues 329-336, spanning over the flavin cofactor. The I335 residue on loop L4 is ∼10 Å away from the active site and ∼3.8 Å from N(1)-C(2)═O atoms of the flavin. In this study, we used molecular dynamics and biochemical techniques to investigate the effect of the mutation of I335 to histidine on the catalytic function of PaDADH. Molecular dynamics showed that the conformational dynamics of PaDADH are shifted to a more closed conformation in the I335H variant. In agreement with an enzyme that samples more in a closed conformation, the kinetic data of the I335H variant showed a 40-fold decrease in the rate constant of substrate association (k₁), a 340-fold reduction in the rate constant of substrate dissociation from the enzyme-substrate complex (k₂), and a 24-fold decrease in the rate constant of product release (k₅), compared to that of the wild-type. Surprisingly, the kinetic data are consistent with the mutation having a negligible effect on the reactivity of the flavin. Altogether, the data indicate that the residue at position 335 has a long-range dynamical effect on the catalytic function in PaDADH.

Molecular Mechanism of Chloramphenicol and Thiamphenicol Resistance Mediated by a Novel Oxidase, CmO, in Sphingomonadaceae

Antibiotic resistance mediated by bacterial enzyme inactivation plays a crucial role in the degradation of antibiotics in the environment. Chloramphenicol (CAP) resistance by enzymatic inactivation comprises nitro reduction, amide bond hydrolysis, and acetylation modification. However, the molecular mechanism of enzymatic oxidation of CAP remains unknown. Here, a novel oxidase gene, cmO, was identified and confirmed biochemically. The encoded CmO oxidase could catalyze the oxidation at the C-1' and C-3' positions of CAP and thiamphenicol (TAP) in Sphingobium sp. strain CAP-1. CmO is highly conserved in members of the family Sphingomonadaceae and shares the highest amino acid similarity of 41.05% with the biochemically identified glucose methanol choline (GMC) oxidoreductases. Molecular docking and site-directed mutagenesis analyses demonstrated that CAP was anchored inside the protein pocket of CmO with the hydrogen bonding of key residues glycine (G) 99, asparagine (N) 518, methionine (M) 474, and tyrosine (Y) 380. CAP sensitivity tests demonstrated that the acetyltransferase and CmO could enable a higher level of resistance to CAP than the amide bond-hydrolyzing esterase and nitroreductase. This study provides a better theoretical basis and a novel diagnostic gene for understanding and assessing the fate and resistance risk of CAP and TAP in the environment. IMPORTANCE Rising levels of antibiotic resistance are undermining ecological and human health as a result of the indiscriminate usage of antibiotics. Various resistance mechanisms have been characterized-for example, genes encoding proteins that degrade antibiotics-and yet, this requires further exploration. In this study, we report a novel gene encoding an oxidase involved in the inactivation of typical amphenicol antibiotics (chloramphenicol and thiamphenicol), and the molecular mechanism is elucidated. The findings provide novel data with which to understand the capabilities of bacteria to tackle antibiotic stress, as well as the complex function of enzymes in the contexts of antibiotic resistance development and antibiotic removal. The reported gene can be further employed as an indicator to monitor amphenicol's fate in the environment, thus benefiting risk assessment in this era of antibiotic resistance.

Biotechnological impacts of Halomonas: a promising cell factory for industrially relevant biomolecules

Extremophiles are the most fascinating life forms for their special adaptations and ability to offer unique extremozymes or bioactive molecules. Halophiles, the natural inhabitants of hypersaline environments, are one among them. Halomonas are the common genus of halophilic bacteria. To support growth in unusual environments, Halomonas produces various hydrolytic enzymes, compatible solutes, biopolymers like extracellular polysaccharides (EPS) and polyhydroxy alkaloates (PHA), antibiotics, biosurfactants, pigments, etc. Many of such molecules are being produced in large-scale bioreactors for commercial use. However, the prospect of the remaining bioactive molecules with industrial relevance is far from their application. Furthermore, the genetic engineering of the respective gene clusters could open up a new path to bio-prospect these molecules by overproducing their products through heterologous expression. The present survey on Halomonas highlights their ecological diversity, application potential of the their various industrially relevant biomolecules and impact of these biomolecules on respective fields.

Exploring Thermal Sensitivities and Adaptations of Oxidative Phosphorylation Pathways

Temperature shifts are a major challenge to animals; they drive adaptations in organisms and species, and affect all physiological functions in ectothermic organisms. Understanding the origin and mechanisms of these adaptations is critical for determining whether ectothermic organisms will be able to survive when faced with global climate change. Mitochondrial oxidative phosphorylation is thought to be an important metabolic player in this regard, since the capacity of the mitochondria to produce energy greatly varies according to temperature. However, organism survival and fitness depend not only on how much energy is produced, but, more precisely, on how oxidative phosphorylation is affected and which step of the process dictates thermal sensitivity. These questions need to be addressed from a new perspective involving a complex view of mitochondrial oxidative phosphorylation and its related pathways. In this review, we examine the effect of temperature on the commonly measured pathways, but mainly focus on the potential impact of lesser-studied pathways and related steps, including the electron-transferring flavoprotein pathway, glycerophosphate dehydrogenase, dihydroorotate dehydrogenase, choline dehydrogenase, proline dehydrogenase, and sulfide:quinone oxidoreductase. Our objective is to reveal new avenues of research that can address the impact of temperature on oxidative phosphorylation in all its complexity to better portray the limitations and the potential adaptations of aerobic metabolism.

Study of osmoadaptation mechanisms of halophilic Halomonas alkaliphila XH26 under salt stress by transcriptome and ectoine analysis

Halophilic bacteria such as the genus Halomonas are promising candidates in diverse industrial, agricultural and biomedical applications. Here, we successfully isolated a halophilic Halomonas alkaliphila strain XH26 from Xiaochaidan Salt Lake, and studied its osmoadaptation strategies using transcriptome and ectoine analysis. Divergent mechanisms were involved in osmoadaptation at different salinities in H. alkaliphila XH26. At moderate salinity (6% NaCl), increased transcriptions of ABC transporters related to iron (III), phosphate, phosphonate, monosaccharide and oligosaccharide import were observed. At high salinity (15% NaCl), transcriptions of flagellum assembly and cell motility were significantly inhibited. The transcriptional levels of ABC transporter genes related to iron (III) and iron³⁺-hydroxamate import, glycine betaine and putrescine uptake, and cytochrome biogenesis and assembly were significantly up-regulated. Ectoine synthesis and accumulation was significantly increased under salt stress, and the increased transcriptional expressions of ectoine synthesis genes ectB and ectC may play a key role in high salinity induced osmoadaptation. At extreme high salinity (18% NaCl), 5-hydroxyectoine and ectoine worked together to maintain cell survival. Together these results give valuable insights into the osmoadaptation mechanisms of H. alkaliphila XH26, and provide useful information for further engineering this specific strain for increased ectoine synthesis and related applications.

Quorum Sensing Controls the CRISPR and Type VI Secretion Systems in Aliivibrio wodanis 06/09/139

For bacteria to thrive in an environment with competitors, phages and environmental cues, they use different strategies, including Type VI Secretion Systems (T6SSs) and Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR) to compete for space. Bacteria often use quorum sensing (QS), to coordinate their behavior as the cell density increases. Like other aliivibrios, Aliivibrio wodanis 06/09/139 harbors two QS systems, the main LuxS/LuxPQ system and an N-acyl homoserine lactone (AHL)-mediated AinS/AinR system and a master QS regulator, LitR. To explore the QS and survival strategies, we performed genome analysis and gene expression profiling on A. wodanis and two QS mutants (ΔainS and ΔlitR) at two cell densities (OD600 2.0 and 6.0) and temperatures (6 and 12°C). Genome analysis of A. wodanis revealed two CRISPR systems, one without a cas loci (CRISPR system 1) and a type I-F CRISPR system (CRISPR system 2). Our analysis also identified three main T6SS clusters (T6SS1, T6SS2, and T6SS3) and four auxiliary clusters, as well about 80 potential Type VI secretion effectors (T6SEs). When comparing the wildtype transcriptome data at different cell densities and temperatures, 13–18% of the genes were differentially expressed. The CRISPR system 2 was cell density and temperature-independent, whereas the CRISPR system 1 was temperature-dependent and cell density-independent. The primary and auxiliary clusters of T6SSs were both cell density and temperature-dependent. In the ΔlitR and ΔainS mutants, several CRISPR and T6SS related genes were differentially expressed. Deletion of litR resulted in decreased expression of CRISPR system 1 and increased expression of CRISPR system 2. The T6SS1 and T6SS2 gene clusters were less expressed while the T6SS3 cluster was highly expressed in ΔlitR. Moreover, in ΔlitR, the hcp1 gene was strongly activated at 6°C compared to 12°C. AinS positively affected the csy genes in the CRISPR system 2 but did not affect the CRISPR arrays. Although AinS did not significantly affect the expression of T6SSs, the hallmark genes of T6SS (hcp and vgrG) were AinS-dependent. The work demonstrates that T6SSs and CRISPR systems in A. wodanis are QS dependent and may play an essential role in survival in its natural environment.

Production of recombinant choline oxidase and its application in betaine production

Choline oxidase catalyzes the oxidation of choline to glycine betaine via betaine aldehyde in glycine betaine biosynthesis and betaine acts as an osmolyte. Choline oxidase has attracted a great deal of attention because of its wide application in clinical and its potential use in enzymatic betaine production. Therefore, the development of efficient methods for overexpression of choline oxidase will be very valuable. In the present study, the choline oxidase gene was amplified from a newly isolated Gram-positive soil Arthrobacter globiformis strain HYJE003 and was cloned into a pET expression vector. Furthermore, the culture conditions were optimized for overexpression of cloned choline oxidase gene in different hosts for periplasmic expression of the enzyme. Expression host system Rosetta-gami2(DE3)pLysS yielded more cell-free protein and 20 fold higher active enzyme compared to any other reported studies. Terrific Broth media were found to be yielding the highest cell biomass, by applying the optimized culture conditions and purification strategy 20,902 U of choline oxidase was produced with a specific activity of 95 U/mg. The optimum pH and temperature for the enzyme activity were found to be 7 and 37 °C, respectively. Finally, we have demonstrated efficient bioconversion of betaine using overexpressed and purified choline oxidase enzyme. The enzymatically produced betaine was estimated by the formation of betaine reineckate and we were able to produce 0.83 molar of betaine from one molar of choline chloride.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s13205-021-02960-z.

The choline dehydrogenase BetA of Acinetobacter baumannii: a flavoprotein responsible for osmotic stress protection

Acinetobacter baumannii is outstanding for its ability to cope with low water activities which significantly contributes to its persistence in hospital environments. The vast majority of bacteria are able to prevent loss of cellular water by amassing osmoactive compatible solutes or their precursors into the cytoplasm. One such precursor of an osmoprotectant is choline that is taken up from the environment and oxidized to the compatible solute glycine betaine. Here, we report the identification of the osmotic stress operon betIBA in A. baumannii. This operon encodes the choline oxidation pathway important for the production of the solute glycine betaine. The salt-sensitive phenotype of a betA deletion strain could not be rescued by addition of choline, which is consistent with the role of BetA in choline oxidation. We found that BetA is a choline dehydrogenase but also mediates in vitro the oxidation of glycine betaine aldehyde to glycine betaine. BetA was found to be associated with the membrane and to contain a flavin, indicative for BetA donating electrons into the respiratory chain. The choline dehydrogenase activity was not salt dependent but was stimulated by the compatible solute glutamate.

Mechanisms of Manganese(II) Oxidation by Filamentous Ascomycete Fungi Vary With Species and Time as a Function of Secretome Composition

Manganese (Mn) oxides are among the strongest oxidants and sorbents in the environment, and Mn(II) oxidation to Mn(III/IV) (hydr)oxides includes both abiotic and microbially-mediated processes. While white-rot Basidiomycete fungi oxidize Mn(II) using laccases and manganese peroxidases in association with lignocellulose degradation, the mechanisms by which filamentous Ascomycete fungi oxidize Mn(II) and a physiological role for Mn(II) oxidation in these organisms remain poorly understood. Here we use a combination of chemical and in-gel assays and bulk mass spectrometry to demonstrate secretome-based Mn(II) oxidation in three phylogenetically diverse Ascomycetes that is mechanistically distinct from hyphal-associated Mn(II) oxidation on solid substrates. We show that Mn(II) oxidative capacity of these fungi is dictated by species-specific secreted enzymes and varies with secretome age, and we reveal the presence of both Cu-based and FAD-based Mn(II) oxidation mechanisms in all 3 species, demonstrating mechanistic redundancy. Specifically, we identify candidate Mn(II)-oxidizing enzymes as tyrosinase and glyoxal oxidase in Stagonospora sp. SRC1lsM3a, bilirubin oxidase in Stagonospora sp. and Paraconiothyrium sporulosum AP3s5-JAC2a, and GMC oxidoreductase in all 3 species, including Pyrenochaeta sp. DS3sAY3a. The diversity of the candidate Mn(II)-oxidizing enzymes identified in this study suggests that the ability of fungal secretomes to oxidize Mn(II) may be more widespread than previously thought.

MtcB, a member of the MttB superfamily from the human gut acetogen Eubacterium limosum, is a cobalamin-dependent carnitine demethylase

The trimethylamine methyltransferase MttB is the first described member of a superfamily comprising thousands of microbial proteins. Most members of the MttB superfamily are encoded by genes that lack the codon for pyrrolysine characteristic of trimethylamine methyltransferases, raising questions about the activities of these proteins. The superfamily member MtcB is found in the human intestinal isolate Eubacterium limosum ATCC 8486, an acetogen that can grow by demethylation of l-carnitine. Here, we demonstrate that MtcB catalyzes l-carnitine demethylation. When growing on l-carnitine, E. limosum excreted the unusual biological product norcarnitine as well as acetate, butyrate, and caproate. Cellular extracts of E. limosum grown on l-carnitine, but not lactate, methylated cob-(I)alamin or tetrahydrofolate using l-carnitine as methyl donor. MtcB, along with the corrinoid protein MtqC and the methylcorrinoid:tetrahydrofolate methyltransferase MtqA, were much more abundant in E. limosum cells grown on l-carnitine than on lactate. Recombinant MtcB methylates either cob(I)alamin or Co(I)-MtqC in the presence of l-carnitine and, to a much lesser extent, γ-butyrobetaine. Other quaternary amines were not substrates. Recombinant MtcB, MtqC, and MtqA methylated tetrahydrofolate via l-carnitine, forming a key intermediate in the acetogenic Wood-Ljungdahl pathway. To our knowledge, MtcB methylation of cobalamin or Co(I)-MtqC represents the first described mechanism of biological l-carnitine demethylation. The conversion of l-carnitine and its derivative γ-butyrobetaine to trimethylamine by the gut microbiome has been linked to cardiovascular disease. The activities of MtcB and related proteins in E. limosum might demethylate proatherogenic quaternary amines and contribute to the perceived health benefits of this human gut symbiont.

Choline oxidases

Choline oxidase catalyzes the four-electron, two-step, flavin-mediated oxidation of choline to glycine betaine. The enzyme is important both for medical and biotechnological reasons, because glycine betaine is one among a limited number of compatible solutes used by cells to counteract osmotic pressure. From a fundamental standpoint, choline oxidase has emerged as one of the paradigm enzymes for the oxidation of alcohols catalyzed by flavoproteins. Mechanistic, structural, and computational studies have elucidated the mechanism of action of the enzyme from Arthrobacter globiformis at the molecular level. Both choline and oxygen access to the active site cavity are gated and tightly controlled. Amino acid residues involved in substrate binding, and their contribution, have been identified. The mechanism of choline oxidation, with a hydride transfer reaction, an asynchronous transition state, the formation and stabilization of an alkoxide transient species, and a quantum mechanical mode of reaction, has been elucidated. The importance of nonpolar side chains for oxygen localization and of the positive charge harbored on the substrate for activation of oxygen for reaction with the reduced flavin have been recognized. Interesting phenomena, like the formation of a metastable photoinduced flavin-protein adduct, the reversible formation of a bicovalent flavoprotein, and the trapping of the enzyme in inactive conformations, have been described. This review summarizes the current status of our understanding on the structure-function-dynamics of choline oxidase.

Substitutions of S101 decrease proton and hydride transfers in the oxidation of betaine aldehyde by choline oxidase

Choline oxidase oxidizes choline to glycine betaine, with two flavin-mediated reactions to convert the alcohol substrate to the carbon acid product. Proton abstraction from choline or hydrated betaine aldehyde in the wild-type enzyme occurs in the mixing time of the stopped-flow spectrophotometer, thereby precluding a mechanistic investigation. Mutagenesis of S101 rendered the proton transfer reaction amenable to study. Here, we have investigated the aldehyde oxidation reaction catalyzed by the mutant enzymes using steady-state and rapid kinetics with betaine aldehyde. Stopped-flow traces for the reductive half-reaction of the S101T/V/C variants were biphasic, corresponding to the reactions of proton abstraction and hydride transfer. In contrast, the S101A enzyme yielded monophasic traces like wild-type choline oxidase. The rate constants for proton transfer in the S101T/C/V variants decreased logarithmically with increasing hydrophobicity of residue 101, indicating a behavior different from that seen previously with choline for which no correlation was determined. The rate constants for hydride transfer also showed a logarithmic decrease with increasing hydrophobicity at position 101, which was similar to previous results with choline as a substrate for the enzyme. Thus, the hydrophilic character of S101 is necessary not only for efficient hydride transfer but also for the proton abstraction reaction.

Treatment of Alkaline Cr(VI)-Contaminated Leachate with an Alkaliphilic Metal-Reducing Bacterium

Chromium in its toxic Cr(VI) valence state is a common contaminant particularly associated with alkaline environments. A well-publicized case of this occurred in Glasgow, United Kingdom, where poorly controlled disposal of a cementitious industrial by-product, chromite ore processing residue (COPR), has resulted in extensive contamination by Cr(VI)-contaminated alkaline leachates. In the search for viable bioremediation treatments for Cr(VI), a variety of bacteria that are capable of reduction of the toxic and highly soluble Cr(VI) to the relatively nontoxic and less mobile Cr(III) oxidation state, predominantly under circumneutral pH conditions, have been isolated. Recently, however, alkaliphilic bacteria that have the potential to reduce Cr(VI) under alkaline conditions have been identified. This study focuses on the application of a metal-reducing bacterium to the remediation of alkaline Cr(VI)-contaminated leachates from COPR. This bacterium, belonging to the Halomonas genus, was found to exhibit growth concomitant to Cr(VI) reduction under alkaline conditions (pH 10). Bacterial cells were able to rapidly remove high concentrations of aqueous Cr(VI) (2.5 mM) under anaerobic conditions, up to a starting pH of 11. Cr(VI) reduction rates were controlled by pH, with slower removal observed at pH 11, compared to pH 10, while no removal was observed at pH 12. The reduction of aqueous Cr(VI) resulted in the precipitation of Cr(III) biominerals, which were characterized using transmission electron microscopy and energy-dispersive X-ray analysis (TEM-EDX) and X-ray photoelectron spectroscopy (XPS). The effectiveness of this haloalkaliphilic bacterium for Cr(VI) reduction at high pH suggests potential for its use as an in situ treatment of COPR and other alkaline Cr(VI)-contaminated environments.

Structural and functional analysis of betaine aldehyde dehydrogenase from Staphylococcus aureus

When exposed to high osmolarity, methicillin-resistant Staphylococcus aureus (MRSA) restores its growth and establishes a new steady state by accumulating the osmoprotectant metabolite betaine. Effective osmoregulation has also been implicated in the acquirement of a profound antibiotic resistance by MRSA. Betaine can be obtained from the bacterial habitat or produced intracellularly from choline via the toxic betaine aldehyde (BA) employing the choline dehydrogenase and betaine aldehyde dehydrogenase (BADH) enzymes. Here, it is shown that the putative betaine aldehyde dehydrogenase SACOL2628 from the early MRSA isolate COL (SaBADH) utilizes betaine aldehyde as the primary substrate and nicotinamide adenine dinucleotide (NAD(+)) as the cofactor. Surface plasmon resonance experiments revealed that the affinity of NAD(+), NADH and BA for SaBADH is affected by temperature, pH and buffer composition. Five crystal structures of the wild type and three structures of the Gly234Ser mutant of SaBADH in the apo and holo forms provide details of the molecular mechanisms of activity and substrate specificity/inhibition of this enzyme.

Human choline dehydrogenase: medical promises and biochemical challenges

Human choline dehydrogenase (CHD) is located in the inner membrane of mitochondria primarily in liver and kidney and catalyzes the oxidation of choline to glycine betaine. Its physiological role is to regulate the concentrations of choline and glycine betaine in the blood and cells. Choline is important for regulation of gene expression, the biosynthesis of lipoproteins and membrane phospholipids and for the biosynthesis of the neurotransmitter acetylcholine; glycine betaine plays important roles as a primary intracellular osmoprotectant and as methyl donor for the biosynthesis of methionine from homocysteine, a required step for the synthesis of the ubiquitous methyl donor S-adenosyl methionine. Recently, CHD has generated considerable medical attention due to its association with various human pathologies, including male infertility, homocysteinuria, breast cancer and metabolic syndrome. Despite the renewed interest, the biochemical characterization of the enzyme has lagged behind due to difficulties in the obtainment of purified, active and stable enzyme. This review article summarizes the medical relevance and the physiological roles of human CHD, highlights the biochemical knowledge on the enzyme, and provides an analysis based on the comparison of the protein sequence with that of bacterial choline oxidase, for which structural and biochemical information is available.

Distribution of serotypes and virulence-associated genes in pathogenic Escherichia coli isolated from ducks

The objective of the present study was to investigate the serotypes and virulence-associated genes of avian pathogenic Escherichia coli (APEC) isolated from duck colibacillosis cases. Two hundred and fifty-four APEC isolates from duck colibacillosis cases were serotyped and amplified for 12 known virulence-associated genes and the betA gene (encoding choline dehydrogenase) by polymerase chain reaction assays. One hundred and forty-three E. coli isolates from cloacal swabs of healthy ducks were also amplified for the same genes. A total of 53 O-serogroups were found in 254 APEC isolates, among which O93, O78 and O92 were predominant serogroups. Polymerase chain reaction results showed that Shiga-toxin-producing E. coli distributed in only 2.4% of ducks compared with 49.2% of the APEC isolates harbouring the irp2 gene, and 44.9% the fyuA gene, respectively. The ibeA gene was only present in 27 APEC isolates and was not found in healthy ducks. The rfaH gene was detected in 20.5% of APEC isolates, whereas 5.6% was found in healthy ducks. A total 79.5% of APEC isolates harboured the betA gene, which was significantly higher than in healthy ducks (16.1%), suggesting that betA may be associated with virulence.

Synthesis of human renalase1 in Escherichia coli and its purification as a FAD-containing holoprotein

Renalase is a protein ubiquitous in vertebrates, which has been proposed to modulate blood pressure and heart rate, and whose downregulation might result in hypertension. Despite its potential relevance for human health, the biochemical characterization of renalase is still lacking, possibly due to difficulties in obtaining it in recombinant form. By expressing two different gene constructs, we found that the major isoform of human renalase, renalase1, is mainly produced in Escherichia coli in inclusion bodies. However, by optimizing the expression conditions, significant amounts of soluble products were obtained. Both soluble renalase forms have been purified to homogeneity exploiting their N-terminal His-tag. Linking of the protein of interest to the SUMO protein did not improve solubility, but yielded untagged renalase1 after proteolytic processing of the fusion product. The two recombinant renalase forms displayed the same molecular properties. They bind equimolar amounts of FAD and appear to be correctly folded by various criteria. The procedures for the production and isolation of recombinant renalase1 here reported are expected to boost the much awaited biochemical studies on this remarkable protein.

Hydride transfer made easy in the reaction of alcohol oxidation catalyzed by flavin-dependent oxidases

Choline oxidase (E.C. 1.1.3.17; choline-oxygen 1-oxidoreductase) catalyzes the two-step, four-electron oxidation of choline to glycine betaine with betaine aldehyde as enzyme-associated intermediate and molecular oxygen as final electron acceptor. Biochemical, structural, and mechanistic studies on the wild-type and a number of mutant forms of choline oxidase from Arthrobacter globiformis have recently been carried out, allowing for the delineation at molecular and atomic levels of the mechanism of alcohol oxidation catalyzed by the enzyme. First, the alcohol substrate is activated to its alkoxide species by the removal of the hydroxyl proton in the enzyme-substrate complex. The resulting activated alkoxide is correctly positioned for catalysis through electrostatic and hydrogen bonding interactions with a number of active site residues. After substrate activation and correct positioning are attained, alcohol oxidation occurs in a highly preorganized enzyme-substrate complex through quantum mechanical transfer of a hydride ion from the alpha-carbon of the chelated, alkoxide species to the N(5) atom of the enzyme-bound flavin. This mechanism in its essence is shared by another class of alcohol oxidizing enzymes that utilize a catalytic zinc to stabilize an alkoxide intermediate and NAD(P)(+) as the organic cofactor that accepts the hydride ion, whose paradigm example is alcohol dehydrogenase. It will be interesting to experimentally evaluate the attractive hypothesis of whether the mechanism of choline oxidase can be extended to other flavin-dependent enzymes as well as enzymes that utilize cofactors other than flavins in the oxidation of alcohols.

Mala s 12 is a major allergen in patients with atopic eczema and has sequence similarities to the GMC oxidoreductase family

BACKGROUND

Atopic eczema (AE) is a chronic inflammatory skin disorder, characterized by impaired skin barrier and itch. The yeast Malassezia belongs to the normal human skin microflora and can induce IgE- and T-cell-mediated allergic reactions in AE patients. Previously, we have identified several IgE-binding components in Malassezia sympodialis extract.

METHODS

Here, we report cloning, production and characterization of a M. sympodialis 67-kDa allergen.

RESULTS

The sequence of the 67-kDa protein, termed Mala s 12, showed sequence similarity to the glucose-methanol-choline (GMC) oxidoreductase enzyme superfamily and was expressed as a recombinant protein in Escherichia coli. The purified protein bound flavin adenine dinucleotide with 1:1 stoichiometry per monomer of protein. The protein-bound flavin showed an extinction coefficient at 451 nm of 11.3 mM(-1)cm(-1). The recombinant 67-kDa protein did not show any enzymatic activity when tested as oxidase or dehydrogenase using choline, glucose, myo-inositol, methanol, ethanol, 1-pentanol, benzyl alcohol, 2-phenylethanol, cholesterol or lauryl alcohol as possible substrates. Recombinant Mala s 12 was recognized by serum IgE from 13 of 21 (62%) M. sympodialis-sensitized AE patients indicating that the 67-kDa component is a major allergen.

CONCLUSIONS

The data show that Mala s 12 has sequence similarity to the GMC oxidoreductase family and is a major allergen in AE patients.

Kinetics of salt-dependent unfolding of [2Fe–2S] ferredoxin of Halobacterium salinarum

The [2Fe-2S] ferredoxin from the extreme haloarchaeon Halobacterium salinarum is stable in high (>1.5 M) salt concentration. At low salt concentration the protein exhibits partial unfolding. The kinetics of unfolding was studied in low salt and in presence of urea in order to investigate the role of salt ions on the stability of the protein. The urea dependent unfolding, monitored by fluorescence of the tryptophan residues and circular dichroism, suggests that the native protein is stable at neutral pH, is destabilized in both acidic and alkaline environment, and involves the formation of kinetic intermediate(s). In contrast, the unfolding kinetics in low salt exhibits enhanced rate of unfolding with increase in pH value and is a two state process without the formation of intermediate. The unfolding at neutral pH is salt concentration dependent and occurs in two stages. The first stage, involves an initial fast phase (indicative of the formation of a hydrophobic collapsed state) followed by a relatively slow phase, and is dependent on the type of cation and anion. The second stage is considerably slower, proceeds with an increase in fluorescence intensity and is largely independent of the nature of salt. Our results thus show that the native form of the haloarchaeal ferredoxin (in high salt concentration) unfolds in low salt concentration through an apparently hydrophobic collapsed form, which leads to a kinetic intermediate. This intermediate then unfolds further to the low salt form of the protein.

Identification of glycine betaine as compatible solute in Synechococcus sp. WH8102 and characterization of its N-methyltransferase genes involved in betaine synthesis

Biosynthesis of glycine betaine from simple carbon sources as compatible solute is rare among aerobic heterotrophic eubacteria, and appears to be almost exclusive to the non-halophilic and slightly halophilic phototrophic cyanobacteria. Although Synechococcus sp. WH8102 (CCMP2370), a unicellular marine cyanobacterium, could grow up to additional 2.5% (w/v) NaCl in SN medium, natural abundance 13C nuclear magnetic resonance spectroscopy identified glycine betaine as its major compatible solute. Intracellular glycine betaine concentrations were dependent on the osmolarity of the growth medium over the range up to additional 2% NaCl in SN medium, increasing from 6.8 +/- 1.5 to 62.3 +/- 5.5 mg/g dw. The ORFs SYNW1914 and SYNW1913 from Synechococcus sp. WH8102 were found as the homologous genes coding for glycine sarcosine N-methyltransferase and sarcosine dimethylglycine N-methyltransferase, heterologously over-expressed respectively as soluble fraction in Escherichia coli BL21(DE3)pLysS and purified by Ni-NTA His x bind resins. Their substrate specificities and the values of the kinetic parameters were determined by TLC and 1H NMR spectroscopy. RT-PCR analysis revealed that the two ORFs were both transcribed in cells of Synechococcus sp. WH8102 growing in SN medium without additional NaCl, which confirmed the pathway of de novo synthesizing betaine from glycine existing in these marine cyanobacteria.

Oxygen- and temperature-dependent kinetic isotope effects in choline oxidase: correlating reversible hydride transfer with environmentally enhanced tunneling

Choline oxidase catalyzes the flavin-linked oxidation of choline to glycine betaine, with betaine aldehyde as intermediate and oxygen as electron acceptor. Here, the effects of oxygen concentration and temperature on the kinetic isotope effects with deuterated choline have been investigated. The D(kcat/Km) and Dkcat values with 1,2-[(2)H4]-choline were pH-independent at saturating oxygen concentrations, whereas they decreased at high pH to limiting values that depended on oxygen concentration at < or = 0.97 mM oxygen. The kcat/Km and kcat pH profiles had similar patterns reaching plateaus at high pH. Both the limiting kcat/Km at high pH and the pKa values were perturbed to lower values with choline and < or = 0.25 mM oxygen. These data suggest that oxygen availability modulates whether the reduced enzyme-betaine aldehyde complex partitions forward to catalysis rather then reverting to the oxidized enzyme-choline alkoxide species. At saturating oxygen concentrations, the D(kcat/Km) was 10.6 +/- 0.6 and temperature independent, and the isotope effect on the preexponential factors (A(H)'/A(D)') was 14 +/- 3, ruling out a classical over-the-barrier behavior for hydride transfer. Similar enthalpies of activation (deltaH(double dagger)) with values of 18 +/- 2 and 18 +/- 5 kJ mol(-1) were determined with choline and 1,2-[(2)H4]-choline. These data suggest that the hydride transfer reaction in which choline is oxidized by choline oxidase occurs quantum mechanically within a preorganized active site, with the reactive configuration for hydride tunneling being minimally affected by environmental vibrations of the reaction coordinate other than those affecting the distance between the donor and acceptor of the hydride.

Organic compatible solutes of halotolerant and halophilic microorganisms

Microorganisms that adapt to moderate and high salt environments use a variety of solutes, organic and inorganic, to counter external osmotic pressure. The organic solutes can be zwitterionic, noncharged, or anionic (along with an inorganic cation such as K(+)). The range of solutes, their diverse biosynthetic pathways, and physical properties of the solutes that effect molecular stability are reviewed.

On the Catalytic Mechanism of Choline Oxidase

Choline oxidase catalyzes the four-electron oxidation of choline to glycine betaine, with betaine aldehyde as an intermediate. In this study, primary deuterium and solvent kinetic isotope effects have been used to elucidate the mechanism for substrate oxidation by choline oxidase using both steady-state kinetics and rapid kinetics techniques. The D(kcat/Km) value with 1,2-[2H4]-choline at saturating oxygen concentration was independent of pH in the range between 6.5 and 10, with a value of approximately 10.6, indicating that CH bond cleavage is not masked by other titratable kinetic steps belonging to the reductive half-reaction. In agreement with this conclusion, a Dkred value of approximately 8.9 was determined at pH 10 for the anaerobic reduction of the flavin by choline, irrespective of whether aqueous or deuterated solvent was used. At pH 10, both the D2(O)(kcat/Km) and the D2(O)kred values were not different from unity with choline or 1,2-[2H4]-choline, while the Dkcat and D2(O)kcat values were 7.3 and 1.1, respectively. The kcat and kred values were 133 s(-1) and 135 s(-1) with betaine aldehyde and 60 s(-1) and 93 s(-1) with choline. These data are consistent with a chemical mechanism in which the choline hydroxyl proton is not in flight in the transition state for CH bond cleavage and with chemical steps of flavin reduction by choline and betaine aldehyde being rate limiting for the overall turnover of the enzyme.