How superoxide reductases and flavodiiron proteins combat oxidative stress in anaerobes

Defense arsenal of the strict anaerobe Clostridioides difficile against reactive oxygen species encountered during its infection cycle

Clostridioides difficile, a strict anaerobe, is the major cause of antibiotic-associated diarrhea. This enteropathogen must adapt to oxidative stress mediated by reactive oxygen species (ROS), notably those released by the neutrophils and macrophages recruited to the site of infection or those endogenously produced upon high oxygen (O2) exposure. C. difficile uses a superoxide reductase, Sor, and several peroxidases to detoxify ROS. We showed that Sor has a superoxide reductase activity in vitro and protects the bacterium from exposure to menadione, a superoxide donor. After confirming the peroxidase activity of the rubrerythrin, Rbr, we showed that this enzyme together with the peroxiredoxin, Bcp, plays a central role in the detoxification of H2O2 and promotes the survival of C. difficile in the presence of not only H2O2 but also air or 4% O2. Under high O2 concentrations encountered in the gastrointestinal tract, the bacterium generated endogenous H2O2. The two O2 reductases, RevRbr2 and FdpF, have also a peroxidase activity and participate in H2O2 resistance. The CD0828 gene, which also contributes to H2O2 protection, forms an operon with rbr, sor, and perR encoding a H2O2-sensing repressor. The expression of the genes encoding the ROS reductases and the CD0828 protein was induced upon exposure to either H2O2 or air. We showed that the induction of the rbr operon is mediated not only by PerR but also by OseR, a recently identified O2-responsive regulator of C. difficile, and indirectly by σB, the sigma factor of the stress response, whereas the expression of bcp is only controlled by σB.

IMPORTANCE

ROS plays a fundamental role in intestinal homeostasis, limiting the proliferation of pathogenic bacteria. Clostridioides difficile is an important enteropathogen that induces an intense immune response, characterized by the massive recruitment of immune cells responsible for secreting ROS, mainly H2O2 and superoxide. We showed in this work that ROS exposure leads to the production of an armada of enzymes involved in ROS detoxification. This includes a superoxide reductase and four peroxidases, Rbr, Bcp, revRbr2, and FdpF. These enzymes likely contribute to the survival of vegetative cells of C. difficile in the colon during the host immune response. Distinct regulations are also observed for the genes encoding the ROS detoxification enzymes allowing a fine tuning of the adaptive response to ROS exposure. Understanding the mechanisms of ROS protection during infection could shed light on how C. difficile survives under conditions of an exacerbated inflammatory response.

Multimodal analysis identifies microbiome changes linked to stem cell transplantation-associated diseases

BACKGROUND

Allogeneic hematopoietic stem cell transplantation (allo-HSCT) is one of the most efficient therapeutic options available to cure many hematological malignancies. However, severe complications derived from this procedure, including graft-versus-host disease (GVHD) and infections, can limit its success and negatively impact survival. Previous studies have shown that alterations in the microbiome are associated with the development of allo-HSCT-derived complications. However, most studies relied on single techniques that can only analyze a unique aspect of the microbiome, which hinders our ability to understand how microbiome alterations drive allo-HSCT-associated diseases.

RESULTS

Here, we have applied multiple "omic" techniques (16S rRNA and shotgun sequencing, targeted and un-targeted metabolomics) in combination with machine learning approaches to define the most significant microbiome changes following allo-HSCT at multiple modalities (bacterial taxa, encoded functions, and derived metabolites). In addition, multivariate approaches were applied to study interactions among the various microbiome modalities (the interactome). Our results show that the microbiome of transplanted patients exhibits substantial changes in all studied modalities. These include depletion of beneficial microbes, mainly from the Clostridiales order, loss of their bacterial encoded functions required for the synthesis of key metabolites, and a reduction in metabolic end products such as short chain fatty acids (SCFAs). These changes were followed by an expansion of bacteria that frequently cause infections after allo-HSCT, including several Staphylococcus species, which benefit from the reduction of bacteriostatic SCFAs. Additionally, we found specific alterations in all microbiome modalities that distinguished those patients who subsequently developed GVHD, including depletion of anti-inflammatory commensals, protective reactive oxygen detoxifying enzymes, and immunoregulatory metabolites such as acetate or malonate. Moreover, extensive shifts in the homeostatic relationship between bacteria and their metabolic products (e.g., Faecalibacterium and butyrate) were detected mainly in patients who later developed GVHD.

CONCLUSIONS

We have identified specific microbiome changes at different modalities (microbial taxa, their encoded genes, and synthetized metabolites) and at the interface between them (the interactome) that precede the development of complications associated with allo-HSCT. These identified microbial features provide novel targets for the design of microbiome-based strategies to prevent diseases associated with stem cell transplantation. Video Abstract.

Physiological role and complex regulation of O2-reducing enzymes in the obligate anaerobe Clostridioides difficile

Clostridioides difficile, the major cause of antibiotic-associated diarrhea, is a strict anaerobic, sporulating Firmicutes. However, during its infectious cycle, this anaerobe is exposed to low oxygen (O2) tensions, with a longitudinal decreasing gradient along the gastrointestinal tract and a second lateral gradient with higher O2 tensions in the vicinity of the cells. A plethora of enzymes involved in oxidative stress detoxication has been identified in C. difficile, including four O2-reducing enzymes: two flavodiiron proteins (FdpA and FdpF) and two reverse rubrerythrins (revRbr1 and revRbr2). Here, we investigated the role of the four O2-reducing enzymes in the tolerance to increasing physiological O2 tensions and air. The four enzymes have different, yet overlapping, spectra of activity. revRbr2 is specific to low O2 tensions (<0.4%), FdpA to low and intermediate O2 tensions (0.4%-1%), revRbr1 has a wider spectrum of activity (0.1%-4%), and finally FdpF is more specific to tensions > 4% and air. These different O2 ranges of action partly arise from differences in regulation of expression of the genes encoding those enzymes. Indeed, we showed that revrbr2 is under the dual control of σA and σB. We also identified a regulator of the Spx family that plays a role in the induction of fdp and revrbr genes upon O2 exposure. Finally, fdpF is regulated by Rex, a regulator sensing the NADH/NAD+ ratio. Our results demonstrate that the multiplicity of O2-reducing enzymes of C. difficile is associated with different roles depending on the environmental conditions, stemming from a complex multi-leveled network of regulation.

IMPORTANCE

The gastrointestinal tract is a hypoxic environment, with the existence of two gradients of O2 along the gut, one longitudinal anteroposterior decreasing gradient and one proximodistal increasing from the lumen to the epithelial cells. O2 is a major source of stress for an obligate anaerobe such as the enteropathogen C. difficile. This bacterium possesses a plethora of enzymes capable of scavenging O2 and reducing it to H2O. In this work, we identified the role of the four O2-reducing enzymes in the tolerance to the physiological O2 tensions faced by C. difficile during its infectious cycle. These four enzymes have different spectra of action and protect the vegetative cells over a large range of O2 tensions. These differences are associated with a distinct regulation of each gene encoding those enzymes. The complex network of regulation is crucial for C. difficile to adapt to the various O2 tensions encountered during infection.

New Electron-Transfer Chain to a Flavodiiron Protein in Fusobacterium nucleatum Couples Butyryl-CoA Oxidation to O2 Reduction

Fusobacterium nucleatum, a Gram-negative obligate anaerobe, is common to the oral microbiota, but the species is known to infect other sites of the body where it is associated with a range of pathologies. At present, little is known about the mechanisms by which F. nucleatum mitigates against oxidative and nitrosative stress. Inspection of the F. nucleatum subsp. polymorphum ATCC 10953 genome reveals that it encodes a flavodiiron protein (FDP; FNP2073) that is known in other organisms to reduce NO to N2O and/or O2 to H2O. FNP2073 is dicistronic with a gene encoding a multicomponent enzyme termed BCR for butyryl-CoA reductase. BCR is composed of a butyryl-CoA dehydrogenase domain (BCD), the C-terminal domain of the α-subunit of the electron-transfer flavoprotein (Etfα), and a rubredoxin domain. We show that BCR and the FDP form an α4β4 heterotetramic complex and use butyryl-CoA to selectively reduce O2 to H2O. The FAD associated with the Etfα domain (α-FAD) forms red anionic semiquinone (FAD•-), whereas the FAD present in the BCD domain (δ-FAD) forms the blue-neutral semiquinone (FADH•), indicating that both cofactors participate in one-electron transfers. This was confirmed in stopped-flow studies where the reduction of oxidized BCR with an excess of butyryl-CoA resulted in rapid (<1.6 ms) interflavin electron transfer evidenced by the formation of the FAD•-. Analysis of bacterial genomes revealed that the dicistron is present in obligate anaerobic gut bacteria considered to be beneficial by virtue of their ability to produce butyrate. Thus, BCR-FDP may help to maintain anaerobiosis in the colon.

Physcomitrium patens flavodiiron proteins form heterotetrametric complexes

Flavodiiron proteins (FLVs) catalyze the reduction of oxygen to water by using electrons from Photosystem I (PSI). In several photosynthetic organisms such as cyanobacteria, green algae, mosses and gymnosperms, FLV-dependent electron flow protects PSI from over-reduction and consequent damage especially under fluctuating light conditions. In this work we investigated biochemical and structural properties of FLVA and FLVB from the model moss Physcomitrium patens. The two proteins, expressed and purified from Escherichia coli, bind both iron and flavin cofactors and show NAD(P)H oxidase activity as well as oxygen reductase capacities. Moreover, the co-expression of both FLVA and FLVB, coupled to a tandem affinity purification procedure with two different affinity tags, enabled the isolation of the stable and catalytically active FLVA/B hetero tetrameric protein complex with cooperative nature. The multimeric organization was shown to be stabilized by inter-subunit disulfide bonds. This investigation provides valuable new information on the biochemical properties of FLVs, with new insights into their in vivo activity.

Different regulation strategies of anaerobic digestion by AC/CaO2 and Fe3O4/CaO2: Reactive oxygen species induction, methanogenic performance, and microbial response

This study examined the combination of activated carbon and magnetite with calcium peroxide in enhancing the anaerobic digestion (AD) performance of food waste (FW). The individual mechanisms of these two approaches were also clarified. The results indicated that AC/CaO2 achieved the highest specific methane yield of 434.4 mL/g VS, followed by Fe3O4/CaO2 (416.9 mL/g VS). Both were significantly higher than other groups (control, AC, Fe3O4, and CaO2 were 330.1, 341.4, 342.8, and 373.2 mL/g VS, respectively). Additionally, compared to Fe3O4/CaO2, AC/CaO2 further increased reactive oxygen species (ROS), thereby enhancing the hydrolytic acidification process. Simultaneously, the higher ROS levels of Fe3O4/CaO2 and AC/CaO2 promoted the formation of microbial aggregates and established a more robust enzymatic defense system and unique damage repair strategy. The research comparatively analyzed the synergistic mechanism of iron-based and carbon-based conductive materials with CaO2, providing new perspectives for optimizing the AD of FW.

Clostridioides difficile superoxide reductase mitigates oxygen sensitivity

Clostridioides difficile causes a serious diarrheal disease and is a common healthcare-associated bacterial pathogen. Although it has a major impact on human health, the mechanistic details of C. difficile intestinal colonization remain undefined. C. difficile is highly sensitive to oxygen and requires anaerobic conditions for in vitro growth. However, the mammalian gut is not devoid of oxygen, and C. difficile tolerates moderate oxidative stress in vivo. The C. difficile genome encodes several antioxidant proteins, including a predicted superoxide reductase (SOR) that is upregulated upon exposure to antimicrobial peptides. The goal of this study was to establish SOR enzymatic activity and assess its role in protecting C. difficile against oxygen exposure. Insertional inactivation of sor rendered C. difficile more sensitive to superoxide, indicating that SOR contributes to antioxidant defense. Heterologous C. difficile sor expression in Escherichia coli conferred protection against superoxide-dependent growth inhibition, and the corresponding cell lysates showed superoxide scavenging activity. Finally, a C. difficile SOR mutant exhibited global proteome changes under oxygen stress when compared to the parent strain. Collectively, our data establish the enzymatic activity of C. difficile SOR, confirm its role in protection against oxidative stress, and demonstrate SOR's broader impacts on the C. difficile vegetative cell proteome.IMPORTANCEClostridioides difficile is an important pathogen strongly associated with healthcare settings and capable of causing severe diarrheal disease. While considered a strict anaerobe in vitro, C. difficile has been shown to tolerate low levels of oxygen in the mammalian host. Among other well-characterized antioxidant proteins, the C. difficile genome encodes a predicted superoxide reductase (SOR), an understudied component of antioxidant defense in pathogens. The significance of the research reported herein is the characterization of SOR's enzymatic activity, including confirmation of its role in protecting C. difficile against oxidative stress. This furthers our understanding of C. difficile pathogenesis and presents a potential new avenue for targeted therapies.

Accelerated dissemination of antibiotic resistant genes via conjugative transfer driven by deficient denitrification in biochar-based biofiltration systems

Biofiltration systems harbored and disseminated antibiotic resistance genes (ARGs), when confronting antibiotic-contained wastewater. Biochar, a widely used environmental remediation material, can mitigate antibiotic stress on adjoining microbes by lowering the availability of sorbed antibiotics, and enhance the attachment of denitrifiers. Herein, bench-scale biofiltration systems, packed with commercial biochars, were established to explore the pivotal drivers affecting ARG emergence. Results showed that biofiltration columns, achieving higher TN removal and denitrification capacity, showed a significant decrease in ARG accumulation (p < 0.05). The relative abundance of ARGs (0.014 ± 0.0008) in the attached biofilms decreased to 1/5-folds of that in the control group (0.065 ± 0.004). Functional analysis indicated ARGs' accumulation was less attributed to ARG activation or horizontal gene transfer (HGT) driven by sorbed antibiotics. Most denitrifiers, like Bradyrhizobium, Geothrix, etc., were found to be enriched and host ARGs. Nitrosative stress from deficient denitrification was demonstrated to be the dominant driver for affecting ARG accumulation and dissemination. Metagenomic and metaproteomic analysis revealed that nitrosative stress promoted the conjugative HGT of ARGs mainly via increasing the transmembrane permeability and enhancing the amino acid transport and metabolism, such as cysteine, methionine, and valine metabolism. Overall, this study highlighted the risks of deficient denitrification in promoting ARG transfer and transmission in biofiltration systems and natural ecosystems.

Reduction of molecular oxygen in flavodiiron proteins - Catalytic mechanism and comparison to heme-copper oxidases

The family of flavodiiron proteins (FDPs) plays an important role in the scavenging and detoxification of both molecular oxygen and nitric oxide. Using electrons from a flavin mononucleotide cofactor molecular oxygen is reduced to water and nitric oxide is reduced to nitrous oxide and water. While the mechanism for NO reduction in FDPs has been studied extensively, there is very little information available about O2 reduction. Here we use hybrid density functional theory (DFT) to study the mechanism for O2 reduction in FDPs. An important finding is that a proton coupled reduction is needed after the O2 molecule has bound to the diferrous diiron active site and before the OO bond can be cleaved. This is in contrast to the mechanism for NO reduction, where both NN bond formation and NO bond cleavage occurs from the same starting structure without any further reduction, according to both experimental and computational results. This computational result for the O2 reduction mechanism should be possible to evaluate experimentally. Another difference between the two substrates is that the actual OO bond cleavage barrier is low, and not involved in rate-limiting the reduction process, while the barrier connected with bond cleavage/formation in the NO reduction process is of similar height as the rate-limiting steps. We suggest that these results may be part of the explanation for the generally higher activity for O2 reduction as compared to NO reduction in most FDPs. Comparisons are also made to the O2 reduction reaction in the family of heme‑copper oxidases.

The flavodiiron protein from Syntrophomonas wolfei has five domains and acts both as an NADH:O2 or an NADH:H2 O2 oxidoreductase

Flavodiiron proteins (FDPs) are a family of enzymes with a significant role in O2 /H2 O2 and/or NO detoxification through the reduction of these species to H2 O or N2 O, respectively. All FDPs contain a minimal catalytic unit of two identical subunits, each one having a metallo-β-lactamase-like domain harboring the catalytic diiron site, and a flavodoxin-like domain. However, more complex and diverse arrangements in terms of domains are found in this family, of which the class H enzymes are among the most complex. One of such FDPs is encoded in the genome of the anaerobic bacterium Syntrophomonas wolfei subsp. wolfei str. Goettingen G311. Besides the core domains, this protein is predicted to have three additional ones after the flavodoxin core domain: two short-chain rubredoxins and a NAD(P)H:rubredoxin oxidoreductase-like domain. This enzyme, FDP_H, was produced and characterized and the presence of the predicted cofactors was investigated by a set of biochemical and spectroscopic methodologies. Syntrophomonas wolfei FDP_H exhibited a remarkable O2 reduction activity with a kcat  = 52.0 ± 1.2 s-1 and a negligible NO reduction activity (~ 100 times lower than with O2 ), with NADH as an electron donor, that is, it is an oxygen-selective FDP. In addition, this enzyme showed the highest turnover value for H2 O2 reduction (kcat  = 19.1 ± 2.2 s-1 ) ever observed among FDPs. Kinetic studies of site-directed mutants of iron-binding cysteines at the two rubredoxin domains demonstrated the essential role of these centers since their absence leads to a significant decrease or even abolishment of O2 and H2 O2 reduction activities.

Microbiome re-assembly boosts anaerobic digestion under volatile fatty acid inhibition: focusing on reactive oxygen species metabolism

The accumulation of volatile fatty acids (VFAs) in anaerobic digestion (AD) systems resulting from food waste overload poses a risk of system collapse. However, limited understanding exists regarding the inhibitory mechanisms and effective strategies to address VFAs-induced stress. This study found that accumulated VFAs exert reactive oxygen species (ROS) stress on indigenous microbiota, particularly impacting methanogens due to their lower antioxidant capability compared to bacteria, which is supposed to be the primary reason for methanogenesis failure. To enhance the VFAs-stressed AD process, microbiome re-assembly using customized propionate-degrading consortia and bioaugmentation with concentrated digestate were implemented. Microbiome re-assembly demonstrated superior efficiency, yielding an average methane yield of 563.6±159.8 mL/L·d and reducing VFAs to undetectable levels for a minimum of 80 days. This strategy improved the abundance of Syntrophomonas, Syntrophobacter and Methanothrix, alleviating ROS stress. Conversely, microbial community in reactor with other strategy experienced an escalating intracellular damage, as indicated by the increase of ROS generation-related genes. This study fills knowledge gaps in stress-related metabolic mechanisms of anaerobic microbiomes exposed to VFAs and microbiome re-assembly to boost methanogenesis process.

Nitrite Formation at a Diiron Dinitrosyl Complex

Pathogenic bacteria employ iron-containing enzymes to detoxify nitric oxide (NO•) produced by mammals as part of their immune response. Two classes of diiron proteins, flavodiiron nitric oxide reductases (FNORs) and the hemerythrin-like proteins from mycobacteria (HLPs), are upregulated in bacteria in response to an increased local NO• concentration. While FNORs reduce NO• to nitrous oxide (N2O), the HLPs have been found to either reduce nitrite to NO• (YtfE), or oxidize NO• to nitrite (Mka-HLP). Various structural and functional models of the diiron site in FNORs have been developed over the years. However, the NO• oxidation reactivity of Mka-HLP has yet to be replicated with a synthetic complex. Compared to the FNORs, the coordination environment of the diiron site in Mka-HLP contains one less carboxylate ligand and, therefore, is expected to be more electron-poor. Herein, we synthesized a new diiron complex that models the electron-poor coordination environment of the Mka-HLP diiron site. The diferrous precursor FeIIFeII reacts with NO• to form a diiron dinitrosyl species ({FeNO}72), which is in equilibrium with a mononitrosyl diiron species (FeII{FeNO}7) in solution. Both complexes can be isolated and fully characterized. However, only oxidation of {FeNO}72 produced nitrite in high yield (71%). Our study provides the first model that reproduces the NO• oxidase reactivity of Mka-HLP and suggests intermediacy of an {FeNO}6/{FeNO}7 species.

Emerging drugs targeting cellular redox homeostasis to eliminate acute myeloid leukemia stem cells

Acute myeloid leukemia (AML) is a very heterogeneous group of disorders with large differences in the percentage of immature blasts that presently are classified according to the specific mutations that trigger malignant proliferation among thousands of mutations reported thus far. It is an aggressive disease for which few targeted therapies are available and still has a high recurrence rate and low overall survival. The main reason for AML relapse is believed to be due to leukemic stem cells (LSCs) that have unlimited self-renewal capacity and long residence in a quiescent state, which promote greater resistance to traditional therapies for this cancer. AML LSCs have low oxidative stress levels, which appear to be caused by a combination of low mitochondrial activity and high activity of ROS-removing pathways. In this sense, oxidative stress has been thought to be an important new potential target for the treatment of AML patients, targeting the eradication of AML LSCs. The aim of this review is to discuss some drugs that induce oxidative stress to direct new goals for future research focusing on redox imbalance as an effective strategy to eliminate AML LSCs.

Proteomic analysis of metronidazole resistance in the human facultative pathogen Bacteroides fragilis

The anaerobic gut bacteria and opportunistic pathogen Bacteroides fragilis can cause life-threatening infections when leaving its niche and reaching body sites outside of the gut. The antimicrobial metronidazole is a mainstay in the treatment of anaerobic infections and also highly effective against Bacteroides spp. Although resistance rates have remained low in general, metronidazole resistance does occur in B. fragilis and can favor fatal disease outcomes. Most metronidazole-resistant Bacteroides isolates harbor nim genes, commonly believed to encode for nitroreductases which deactivate metronidazole. Recent research, however, suggests that the mode of resistance mediated by Nim proteins might be more complex than anticipated because they affect the cellular metabolism, e.g., by increasing the activity of pyruvate:ferredoxin oxidoreductase (PFOR). Moreover, although nim genes confer only low-level metronidazole resistance to Bacteroides, high-level resistance can be much easier induced in the laboratory in the presence of a nim gene than without. Due to these observations, we hypothesized that nim genes might induce changes in the B. fragilis proteome and performed comparative mass-spectrometric analyses with B. fragilis 638R, either with or without the nimA gene. Further, we compared protein expression profiles in both strains after induction of high-level metronidazole resistance. Interestingly, only few proteins were repeatedly found to be differentially expressed in strain 638R with the nimA gene, one of them being the flavodiiron protein FprA, an enzyme involved in oxygen scavenging. After induction of metronidazole resistance, a far higher number of proteins were found to be differentially expressed in 638R without nimA than in 638R with nimA. In the former, factors for the import of hemin were strongly downregulated, indicating impaired iron import, whereas in the latter, the observed changes were not only less numerous but also less specific. Both resistant strains, however, displayed a reduced capability of scavenging oxygen. Susceptibility to metronidazole could be widely restored in resistant 638R without nimA by supplementing growth media with ferrous iron sulfate, but not so in resistant 638R with the nimA gene. Finally, based on the results of this study, we present a novel hypothetic model of metronidazole resistance and NimA function.

Why is manganese so valuable to bacterial pathogens?

Apart from oxygenic photosynthesis, the extent of manganese utilization in bacteria varies from species to species and also appears to depend on external conditions. This observation is in striking contrast to iron, which is similar to manganese but essential for the vast majority of bacteria. To adequately explain the role of manganese in pathogens, we first present in this review that the accumulation of molecular oxygen in the Earth's atmosphere was a key event that linked manganese utilization to iron utilization and put pressure on the use of manganese in general. We devote a large part of our contribution to explanation of how molecular oxygen interferes with iron so that it enhances oxidative stress in cells, and how bacteria have learned to control the concentration of free iron in the cytosol. The functioning of iron in the presence of molecular oxygen serves as a springboard for a fundamental understanding of why manganese is so valued by bacterial pathogens. The bulk of this review addresses how manganese can replace iron in enzymes. Redox-active enzymes must cope with the higher redox potential of manganese compared to iron. Therefore, specific manganese-dependent isoenzymes have evolved that either lower the redox potential of the bound metal or use a stronger oxidant. In contrast, redox-inactive enzymes can exchange the metal directly within the individual active site, so no isoenzymes are required. It appears that in the physiological context, only redox-inactive mononuclear or dinuclear enzymes are capable of replacing iron with manganese within the same active site. In both cases, cytosolic conditions play an important role in the selection of the metal used. In conclusion, we summarize both well-characterized and less-studied mechanisms of the tug-of-war for manganese between host and pathogen.

Proteiniphilum and Methanothrix harundinacea became dominant acetate utilizers in a methanogenic reactor operated under strong ammonia stress

Microorganisms in anaerobic digestion (AD) are easily affected by ammonia, especially acetoclastic methanogens. Thus, in ammonia-suppressed AD systems, acetate degradation is reported to be carried out mainly by the cooperation of syntrophic acetate oxidizers and hydrogenotrophic methanogens. Previous studies have revealed ammonia inhibition on microbial flora by AD performance, but the effect mechanism of ammonia on microbial metabolism remains poorly understood. In this study, we constructed a mesophilic chemostat fed with acetate as the sole carbon source, gradually increased the total ammonia nitrogen (TAN) concentration from 1 g L^-1 to 6 g L^-1, and employed the 16S rRNA gene, metagenomics, and metatranscriptomics analysis to characterize the microbial community structure and metabolic behavior. The results showed that even at the TAN of 6 g L^-1 (pH 7.5), the methanogenesis kept normal, the biogas production was approximately 92% of that at TAN of 1 g L^-1 and the acetate degradation ratio reached 99%, suggesting the strong TAN tolerance of the microbial community enriched. 16S rRNA gene analysis suggested that the microbial community structure changed along with the TAN concentration. Methanothrix predominated in methanogens all the time, in which the dominant species was gradually replaced from M. soehngenii to M. harundinacea with the increased TAN. Dominant bacterial species also changed and Proteiniphilum showed a significant positive correlation with increased TAN. Meta-omics analysis showed that the absolute dominant microorganisms at TAN of 6 g L^-1 were M. harundinacea and Proteiniphilum, both of which highly expressed genes for anti-oxidative stress. M. harundinacea and the second dominant methanogen Methanosarcina highly expressed both acetate cleavage and CO₂ reduction pathways, suggesting the possibility that these two pathways contributed to methanogenesis together. Proteiniphilum and some other species in Firmicutes and Synergistetes were likely acetate oxidizers in the community as they highly expressed genes for syntrophic acetate oxidization, H₂ generation, and electron transfer. These results suggested that Proteiniphilum as well as M. harundinacea have strong ammonia tolerance and played critical roles in acetate degradation under ammonia-suppressed conditions. The achievements of the study would contribute to the regulation and management of the AD process.

Exploring optimal Taxol® CYP725A4 activity in Saccharomyces cerevisiae

Background

CYP725A4 catalyses the conversion of the first Taxol® precursor, taxadiene, to taxadiene-5α-ol (T5α-ol) and a range of other mono- and di-hydroxylated side products (oxygenated taxanes). Initially known to undergo a radical rebound mechanism, the recent studies have revealed that an intermediate epoxide mediates the formation of the main characterised products of the enzyme, being T5α-ol, 5(12)-oxa-3(11)-cyclotaxane (OCT) and its isomer, 5(11)-oxa-3(11)-cyclotaxane (iso-OCT) as well as taxadienediols. Besides the high side product: main product ratio and the low main product titre, CYP725A4 is also known for its slow enzymatic activity, massively hindering further progress in heterologous production of Taxol® precursors. Therefore, this study aimed to systematically explore the key parameters for improving the regioselectivity and activity of eukaryotic CYP725A4 enzyme in a whole-cell eukaryotic biocatalyst, Saccharomyces cerevisiae.

Results

Investigating the impact of CYP725A4 and reductase gene dosages along with construction of self-sufficient proteins with strong prokaryotic reductases showed that a potential uncoupling event accelerates the formation of oxygenated taxane products of this enzyme, particularly the side products OCT and iso-OCT. Due to the harmful effect of uncoupling products and the reactive metabolites on the enzyme, the impact of flavins and irons, existing as prosthetic groups in CYP725A4 and reductase, were examined in both their precursor and ready forms, and to investigate the changes in product distribution. We observed that the flavin adenine dinucleotide improved the diterpenoids titres and biomass accumulation. Hemin was found to decrease the titre of iso-OCT and T5α-ol, without impacting the side product OCT, suggesting the latter being the major product of CYP725A4. The interaction between this iron and the iron precursor, δ-Aminolevulinic acid, seemed to improve the production of these diterpenoids, further denoting that iso-OCT and T5α-ol were the later products. While no direct correlation between cellular-level oxidative stress and oxygenated taxanes was observed, investigating the impact of salt and antioxidant on CYP725A4 further showed the significant drop in OCT titre, highlighting the possibility of enzymatic-level uncoupling event and reactivity as the major mechanism behind the enzyme activity. To characterise the product spectrum and production capacity of CYP725A4 in the absence of cell growth, resting cell assays with optimal neutral pH revealed an array of novel diterpenoids along with higher quantities of characterised diterpenoids and independence of the oxygenated product spectra from the acidity effect. Besides reporting on the full product ranges of CYP725A4 in yeast for the first time, the highest total taxanes of around 361.4 ± 52.4 mg/L including 38.1 ± 8.4 mg/L of T5α-ol was produced herein at a small, 10-mL scale by resting cell assay, where the formation of some novel diterpenoids relied on the prior existence of other diterpenes/diterpenoids as shown by statistical analyses.

Conclusions

This study shows how rational strain engineering combined with an efficient design of experiment approach systematically uncovered the promoting effect of uncoupling for optimising the formation of the early oxygenated taxane precursors of Taxol®. The provided strategies can effectively accelerate the design of more efficient Taxol®-producing yeast strains.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12934-022-01922-1.

Oxidative stress and antioxidant mechanisms of obligate anaerobes involved in biological waste treatment processes: A review

In-depth understanding of the molecular mechanisms and physiological consequences of oxidative stress is still limited for anaerobes. Anaerobic biotechnology has become widely accepted by the wastewater/sludge industry as a better alternative to more conventional but costly aerobic processes. However, the functional anaerobic microorganisms used in anaerobic biotechnology are frequently hampered by reactive oxygen/nitrogen species (ROS/RNS)-mediated oxidative stress caused by exposure to stressful factors (e.g., oxygen and heavy metals), which negatively impact treatment performance. Thus, identifying stressful factors and understanding antioxidative defense mechanisms of functional obligate anaerobes are crucial for the optimization of anaerobic bioprocesses. Herein, we present a comprehensive overview of oxidative stress and antioxidant mechanisms of obligate anaerobes involved in anaerobic bioprocesses; as examples, we focus on anaerobic ammonium oxidation bacteria and methanogenic archaea. We summarize the primary stress factors in anaerobic bioprocesses and the cellular antioxidant defense systems of functional anaerobes, a consortia of enzymatic and nonenzymatic mechanisms. The dual role of ROS/RNS in cellular processes is elaborated; at low concentrations, they have vital cell signaling functions, but at high concentrations, they cause oxidative damage. Finally, we highlight gaps in knowledge and future work to uncover antioxidant and damage repair mechanisms in obligate anaerobes. This review provides in-depth insights and guidance for future research on oxidative stress of obligate anaerobes to boost the accurate regulation of anaerobic bioprocesses in challenging and changing operating conditions.

Reversible Glutamate Coordination to High-Valent Nickel Protects the Active Site of a [NiFe] Hydrogenase from Oxygen

NAD⁺-reducing [NiFe] hydrogenases are valuable biocatalysts for H₂-based energy conversion and the regeneration of nucleotide cofactors. While most hydrogenases are sensitive toward O₂ and elevated temperatures, the soluble NAD⁺-reducing [NiFe] hydrogenase from Hydrogenophilus thermoluteolus (HtSH) is O₂-tolerant and thermostable. Thus, it represents a promising candidate for biotechnological applications. Here, we have investigated the catalytic activity and active-site structure of native HtSH and variants in which a glutamate residue in the active-site cavity was replaced by glutamine, alanine, and aspartate. Our biochemical, spectroscopic, and theoretical studies reveal that at least two active-site states of oxidized HtSH feature an unusual architecture in which the glutamate acts as a terminal ligand of the active-site nickel. This observation demonstrates that crystallographically observed glutamate coordination represents a native feature of the enzyme. One of these states is diamagnetic and characterized by a very high stretching frequency of an iron-bound active-site CO ligand. Supported by density-functional-theory calculations, we identify this state as a high-valent species with a biologically unprecedented formal Ni(IV) ground state. Detailed insights into its structure and dynamics were obtained by ultrafast and two-dimensional infrared spectroscopy, demonstrating that it represents a conformationally strained state with unusual bond properties. Our data further show that this state is selectively and reversibly formed under oxic conditions, especially upon rapid exposure to high O₂ levels. We conclude that the kinetically controlled formation of this six-coordinate high-valent state represents a specific and precisely orchestrated stereoelectronic response toward O₂ that could protect the enzyme from oxidative damage.

An Abundant and Diverse New Family of Electron Bifurcating Enzymes With a Non-canonical Catalytic Mechanism

Microorganisms utilize electron bifurcating enzymes in metabolic pathways to carry out thermodynamically unfavorable reactions. Bifurcating FeFe-hydrogenases (HydABC) reversibly oxidize NADH (E′∼−280 mV, under physiological conditions) and reduce protons to H2 gas (E°′−414 mV) by coupling this endergonic reaction to the exergonic reduction of protons by reduced ferredoxin (Fd) (E′∼−500 mV). We show here that HydABC homologs are surprisingly ubiquitous in the microbial world and are represented by 57 phylogenetically distinct clades but only about half are FeFe-hydrogenases. The others have replaced the hydrogenase domain with another oxidoreductase domain or they contain additional subunits, both of which enable various third reactions to be reversibly coupled to NAD+ and Fd reduction. We hypothesize that all of these enzymes carry out electron bifurcation and that their third substrates can include hydrogen peroxide, pyruvate, carbon monoxide, aldehydes, aryl-CoA thioesters, NADP+, cofactor F420, formate, and quinones, as well as many yet to be discovered. Some of the enzymes are proposed to be integral membrane-bound proton-translocating complexes. These different functionalities are associated with phylogenetically distinct clades and in many cases with specific microbial phyla. We propose that this new and abundant class of electron bifurcating enzyme be referred to as the Bfu family whose defining feature is a conserved bifurcating BfuBC core. This core contains FMN and six iron sulfur clusters and it interacts directly with ferredoxin (Fd) and NAD(H). Electrons to or from the third substrate are fed into the BfuBC core via BfuA. The other three known families of electron bifurcating enzyme (abbreviated as Nfn, EtfAB, and HdrA) contain a special FAD that bifurcates electrons to high and low potential pathways. The Bfu family are proposed to use a different electron bifurcation mechanism that involves a combination of FMN and three adjacent iron sulfur clusters, including a novel [2Fe-2S] cluster with pentacoordinate and partial non-Cys coordination. The absolute conservation of the redox cofactors of BfuBC in all members of the Bfu enzyme family indicate they have the same non-canonical mechanism to bifurcate electrons. A hypothetical catalytic mechanism is proposed as a basis for future spectroscopic analyses of Bfu family members.

Defenses of multidrug resistant pathogens against reactive nitrogen species produced in infected hosts

Bacterial pathogens have sophisticated systems that allow them to survive in hosts in which innate immunity is the frontline of defense. One of the substances produced by infected hosts is nitric oxide (NO) that together with its derived species leads to the so-called nitrosative stress, which has antimicrobial properties. In this review, we summarize the current knowledge on targets and protective systems that bacteria have to survive host-generated nitrosative stress. We focus on bacterial pathogens that pose serious health concerns due to the growing increase in resistance to currently available antimicrobials. We describe the role of nitrosative stress as a weapon for pathogen eradication, the detoxification enzymes, protein/DNA repair systems and metabolic strategies that contribute to limiting NO damage and ultimately allow survival of the pathogen in the host. Additionally, this systematization highlights the lack of available data for some of the most important human pathogens, a gap that urgently needs to be addressed.

Practical observations on the use of fluorescent reporter systems in Clostridioides difficile

Fluorescence microscopy is a valuable tool to study a broad variety of bacterial cell components and dynamics thereof. For Clostridioides difficile, the fluorescent proteins CFP^opt, mCherry^Opt and phiLOV2.1, and the self-labelling tags SNAP^Cd and HaloTag, hereafter collectively referred as fluorescent systems, have been described to explore different cellular pathways. In this study, we sought to characterize previously used fluorescent systems in C. difficile cells. We performed single cell analyses using fluorescence microscopy of exponentially growing C. difficile cells harbouring different fluorescent systems, either expressing these separately in the cytosol or fused to the C-terminus of HupA, under defined conditions. We show that the intrinsic fluorescence of C. difficile cells increases during growth, independent of sigB or spo0A. However, when C. difficile cells are exposed to environmental oxygen autofluorescence is enhanced. Cytosolic overexpression of the different fluorescent systems alone, using the same expression signals, showed heterogeneous expression of the fluorescent systems. High levels of mCherry^Opt were toxic for C. difficile cells limiting the applicability of this fluorophore as a transcriptional reporter. When fused to HupA, a C. difficile histone-like protein, the fluorescent systems behaved similarly and did not affect the HupA overproduction phenotype. The present study compares several commonly used fluorescent systems for application as transcriptional or translational reporters in microscopy and summarizes the limitations and key challenges for live-cell imaging of C. difficile. Due to independence of molecular oxygen and fluorescent signal, SNAP^Cd appears the most suitable candidate for live-cell imaging in C. difficile to date.

A Large-Scale Multiple Genome Comparison of Acidophilic Archaea (pH ≤ 5.0) Extends Our Understanding of Oxidative Stress Responses in Polyextreme Environments

Acidophilic archaea thrive in anaerobic and aerobic low pH environments (pH < 5) rich in dissolved heavy metals that exacerbate stress caused by the production of reactive oxygen species (ROS) such as hydrogen peroxide (H2O2), hydroxyl radical (OH) and superoxide (O2−). ROS react with lipids, proteins and nucleic acids causing oxidative stress and damage that can lead to cell death. Herein, genes and mechanisms potentially involved in ROS mitigation are predicted in over 200 genomes of acidophilic archaea with sequenced genomes. These organisms are often be subjected to simultaneous multiple stresses such as high temperature, high salinity, low pH and high heavy metal loads. Some of the topics addressed include: (1) the phylogenomic distribution of these genes and what this can tell us about the evolution of these mechanisms in acidophilic archaea; (2) key differences in genes and mechanisms used by acidophilic versus non-acidophilic archaea and between acidophilic archaea and acidophilic bacteria and (3) how comparative genomic analysis predicts novel genes or pathways involved in oxidative stress responses in archaea and likely horizontal gene transfer (HGT) events.

The Biologically Relevant Coordination Chemistry of Iron and Nitric Oxide: Electronic Structure and Reactivity

Nitric oxide (NO) is an important signaling molecule that is involved in a wide range of physiological and pathological events in biology. Metal coordination chemistry, especially with iron, is at the heart of many biological transformations involving NO. A series of heme proteins, nitric oxide synthases (NOS), soluble guanylate cyclase (sGC), and nitrophorins, are responsible for the biosynthesis, sensing, and transport of NO. Alternatively, NO can be generated from nitrite by heme- and copper-containing nitrite reductases (NIRs). The NO-bearing small molecules such as nitrosothiols and dinitrosyl iron complexes (DNICs) can serve as an alternative vehicle for NO storage and transport. Once NO is formed, the rich reaction chemistry of NO leads to a wide variety of biological activities including reduction of NO by heme or non-heme iron-containing NO reductases and protein post-translational modifications by DNICs. Much of our understanding of the reactivity of metal sites in biology with NO and the mechanisms of these transformations has come from the elucidation of the geometric and electronic structures and chemical reactivity of synthetic model systems, in synergy with biochemical and biophysical studies on the relevant proteins themselves. This review focuses on recent advancements from studies on proteins and model complexes that not only have improved our understanding of the biological roles of NO but also have provided foundations for biomedical research and for bio-inspired catalyst design in energy science.

Oxygen response and tolerance mechanisms in Clostridioides difficile

While the gut is typically thought of as anoxic, there are two intersecting and decreasing oxygen gradients that are observed in the gut: oxygen decreases from the small to the large intestine and from the intestinal epithelium toward the colon lumen. Gut oxygen levels also increase following antibiotic induced-dysbiosis. While dysbiosis favors growth of Clostridioides difficile, the oxygen increase also causes stress to this anaerobic enteropathogen. To circumvent oxygen threat, C. difficile has developed efficient strategies: sporulation, biofilm formation, the rerouting of central metabolism and the production of oxygen detoxification enzymes. Especially, reverse rubrerythrins and flavodiiron proteins involved in oxygen reduction are crucial in C. difficile ability to tolerate and survive the oxygen concentrations encountered in the gastrointestinal tract. Two regulators, σ^B and PerR, play pivotal role in the mastering of these adaptive responses by controlling the various systems that protect cells from oxidative damages.

Functional and protective hole hopping in metalloenzymes

Electrons can tunnel through proteins in microseconds with a modest release of free energy over distances in the 15 to 20 Å range. To span greater distances, or to move faster, multiple charge transfers (hops) are required. When one of the reactants is a strong oxidant, it is convenient to consider the movement of a positively charged "hole" in a direction opposite to that of the electron. Hole hopping along chains of tryptophan (Trp) and tyrosine (Tyr) residues is a critical function in several metalloenzymes that generate high-potential intermediates by reactions with O2 or H2O2, or by activation with visible light. Examination of the protein structural database revealed that Tyr/Trp chains are common protein structural elements, particularly among enzymes that react with O2 and H2O2. In many cases these chains may serve a protective role in metalloenzymes by deactivating high-potential reactive intermediates formed in uncoupled catalytic turnover.

Responses of Clostridia to oxygen: from detoxification to adaptive strategies

Clostridia comprise bacteria of environmental, biotechnological and medical interest and many commensals of the gut microbiota. Because of their strictly anaerobic lifestyle, oxygen is a major stress for Clostridia. However, recent data showed that these bacteria can cope with O₂ better than expected for obligate anaerobes through their ability to scavenge, detoxify and consume O₂ . Upon O₂ exposure, Clostridia redirect their central metabolism onto pathways less O₂ -sensitive and induce the expression of genes encoding enzymes involved in O₂ -reduction and in the repair of oxidized damaged molecules. While Faecalibacterium prausnitzii efficiently consumes O₂ through a specific extracellular electron shuttling system requiring riboflavin, enzymes such as rubrerythrins and flavodiiron proteins with NAD(P)H-dependent O₂ - and/or H₂ O₂ -reductase activities are usually encoded in other Clostridia. These two classes of enzymes play indeed a pivotal role in O₂ tolerance in Clostridioides difficile and Clostridium acetobutylicum. Two main signalling pathways triggering O₂ -induced responses have been described so far in Clostridia. PerR acts as a key regulator of the O₂ - and/or reactive oxygen species-defence machinery while in C. difficile, σ^B , the sigma factor of the general stress response also plays a crucial role in O₂ tolerance by controlling the expression of genes involved in O₂ scavenging and repair systems.

The Amino Acids Motif -32GSSYN36- in the Catalytic Domain of E. coli Flavorubredoxin NO Reductase Is Essential for Its Activity

Flavodiiron proteins (FDPs) are a family of modular and soluble enzymes endowed with nitric oxide and/or oxygen reductase activities, producing N2O or H2O, respectively. The FDP from Escherichia coli, which, apart from the two core domains, possesses a rubredoxin-like domain at the C-terminus (therefore named flavorubredoxin (FlRd)), is a bona fide NO reductase, exhibiting O2 reducing activity that is approximately ten times lower than that for NO. Among the flavorubredoxins, there is a strictly conserved amino acids motif, -G[S,T]SYN-, close to the catalytic diiron center. To assess its role in FlRd’s activity, we designed several site-directed mutants, replacing the conserved residues with hydrophobic or anionic ones. The mutants, which maintained the general characteristics of the wild type enzyme, including cofactor content and integrity of the diiron center, revealed a decrease of their oxygen reductase activity, while the NO reductase activity—specifically, its physiological function—was almost completely abolished in some of the mutants. Molecular modeling of the mutant proteins pointed to subtle changes in the predicted structures that resulted in the reduction of the hydration of the regions around the conserved residues, as well as in the elimination of hydrogen bonds, which may affect proton transfer and/or product release.

Conformational H-bonding modulation of the iron active site cysteine ligand of superoxide reductase: absorption and resonance Raman studies

Superoxide reductases (SORs) are mononuclear non-heme iron enzymes involved in superoxide radical detoxification in some microorganisms. Their atypical active site is made of an iron atom pentacoordinated by four equatorial nitrogen atoms from histidine residues and one axial sulfur atom from a cysteinate residue, which plays a central role in catalysis. In most SORs, the residue immediately following the cysteinate ligand is an asparagine, which belongs to the second coordination sphere and is expected to have a critical influence on the properties of the active site. In this work, in order to investigate the role of this asparagine residue in the Desulfoarculus baarsii enzyme (Asn117), we carried out, in comparison with the wild-type enzyme, absorption and resonance Raman (RR) studies on a SOR mutant in which Asn117 was changed into an alanine. RR analysis was developed in order to assign the different bands using excitation in the (Cys116)-S^-→ Fe³⁺ charge transfer band. By investigating the correlation between the (Cys116)-S^-→ Fe³⁺ charge transfer band maximum with the frequency of each RR band in different SOR forms, we assessed the contribution of the ν(Fe-S) vibration among the different RR bands. The data showed that Asn117, by making hydrogen bond interactions with Lys74 and Tyr76, allows a rigidification of the backbone of the Cys116 ligand, as well as that of the neighboring residues Ile118 and His119. Such a structural role of Asn117 has a deep impact on the S-Fe bond. It results in a tight control of the H-bond distance between the Ile118 and His119 NH peptidic moiety with the cysteine sulfur ligand, which in turn enables fine-tuning of the S-Fe bond strength, an essential property for the SOR active site. This study illustrates the intricate roles of second coordination sphere residues to adjust the ligand to metal bond properties in the active site of metalloenzymes.

Synergistic association between cytochrome bd-encoded Proteiniphilum and reactive oxygen species (ROS)-scavenging methanogens in microaerobic-anaerobic digestion of lignocellulosic biomass

Intermittent (every other day) microaerobic [picomolar oxygen by oxidation-reduction potential (ORP) set at +25 mV above anaerobic baseline] digestion of lignocellulosic biomass showed higher digestibility and better stability at a high organic loading rate (OLR) of 5 g volatile solids (VS)/L/d than that under strict anaerobic conditions. However, the microbial mechanisms supporting the delicate balance under microaeration remain underexplored. On the basis of our previous findings that microbial communities in replicate experiments were dominated by strains of the genus Proteiniphilum but contained diverse taxa of methanogenic archaea, here we recovered related genomes and reconstructed the putative metabolic pathways using a genome-centric metagenomic approach. The highly enriched Proteiniphilum strains were identified as efficient cellulolytic facultative bacterium, which directly degraded lignocellulose to carbon dioxide, formate, and acetate via aerobic respiration and anaerobic fermentation, alternatively. Moreover, high oxygen affinity cytochromes, bd-type terminal oxidases, in Proteiniphilum strains were found to be closely associated with such picomolar oxygen conditions, which has long been overlooked in anaerobic digestion. Furthermore, hydrogenotrophic methanogenesis was the prevalent pathway for methane production while Methanosarcina, Methanobrevibacter, and Methanocorpusculum were the dominant methanogens in the replicate experiments. Importantly, the two functional groups, namely cellulolytic facultative Proteiniphilum strains and methanogens, encoded various antioxidant enzymes. Energy-dependent reactive oxygen species (ROS) scavengers (superoxide reductase (SOR) and rubrerythrin (rbr) were ubiquitously present in different methanogenic taxa in response to replicate-specific ORP levels (-470, -450 and -475 mV). Collectively, cytochrome bd oxidase and ROS defenders may play roles in improving the digestibility and stability observed in intermittent microaerobic digestion.

Unlocking Survival Mechanisms for Metal and Oxidative Stress in the Extremely Acidophilic, Halotolerant Acidihalobacter Genus

Microorganisms used for the biohydrometallurgical extraction of metals from minerals must be able to survive high levels of metal and oxidative stress found in bioleaching environments. The Acidihalobacter genus consists of four species of halotolerant, iron–sulfur-oxidizing acidophiles that are unique in their ability to tolerate chloride and acid stress while simultaneously bioleaching minerals. This paper uses bioinformatic tools to predict the genes and mechanisms used by Acidihalobacter members in their defense against a wide range of metals and oxidative stress. Analysis revealed the presence of multiple conserved mechanisms of metal tolerance. Ac. yilgarnensis F5^T, the only member of this genus that oxidizes the mineral chalcopyrite, contained a 39.9 Kb gene cluster consisting of 40 genes encoding mobile elements and an array of proteins with direct functions in copper resistance. The analysis also revealed multiple strategies that the Acidihalobacter members can use to tolerate high levels of oxidative stress. Three of the Acidihalobacter genomes were found to contain genes encoding catalases, which are not common to acidophilic microorganisms. Of particular interest was a rubrerythrin genomic cluster containing genes that have a polyphyletic origin of stress-related functions.

Rubredoxin from the green sulfur bacterium Chlorobaculum tepidum donates a redox equivalent to the flavodiiron protein in an NAD(P)H dependent manner via ferredoxin-NAD(P)+ oxidoreductase

The green sulfur bacterium, Chlorobaculum tepidum, is an anaerobic photoautotroph that performs anoxygenic photosynthesis. Although genes encoding rubredoxin (Rd) and a putative flavodiiron protein (FDP) were reported in the genome, a gene encoding putative NADH-Rd oxidoreductase is not identified. In this work, we expressed and purified the recombinant Rd and FDP and confirmed dioxygen reductase activity in the presence of ferredoxin-NAD(P)⁺ oxidoreductase (FNR). FNR from C. tepidum and Bacillus subtilis catalyzed the reduction of Rd at rates comparable to those reported for NADH-Rd oxidoreductases. Also, we observed substrate inhibition at high concentrations of NADPH similar to that observed with ferredoxins. In the presence of NADPH, B. subtilis FNR and Rd, FDP promoted dioxygen reduction at rates comparable to those reported for other bacterial FDPs. Taken together, our results suggest that Rd and FDP participate in the reduction of dioxygen in C. tepidum and that FNR can promote the reduction of Rd in this bacterium.

How the Anaerobic Enteropathogen Clostridioides difficile Tolerates Low O2 Tensions

Although the gastrointestinal tract is regarded as mainly anoxic, low O₂ tension is present in the gut and tends to increase following antibiotic-induced disruption of the host microbiota. Two decreasing O₂ gradients are observed, a longitudinal one from the small to the large intestine and a second one from the intestinal epithelium toward the colon lumen. Thus, O₂ concentration fluctuations within the gastrointestinal tract are a challenge for anaerobic bacteria such as C. difficile. This enteropathogen has developed efficient strategies to detoxify O₂. In this work, we identified reverse rubrerythrins and flavodiiron proteins as key actors for O₂ tolerance in C. difficile. These enzymes are responsible for the reduction of O₂ protecting C. difficile vegetative cells from associated damages. Original and complex detoxification pathways involving O₂-reductases are crucial in the ability of C. difficile to tolerate O₂ and survive to O₂ concentrations encountered in the gastrointestinal tract.

Clostridioides difficile is a major cause of diarrhea associated with antibiotherapy. After germination of C. difficile spores in the small intestine, vegetative cells are exposed to low oxygen (O₂) tensions. While considered strictly anaerobic, C. difficile is able to grow in nonstrict anaerobic conditions (1 to 3% O₂) and tolerates brief air exposure indicating that this bacterium harbors an arsenal of proteins involved in O₂ detoxification and/or protection. Tolerance of C. difficile to low O₂ tensions requires the presence of the alternative sigma factor, σ^B, involved in the general stress response. Among the genes positively controlled by σ^B, four encode proteins likely involved in O₂ detoxification: two flavodiiron proteins (FdpA and FdpF) and two reverse rubrerythrins (revRbr1 and revRbr2). As previously observed for FdpF, we showed that both purified revRbr1 and revRbr2 harbor NADH-linked O₂- and H₂O₂-reductase activities in vitro, while purified FdpA mainly acts as an O₂-reductase. The growth of a fdpA mutant is affected at 0.4% O₂, while inactivation of both revRbrs leads to a growth defect above 0.1% O₂. O₂-reductase activities of these different proteins are additive since the quadruple mutant displays a stronger phenotype when exposed to low O₂ tensions compared to the triple mutants. Our results demonstrate a key role for revRbrs, FdpF, and FdpA proteins in the ability of C. difficile to grow in the presence of physiological O₂ tensions such as those encountered in the colon.

Identification of the electron donor to flavodiiron proteins in Synechocystis sp. PCC 6803 by in vivo spectroscopy

Flavodiiron proteins (FDPs) of photosynthetic organisms play a photoprotective role by reducing oxygen to water and thus avoiding the accumulation of excess electrons on the photosystem I (PSI) acceptor side under stress conditions. In Synechocystis sp. PCC 6803 grown under high CO₂, both FDPs Flv1 and Flv3 are indispensable for oxygen reduction. We performed a detailed in vivo kinetic study of wild-type (WT) and Δflv1/3 strains of Synechocystis using light-induced NADPH fluorescence and near-infrared absorption of iron-sulfur clusters from ferredoxin and the PSI acceptors (F_AF_B), collectively named FeS. These measurements were performed under conditions where the Calvin-Benson cycle is inactive or poorly activated. Under such conditions, the NADPH decay following a short illumination decays in parallel in both strains and exhibits a time lag which is correlated to the presence of reduced FeS. On the contrary, reduced FeS decays much faster in WT than in Δflv1/3 (13 vs 2 s^-1). These data unambiguously show that reduced ferredoxin, or possibly reduced F_AF_B, is the direct electron donor to the Flv1/Flv3 heterodimer. Evidences for large reduction of (F_AF_B) and recombination reactions within PSI were also provided by near-infrared absorption. Mutants lacking either the NDH1-L complex, the homolog of complex I of respiration, or the Pgr5 protein show no difference with WT in the oxidation of reduced FeS following a short illumination. These observations question the participation of a significant cyclic electron flow in cyanobacteria during the first seconds of the induction phase of photosynthesis.

Biological and Bioinspired Inorganic N-N Bond-Forming Reactions

The metallobiochemistry underlying the formation of the inorganic N-N-bond-containing molecules nitrous oxide (N2O), dinitrogen (N2), and hydrazine (N2H4) is essential to the lifestyles of diverse organisms. Similar reactions hold promise as means to use N-based fuels as alternative carbon-free energy sources. This review discusses research efforts to understand the mechanisms underlying biological N-N bond formation in primary metabolism and how the associated reactions are tied to energy transduction and organismal survival. These efforts comprise studies of both natural and engineered metalloenzymes as well as synthetic model complexes.

Rational redox tuning of transition metal sites: learning from superoxide reductase

Using superoxide reductase as a model system, a computational approach reveals how histidine tautomerism tunes the redox properties of metalloenzymes to enable their catalytic function. Inspired by these experimentally inaccessible insights, non-canonical histidine congeners are introduced as new versatile tools for the rational engineering of biological transition metal sites.

Quantitative Proteomic Analysis of the Response of Probiotic Putative Lactococcus lactis NCDO 2118 Strain to Different Oxygen Availability Under Temperature Variation

Lactococcus lactis is a gram positive facultative anaerobe widely used in the dairy industry and human health. L. lactis subsp. lactis NCDO 2118 is a strain that exhibits anti-inflammatory and immunomodulatory properties. In this study, we applied a label-free shotgun proteomic approach to characterize and quantify the NCDO 2118 proteome in response to variations of temperature and oxygen bioavailability, which constitute the environmental conditions found by this bacterium during its passage through the host gastro-intestinal tract and in other industrial processes. From this proteomic analysis, a total of 1,284 non-redundant proteins of NCDO 2118 were characterized, which correspond to approximately 54% of its predicted proteome. Comparative proteomic analysis identified 149 and 136 proteins in anaerobic (30°C and 37°C) and non-aerated (30°C and 37°C) conditions, respectively. Our label-free proteomic analysis quantified a total of 1,239 proteins amongst which 161 proteins were statistically differentially expressed. Main differences were observed in cellular metabolism, stress response, transcription and proteins associated to cell wall. In addition, we identified six strain-specific proteins of NCDO 2118. Altogether, the results obtained in our study will help to improve the understanding about the factors related to both physiology and adaptive processes of L. lactis NCDO 2118 under changing environmental conditions.