A Streptococcus mutans superoxide dismutase that is active with either manganese or iron as a cofactor

SloR-SRE binding to the S. mutans mntH promoter is cooperative

Streptococcus mutans is a commensal member of the plaque microbiome. It is especially prevalent when dietary sugars are available for S. mutans fermentation, generating acid byproducts that lower plaque pH and foster tooth decay. S. mutans can survive in the transient conditions of the mouth, in part because it can regulate the uptake of manganese and iron during periods of feast when metal ions are available, and famine when they are limited. S. mutans depends on a 25kDa metalloregulatory protein, called SloR, to modulate the uptake of these cations across the bacterial cell surface. When bound to manganese, SloR binds to palindromic recognition elements in the promoter of the sloABC genes that encode the major manganese transporter in S. mutans. Reports in the literature describe MntH, an ancillary manganese transporter in S. mutans, that is also subject to SloR control. In the present study, we performed expression profiling experiments that reveal coordinate regulation of the sloABC and mntH genes at the level of transcription. In addition, we describe a role for the mntH gene product that is redundant with that of the sloABC-encoded metal ion uptake machinery. The results of DNA-binding studies support direct SloR binding to the mntH promoter region which, like that at the sloABC promoter, harbors three palindromic recognition elements to which SloR binds cooperatively to repress downstream transcription. These findings expand our understanding of the SloR metalloregulome and elucidate SloR-DNA binding that is essential for S. mutans metal ion homeostasis and fitness in the oral cavity.

IMPORTANCE

Dental caries disproportionately impacts low-income socioeconomic groups in the United States and abroad. Research that is focused on S. mutans, the primary causative agent of dental caries in humans, is significant to mitigation efforts aimed at alleviating or preventing dental caries. The SloR protein is a major regulator of the S. mutans metal ion uptake machinery encoded by the sloABC- and mntH genes. This SloR-mediated gene control is essential for maintaining intracellular metal ion homeostasis, and hence S. mutans fitness in the plaque microbiome. An improved understanding of the sloABC and mntH metal ion transporters and their regulation by SloR can guide rational drug design that, by targeting the SloR-DNA-binding interface, can alleviate or prevent S. mutans-induced disease.

SloR-SRE binding to the S. mutans mntH promoter is cooperative

Streptococcus mutans is a commensal member of the plaque microbiome. It is especially prevalent when dietary sugars are available for S. mutans fermentation, generating acid byproducts that lower plaque pH and foster tooth decay. S. mutans can survive in the transient conditions of the mouth, in part because it can regulate the uptake of manganese and iron during periods of feast when metal ions are available, and famine when they are limiting. S. mutans depends on a 25kDa metalloregulatory protein, called SloR, to modulate uptake of these cations across the bacterial cell surface. When bound to manganese, SloR, binds to palindromic recognition elements in the promoter of the sloABC genes that encode the major manganese transporter in S. mutans. Reports in the literature describ MntH, an ancillary manganese transporter in S. mutans, that is also subject to SloR control. In the present study, we performed expression profiling experiments that reveal coordinate regulation of the sloABC and mntH genes at the level of transcription. In addition, we describe a role for the mntH gene product that is redundant with that of the sloABC-encoded metal ion uptake machinery. The results of DNA binding studies support direct SloR binding to the mntH promoter region which, like that at the sloABC promoter, harbors three palindromic recognition elements to which SloR binds cooperatively to repress downstream transcription. These findings expand our understanding of the SloR metalloregulome and elucidate SloR-DNA binding that is essential for S. mutans metal ion homeostasis and fitness in the oral cavity.

Human saliva modifies growth, biofilm architecture, and competitive behaviors of oral streptococci

The bacteria within supragingival biofilms participate in complex exchanges with other microbes inhabiting the same niche. One example is the mutans group streptococci (Streptococcus mutans), implicated in the development of tooth decay, and other health-associated commensal streptococci species. Previously, our group transcriptomically characterized intermicrobial interactions between S. mutans and several species of oral bacteria. However, these experiments were carried out in a medium without human saliva. To better mimic their natural environment, we first evaluated how inclusion of saliva affected growth and biofilm formation of eight Streptococcus species individually and found saliva to positively benefit growth rates while negatively influencing biofilm biomass accumulation and altering spatial arrangement. These results carried over during evaluation of 29 saliva-derived isolates of various species. Surprisingly, we also found that addition of saliva increased the competitive behaviors of S. mutans in coculture competitions against commensal streptococci that led to increases in biofilm microcolony volumes. Through transcriptomically characterizing mono- and cocultures of S. mutans and Streptococcus oralis with and without saliva, we determined that each species developed a nutritional niche under mixed-species growth, with S. mutans upregulating carbohydrate uptake and utilization pathways while S. oralis upregulated genome features related to peptide uptake and glycan foraging. S. mutans also upregulated genes involved in oxidative stress tolerance, particularly manganese uptake, which we could artificially manipulate by supplementing in manganese leading to an advantage over its opponent. Our report highlights observable changes in microbial behaviors through leveraging environmental- and host-supplied resources over their competitors.

IMPORTANCE

Dental caries (tooth decay) is the most prevalent disease for both children and adults nationwide. Caries are initiated from demineralization of the enamel due to organic acid production through the metabolic activity of oral bacteria growing in biofilm communities attached to the tooth's surface. Mutans group streptococci are closely associated with caries development and initiation of the cariogenic cycle, which decreases the amount of acid-sensitive, health-associated commensal bacteria while selecting for aciduric and acidogenic species that then further drives the disease process. Defining the exchanges that occur between mutans group streptococci and oral commensals in a condition that closely mimics their natural environment is of critical need toward identifying factors that can influence odontopathogen establishment, persistence, and outgrowth. The goal of our research is to develop strategies, potentially through manipulation of microbial interactions characterized here, that prevent the emergence of mutans group streptococci while keeping the protective flora intact.

Oxidative stress response: a critical factor affecting the ecological competitiveness of Streptococcus mutans

Oral microecological balance is closely associated with the development of dental caries. Oxidative stress is one of the important factors regulating the composition and structure of the oral microbial community. Streptococcus mutans is linked to the occurrence and development of dental caries. The ability of S. mutans to withstand oxidative stress affects its survival competitiveness in biofilms. The oxidative stress regulatory mechanisms of S. mutans include synthesis of reductase, regulation of metal ions uptake, regulator PerR, transcription regulator Spx, extracellular uptake of glutathione, and other related signal transduction systems. Here, we provide an overview of how S. mutans adapts to oxidative stress and its influence on oral microecology, which may offer novel options to investigate the cariogenic mechanisms of S. mutans in the oral microenvironment, and new targets for the ecological prevention and treatment of dental caries.

Small regulatory RNAs are mediators of the Streptococcus mutans SloR regulon

Dental caries is among the most prevalent chronic diseases worldwide. Streptococcus mutans, the chief causative agent of caries, uses a 25-kDa manganese-dependent SloR protein to coordinate the uptake of essential manganese with the transcription of its virulence attributes. Small non-coding RNAs (sRNAs) can either enhance or repress gene expression, and reports in the literature ascribe an emerging role for sRNAs in the environmental stress response. Herein, we focused our attention on 18-50 nt sRNAs as mediators of the S. mutans SloR and manganese regulons. Specifically, the results of RNA sequencing revealed 19 sRNAs in S. mutans, which were differentially transcribed in the SloR-proficient UA159 and SloR-deficient GMS584 strains, and 10 sRNAs that were differentially expressed in UA159 cells grown in the presence of low vs high manganese. We describe SmsR1532 and SmsR1785 as SloR- and manganese-responsive sRNAs that are processed from large transcripts and that bind SloR directly in their promoter regions. The predicted targets of these sRNAs include regulators of metal ion transport, growth management via a toxin-antitoxin operon, and oxidative stress tolerance. These findings support a role for sRNAs in coordinating intracellular metal ion homeostasis with virulence gene control in an important oral cariogen. IMPORTANCE Small regulatory RNAs (sRNAs) are critical mediators of environmental signaling, particularly in bacterial cells under stress, but their role in Streptococcus mutans is poorly understood. S. mutans, the principal causative agent of dental caries, uses a 25-kDa manganese-dependent protein, called SloR, to coordinate the regulated uptake of essential metal ions with the transcription of its virulence genes. In the present study, we identified and characterized sRNAs that are both SloR and manganese responsive. Taken together, this research can elucidate the details of regulatory networks that engage sRNAs in an important oral pathogen and that can enable the development of an effective anti-caries therapeutic.

Human Saliva Modifies Growth, Biofilm Architecture and Competitive Behaviors of Oral Streptococci

The bacteria within supragingival biofilms participate in complex exchanges with other microbes inhabiting the same niche. One example are the mutans group streptococci (Streptococcus mutans), implicated in the development of tooth decay, and other health-associated commensal streptococci species. Previously, our group transcriptomically characterized intermicrobial interactions between S. mutans and several species of oral bacteria. However, these experiments were carried out in a medium that was absent of human saliva. To better mimic their natural environment, we first evaluated how inclusion of saliva affected growth and biofilm formation of eight streptococci species individually, and found saliva to positively benefit growth rates while negatively influencing biomass accumulation and altering spatial arrangement. These results carried over during evaluation of 29 saliva-derived isolates of various species. Surprisingly, we also found that addition of saliva increased the competitive behaviors of S. mutans in coculture competitions against commensal streptococci that led to increases in biofilm microcolony volumes. Through transcriptomically characterizing mono- and cocultures of S. mutans and Streptococcus oralis with and without saliva, we determined that each species developed a nutritional niche under mixed-species growth, with S. mutans upregulating carbohydrate uptake and utilization pathways while S. oralis upregulated genome features related to peptide uptake and glycan foraging. S. mutans also upregulated genes involved in oxidative stress tolerance, particularly manganese uptake, which we could artificially manipulate by supplementing in manganese to give it an advantage over its opponent. Our report highlights observable changes in microbial behaviors via leveraging environmental- and host-supplied resources over their competitors.

Small regulatory RNAs are mediators of the Streptococcus mutans SloR regulon

Dental caries is among the most prevalent chronic infectious diseases worldwide. Streptococcus mutans , the chief causative agent of caries, uses a 25 kDa manganese dependent SloR protein to coordinate the uptake of essential manganese with the transcription of its virulence attributes. Small non-coding RNAs (sRNAs) can either enhance or repress gene expression and reports in the literature ascribe an emerging role for sRNAs in the environmental stress response. Herein, we identify 18-50 nt sRNAs as mediators of the S. mutans SloR and manganese regulons. Specifically, the results of sRNA-seq revealed 56 sRNAs in S. mutans that were differentially transcribed in the SloR-proficient UA159 and SloR-deficient GMS584 strains, and 109 sRNAs that were differentially expressed in UA159 cells grown in the presence of low versus high manganese. We describe SmsR1532 and SmsR1785 as SloR- and/or manganese-responsive sRNAs that are processed from large transcripts, and that bind SloR directly in their promoter regions. The predicted targets of these sRNAs include regulators of metal ion transport, growth management via a toxin-antitoxin operon, and oxidative stress tolerance. These findings support a role for sRNAs in coordinating intracellular metal ion homeostasis with virulence gene control in an important oral cariogen.

IMPORTANCE

Small regulatory RNAs (sRNAs) are critical mediators of environmental signaling, particularly in bacterial cells under stress, but their role in Streptococcus mutans is poorly understood. S. mutans, the principal causative agent of dental caries, uses a 25 kDa manganese-dependent protein, called SloR, to coordinate the regulated uptake of essential metal ions with the transcription of its virulence genes. In the present study, we identified and characterize sRNAs that are both SloR- and manganese-responsive. Taken together, this research can elucidate the details of regulatory networks that engage sRNAs in an important oral pathogen, and that can enable the development of an effective anti-caries therapeutic.

Underlying Role of Hydrophobic Environments in Tuning Metal Elements for Efficient Enzyme Catalysis

The catalytic functions of metalloenzymes are often strongly correlated with metal elements in the active sites. However, dioxygen-activating nonheme quercetin dioxygenases (QueD) are found with various first-row transition-metal ions when metal swapping inactivates their innate catalytic activity. To unveil the molecular basis of this seemingly promiscuous yet metal-specific enzyme, we transformed manganese-dependent QueD into a nickel-dependent enzyme by sequence- and structure-based directed evolution. Although the net effect of acquired mutations was primarily to rearrange hydrophobic residues in the active site pocket, biochemical, kinetic, X-ray crystallographic, spectroscopic, and computational studies suggest that these modifications in the secondary coordination spheres can adjust the electronic structure of the enzyme-substrate complex to counteract the effects induced by the metal substitution. These results explicitly demonstrate that such noncovalent interactions encrypt metal specificity in a finely modulated manner, revealing the underestimated chemical power of the hydrophobic sequence network in enzyme catalysis.

Characterization of ancestral Fe/Mn superoxide dismutases indicates their cambialistic origin

Superoxide dismutases (SODs) are critical metalloenzymes mitigating the damages of the modern oxygenated world. However, the emergence of one family of SODs, the Fe/Mn SOD, has been recurrently proposed to predate the great oxygenation event (GOE). This ancient family lacks metal binding selectivity, but displays strong catalytic selectivity. Therefore, some homologues would only be active when bound to Fe or Mn, although others, dubbed cambialistic, would function when loaded with either ion. This posed the longstanding question about the identity of the cognate metal ion of the first SODs to emerge. In this work, we utilize ancestral sequence reconstruction techniques to infer the earliest SODs. We show that the "ancestors" are active in vivo and in vitro. Further, we test their metal specificity and demonstrate that they are cambialistic in nature. Our findings shed light on how the predicted Last Common Universal Ancestor was capable of dealing with decomposition of the superoxide anion, and the early relationship between life, oxygen, and metal ion availability.

Old dogs, new tricks: New insights into the iron/manganese superoxide dismutase family

Superoxide dismutases (SODs) are ancient enzymes of widespread importance present in all domains of life. Many insights have been gained into these important enzymes over the 50 years since their initial description, but recent studies in the context of microbial pathogenesis have resulted in findings that challenge long established dogmas. The repertoire of SODs that bacterial pathogens encode is diverse both in number and in metal dependencies, including copper, copper and zinc, manganese, iron, and cambialistic enzymes. Other bacteria also possess nickel dependent SODs. Compartmentalization of SODs only partially explains their diversity. The need for pathogens to maintain SOD activity across distinct hostile environments encountered during infection, including those limited for essential metals, is also a driver of repertoire diversity. SOD research using pathogenic microbes has also revealed the apparent biochemical ease with which metal specificity can change within the most common family of SODs. Collectively, these studies are revealing the dynamic nature of SOD evolution, both that of individual SOD enzymes that can change their metal specificity to adapt to fluctuating cellular metal availability, and of a cell's repertoire of SOD isozymes that can be differentially expressed to adapt to fluctuating environmental metal availability in a niche.

Distribution of a Sulfolane-Metabolizing Rhodoferax sp. Throughout a Contaminated Subarctic Aquifer and Two Groundwater Treatment Systems

An extensive plume of the emerging contaminant sulfolane has been found emanating from a refinery in Interior Alaska, raising questions about the microbial potential for natural attenuation and bioremediation in this subarctic aquifer. Previously, an aerobic sulfolane-assimilating Rhodoferax sp. was identified from the aquifer using stable isotope probing. Here, we assessed the distribution of known sulfolane-assimilating bacteria throughout the contaminated subarctic aquifer using 16S-rRNA-amplicon analyses of ~100 samples collected from groundwater monitoring wells and two groundwater treatment systems. One treatment system was an in situ air sparging system where air was injected directly into the aquifer. The other was an ex situ granular activated carbon (GAC) filtration system for the treatment of private well water. We found that the sulfolane-assimilating Rhodoferax sp. was present throughout the aquifer but was significantly more abundant in groundwater associated with the air sparge system. The reduction of sulfolane concentrations combined with the apparent enrichment of sulfolane degraders in the air sparging zone suggests that the addition of oxygen facilitated sulfolane biodegradation. To investigate other environmental controls on Rhodoferax populations, we also examined correlations between groundwater geochemical parameters and the relative abundance of the Rhodoferax sp. and found only manganese to be significantly positively correlated. The sulfolane-assimilating Rhodoferax sp. was not a major component of the GAC filtration system, suggesting that biodegradation is not an important contributor to sulfolane removal in these systems. We conclude that air sparging is a promising approach for enhancing the abundance and activity of aerobic sulfolane-degraders like Rhodoferax to locally stimulate sulfolane biodegradation in situ.

Calculating metalation in cells reveals CobW acquires CoII for vitamin B12 biosynthesis while related proteins prefer ZnII

Protein metal-occupancy (metalation) in vivo has been elusive. To address this challenge, the available free energies of metals have recently been determined from the responses of metal sensors. Here, we use these free energy values to develop a metalation-calculator which accounts for inter-metal competition and changing metal-availabilities inside cells. We use the calculator to understand the function and mechanism of GTPase CobW, a predicted Co^II-chaperone for vitamin B₁₂. Upon binding nucleotide (GTP) and Mg^II, CobW assembles a high-affinity site that can obtain Co^II or Zn^II from the intracellular milieu. In idealised cells with sensors at the mid-points of their responses, competition within the cytosol enables Co^II to outcompete Zn^II for binding CobW. Thus, Co^II is the cognate metal. However, after growth in different [Co^II], Co^II-occupancy ranges from 10 to 97% which matches CobW-dependent B₁₂ synthesis. The calculator also reveals that related GTPases with comparable Zn^II affinities to CobW, preferentially acquire Zn^II due to their relatively weaker Co^II affinities. The calculator is made available here for use with other proteins.

An evolutionary path to altered cofactor specificity in a metalloenzyme

Almost half of all enzymes utilize a metal cofactor. However, the features that dictate the metal utilized by metalloenzymes are poorly understood, limiting our ability to manipulate these enzymes for industrial and health-associated applications. The ubiquitous iron/manganese superoxide dismutase (SOD) family exemplifies this deficit, as the specific metal used by any family member cannot be predicted. Biochemical, structural and paramagnetic analysis of two evolutionarily related SODs with different metal specificity produced by the pathogenic bacterium Staphylococcus aureus identifies two positions that control metal specificity. These residues make no direct contacts with the metal-coordinating ligands but control the metal’s redox properties, demonstrating that subtle architectural changes can dramatically alter metal utilization. Introducing these mutations into S. aureus alters the ability of the bacterium to resist superoxide stress when metal starved by the host, revealing that small changes in metal-dependent activity can drive the evolution of metalloenzymes with new cofactor specificity.

Many metalloenzymes are highly specific for their cognate metal ion but the molecular principles underlying this specificity often remain unclear. Here, the authors characterize the structural and biochemical basis for the different metal specificity of two evolutionarily related superoxide dismutases.

Disruption of l-Rhamnose Biosynthesis Results in Severe Growth Defects in Streptococcus mutans

The rhamnose-glucose cell wall polysaccharide (RGP) of Streptococcus mutans plays a significant role in cell division, virulence, and stress protection. Prior studies examined function of the RGP using strains carrying deletions in the machinery involved in RGP assembly. In this study, we explored loss of the substrate for RGP, l-rhamnose, via deletion of rmlD (encoding the protein responsible for the terminal step in l-rhamnose biosynthesis). We demonstrate that loss of rhamnose biosynthesis causes a phenotype similar to strains with disrupted RGP assembly (ΔrgpG and ΔrgpF strains). Deletion of rmlD not only caused a severe growth defect under nonstress growth conditions but also elevated susceptibility of the strain to acid and oxidative stress, common conditions found in the oral cavity. A genetic complement of the ΔrmlD strain completely restored wild-type levels of growth, whereas addition of exogenous rhamnose did not. The loss of rhamnose production also significantly disrupted biofilm formation, an important aspect of S. mutans growth in the oral cavity. Further, we demonstrate that loss of either rmlD or rgpG results in ablation of rhamnose content in the S. mutans cell wall. Taken together, these results highlight the importance of rhamnose production in both the fitness and the ability of S. mutans to overcome environmental stresses.IMPORTANCE Streptococcus mutans is a pathogenic bacterium that is the primary etiologic agent of dental caries, a disease that affects billions yearly. Rhamnose biosynthesis is conserved not only in streptococcal species but in other Gram-positive, as well as Gram-negative, organisms. This study highlights the importance of rhamnose biosynthesis in RGP production for protection of the organism against acid and oxidative stresses, the two major stressors that the organism encounters in the oral cavity. Loss of RGP also severely impacts biofilm formation, the first step in the onset of dental caries. The high conservation of the rhamnose synthesis enzymes, as well as their importance in S. mutans and other organisms, makes them favorable antibiotic targets for the treatment of disease.

The Crystal Structure of the Manganese Superoxide Dismutase from Geobacillus stearothermophilus: Parker and Blake (1988) Revisited

Superoxide dismutase (SOD) is an almost ubiquitous metalloenzyme in aerobic organisms that catalyses the disproportionation of superoxide. Geobacillus stearothermophilus MnSOD is the only published MnSOD structure that does not have its coordinates publicly available, yet it is one of the more cited structures in the SOD literature. The structure has now been refined with modern programs, yielding a significantly improved structure which has been deposited in the Protein Data Bank. Importantly, the further refined structure reveals the presence of a catalytically important fifth ligand, water, to the metal centre, as observed in other SOD structures.

Chemical Warfare at the Microorganismal Level: A Closer Look at the Superoxide Dismutase Enzymes of Pathogens

Superoxide anion radical is generated as a natural byproduct of aerobic metabolism but is also produced as part of the oxidative burst of the innate immune response design to kill pathogens. In living systems, superoxide is largely managed through superoxide dismutases (SODs), families of metalloenzymes that use Fe, Mn, Ni, or Cu cofactors to catalyze the disproportionation of superoxide to oxygen and hydrogen peroxide. Given the bursts of superoxide faced by microbial pathogens, it comes as no surprise that SOD enzymes play important roles in microbial survival and virulence. Interestingly, microbial SOD enzymes not only detoxify host superoxide but also may participate in signaling pathways that involve reactive oxygen species derived from the microbe itself, particularly in the case of eukaryotic pathogens. In this Review, we will discuss the chemistry of superoxide radicals and the role of diverse SOD metalloenzymes in bacterial, fungal, and protozoan pathogens. We will highlight the unique features of microbial SOD enzymes that have evolved to accommodate the harsh lifestyle at the host-pathogen interface. Lastly, we will discuss key non-SOD superoxide scavengers that specific pathogens employ for defense against host superoxide.

Carboxylic acid production from ensiled crops in anaerobic solid-state fermentation - trace elements as pH controlling agents support microbial chain elongation with lactic acid

BACKGROUND

For the production of carboxylic acid platform chemicals like medium-chain fatty acids (MCFA) by anaerobic fermentation, pH control is required. However, adding buffer solutions is ineffective in leach-bed reactors.

AIM

In order to increase the MCFA production by maize silage fermentation and to engineer the process we investigated the effect of solid alkaline iron and manganese additives on the process performance and microbial community dynamics.

RESULTS

Without additives, the pH dropped to 3.9 and lactic acid bacteria were favored. Total product yields of 207 ± 5.4 g organic acids (C₂-C₆) and alcohols per kg volatile solids were reached. The addition of trace elements increased the pH value and the product spectrum and yields changed. With a commercial iron additive, the product yields were higher (293 ± 15.2 g/kg_{volatile solids}) and supposedly clostridia used lactic acid for microbial chain elongation of acetic acid producing n-butyric acid. With the addition of pure Fe(OH)₃ or Mn(OH)₂, the total product yields were lower than in the other reactors. However, increased production of MCFA and the occurrence of distinct bacterial taxa (Lachnospiraceae, Ruminococcaceae and Megasphaera) related to this metabolic function were observed.

CONCLUSIONS

The application of alkaline trace metal additives as pH stabilizing agents can mitigate spatial metabolic heterogeneities when trace metal deficient substrates like specific crops or residues thereof are applied.

Oxidative Stressors Modify the Response of Streptococcus mutans to Its Competence Signal Peptides

The dental caries pathogen Streptococcus mutans is continually exposed to several types of stress in the oral biofilm environment. Oxidative stress generated by reactive oxygen species has a major impact on the establishment, persistence, and virulence of S. mutans Here, we combined fluorescent reporter-promoter fusions with single-cell imaging to study the effects of reactive oxygen species on activation of genetic competence in S. mutans Exposure to paraquat, which generates superoxide anion, produced a qualitatively different effect on activation of expression of the gene for the master competence regulator, ComX, than did treatment with hydrogen peroxide (H₂O₂), which can yield hydroxyl radical. Paraquat suppressed peptide-mediated induction of comX in a progressive and cumulative fashion, whereas the response to H₂O₂ displayed a strong threshold behavior. Low concentrations of H₂O₂ had little effect on induction of comX or the bacteriocin gene cipB, but expression of these genes declined sharply if extracellular H₂O₂ exceeded a threshold concentration. These effects were not due to decreased reporter gene fluorescence. Two different threshold concentrations were observed in the response to H₂O₂, depending on the gene promoter that was analyzed and the pathway by which the competence regulon was stimulated. The results show that paraquat and H₂O₂ affect the S. mutans competence signaling pathway differently, and that some portions of the competence signaling pathway are more sensitive to oxidative stress than others.IMPORTANCEStreptococcus mutans inhabits the oral biofilm, where it plays an important role in the development of dental caries. Environmental stresses such as oxidative stress influence the growth of S. mutans and its important virulence-associated behaviors, such as genetic competence. S. mutans competence development is a complex behavior that involves two different signaling peptides and can exhibit cell-to-cell heterogeneity. Although oxidative stress is known to influence S. mutans competence, it is not understood how oxidative stress interacts with the peptide signaling or affects heterogeneity. In this study, we used fluorescent reporters to probe the effect of reactive oxygen species on competence signaling at the single-cell level. Our data show that different reactive oxygen species have different effects on S. mutans competence, and that some portions of the signaling pathway are more acutely sensitive to oxidative stress than others.

Inorganic pyrophosphatases of Family II-two decades after their discovery

Inorganic pyrophosphatases (PPases) convert pyrophosphate (PP_i ) to phosphate and are present in all cell types. Soluble PPases belong to three nonhomologous families, of which Family II is found in approximately a quarter of prokaryotic organisms, often pathogenic ones. Each subunit of dimeric canonical Family II PPases is formed by two domains connected by a flexible linker, with the active site located between the domains. These enzymes require both magnesium and a transition metal ion (manganese or cobalt) for maximal activity and are the most active (k_cat ≈ 10⁴ s^-1 ) among all PPase types. Catalysis by Family II PPases requires four metal ions per substrate molecule, three of which form a unique trimetal center that coordinates the nucleophilic water and converts it to a reactive hydroxide ion. A quarter of Family II PPases contain an autoinhibitory regulatory insert formed by two cystathionine β-synthase (CBS) domains and one DRTGG domain. Adenine nucleotide binding either activates or inhibits the CBS domain-containing PPases, thereby tuning their activity and, hence, PP_i levels, in response to changes in cell energy status (ATP/ADP ratio).

The Role of Intermetal Competition and Mis-Metalation in Metal Toxicity

The metals manganese, iron, cobalt, nickel, copper and zinc are essential for almost all bacteria, but their precise metal requirements vary by species, by ecological niche and by growth condition. Bacteria thus must acquire each of these essential elements in sufficient quantity to satisfy their cellular demand, but in excess these same elements are toxic. Metal toxicity has been exploited by humanity for centuries, and by the mammalian immune system for far longer, yet the mechanisms by which these elements cause toxicity to bacteria are not fully understood. There has been a resurgence of interest in metal toxicity in recent decades due to the problematic spread of antibiotic resistance amongst bacterial pathogens, which has led to an increased research effort to understand these toxicity mechanisms at the molecular level. A recurring theme from these studies is the role of intermetal competition in bacterial metal toxicity. In this review, we first survey biological metal usage and introduce some fundamental chemical concepts that are important for understanding bacterial metal usage and toxicity. Then we introduce a simple model by which to understand bacterial metal homeostasis in terms of the distribution of each essential metal ion within cellular 'pools', and dissect how these pools interact with each other and with key proteins of bacterial metal homeostasis. Finally, using a number of key examples from the recent literature, we look at specific metal toxicity mechanisms in model bacteria, demonstrating the role of metal-metal competition in the toxicity mechanisms of diverse essential metals.

A Superoxide Dismutase Capable of Functioning with Iron or Manganese Promotes the Resistance of Staphylococcus aureus to Calprotectin and Nutritional Immunity

Staphylococcus aureus is a devastating mammalian pathogen for which the development of new therapeutic approaches is urgently needed due to the prevalence of antibiotic resistance. During infection pathogens must overcome the dual threats of host-imposed manganese starvation, termed nutritional immunity, and the oxidative burst of immune cells. These defenses function synergistically, as host-imposed manganese starvation reduces activity of the manganese-dependent enzyme superoxide dismutase (SOD). S. aureus expresses two SODs, denoted SodA and SodM. While all staphylococci possess SodA, SodM is unique to S. aureus, but the advantage that S. aureus gains by expressing two apparently manganese-dependent SODs is unknown. Surprisingly, loss of both SODs renders S. aureus more sensitive to host-imposed manganese starvation, suggesting a role for these proteins in overcoming nutritional immunity. In this study, we have elucidated the respective contributions of SodA and SodM to resisting oxidative stress and nutritional immunity. These analyses revealed that SodA is important for resisting oxidative stress and for disease development when manganese is abundant, while SodM is important under manganese-deplete conditions. In vitro analysis demonstrated that SodA is strictly manganese-dependent whereas SodM is in fact cambialistic, possessing equal enzymatic activity when loaded with manganese or iron. Cumulatively, these studies provide a mechanistic rationale for the acquisition of a second superoxide dismutase by S. aureus and demonstrate an important contribution of cambialistic SODs to bacterial pathogenesis. Furthermore, they also suggest a new mechanism for resisting manganese starvation, namely populating manganese-utilizing enzymes with iron.

The Role of Oxygen Sensors, Hydroxylases, and HIF in Cardiac Function and Disease

Ischemic heart disease is the leading cause of death worldwide. Oxygen-sensing proteins are critical components of the physiological response to hypoxia and reperfusion injury, but the role of oxygen and oxygen-mediated effects is complex in that they can be cardioprotective or deleterious to the cardiac tissue. Over 200 oxygen-sensing proteins mediate the effects of oxygen tension and use oxygen as a substrate for posttranslational modification of other proteins. Hydroxylases are an essential component of these oxygen-sensing proteins. While a major role of hydroxylases is regulating the transcription factor HIF, we investigate the increasing scope of hydroxylase substrates. This review discusses the importance of oxygen-mediated effects in the heart as well as how the field of oxygen-sensing proteins is expanding, providing a more complete picture into how these enzymes play a multifaceted role in cardiac function and disease. We also review how oxygen-sensing proteins and hydroxylase function could prove to be invaluable in drug design and therapeutic targets for heart disease.

The copYAZ Operon Functions in Copper Efflux, Biofilm Formation, Genetic Transformation, and Stress Tolerance in Streptococcus mutans

In bacteria, copper homeostasis is closely monitored to ensure proper cellular functions while avoiding cell damage. Most Gram-positive bacteria utilize the copYABZ operon for copper homeostasis, where copA and copB encode copper-transporting P-type ATPases, whereas copY and copZ regulate the expression of the cop operon. Streptococcus mutans is a biofilm-forming oral pathogen that harbors a putative copper-transporting copYAZ operon. Here, we characterized the role of copYAZ operon in the physiology of S. mutans and delineated the mechanisms of copper-induced toxicity in this bacterium. We observed that copper induced toxicity in S. mutans cells by generating oxidative stress and disrupting their membrane potential. Deletion of the copYAZ operon in S. mutans strain UA159 resulted in reduced cell viability under copper, acid, and oxidative stress relative to the viability of the wild type under these conditions. Furthermore, the ability of S. mutans to form biofilms and develop genetic competence was impaired under copper stress. Briefly, copper stress significantly reduced cell adherence and total biofilm biomass, concomitantly repressing the transcription of the gtfB, gtfC, gtfD, gbpB, and gbpC genes, whose products have roles in maintaining the structural and/or functional integrity of the S. mutans biofilm. Furthermore, supplementation with copper or loss of copYAZ resulted in significant reductions in transformability and in the transcription of competence-associated genes. Copper transport assays revealed that the ΔcopYAZ strain accrued significantly large amounts of intracellular copper compared with the amount of copper accumulation in the wild-type strain, thereby demonstrating a role for CopYAZ in the copper efflux of S. mutans. The complementation of the CopYAZ system restored copper expulsion, membrane potential, and stress tolerance in the copYAZ-null mutant. Taking these results collectively, we have established the function of the S. mutans CopYAZ system in copper export and have further expanded knowledge on the importance of copper homeostasis and the CopYAZ system in modulating streptococcal physiology, including stress tolerance, membrane potential, genetic competence, and biofilm formation.

IMPORTANCE

S. mutans is best known for its role in the initiation and progression of human dental caries, one of the most common chronic diseases worldwide. S. mutans is also implicated in bacterial endocarditis, a life-threatening inflammation of the heart valve. The core virulence factors of S. mutans include its ability to produce and sustain acidic conditions and to form a polysaccharide-encased biofilm that provides protection against environmental insults. Here, we demonstrate that the addition of copper and/or deletion of copYAZ (the copper homeostasis system) have serious implications in modulating biofilm formation, stress tolerance, and genetic transformation in S. mutans. Manipulating the pathways affected by copper and the copYAZ system may help to develop potential therapeutics to prevent S. mutans infection in and beyond the oral cavity.

Structure of inorganic pyrophosphatase from Staphylococcus aureus reveals conformational flexibility of the active site

Cytoplasmic inorganic pyrophosphatase (PPiase) is an enzyme essential for survival of organisms, from bacteria to human. PPiases are divided into two structurally distinct families: family I PPiases are Mg(2+)-dependent and present in most archaea, eukaryotes and prokaryotes, whereas the relatively less understood family II PPiases are Mn(2+)-dependent and present only in some archaea, bacteria and primitive eukaryotes. Staphylococcus aureus (SA), a dangerous pathogen and a frequent cause of hospital infections, contains a family II PPiase (PpaC), which is an attractive potential target for development of novel antibacterial agents. We determined a crystal structure of SA PpaC in complex with catalytic Mn(2+) at 2.1Å resolution. The active site contains two catalytic Mn(2+) binding sites, each half-occupied, reconciling the previously observed 1:1 Mn(2+):enzyme stoichiometry with the presence of two divalent metal ion sites in the apo-enzyme. Unexpectedly, despite the absence of the substrate or products in the active site, the two domains of SA PpaC form a closed active site, a conformation observed in structures of other family II PPiases only in complex with substrate or product mimics. A region spanning residues 295-298, which contains a conserved substrate binding RKK motif, is flipped out of the active site, an unprecedented conformation for a PPiase. Because the mutant of Arg295 to an alanine is devoid of activity, this loop likely undergoes an induced-fit conformational change upon substrate binding and product dissociation. This closed conformation of SA PPiase may serve as an attractive target for rational design of inhibitors of this enzyme.

The relationship of the lipoprotein SsaB, manganese and superoxide dismutase in Streptococcus sanguinis virulence for endocarditis

Streptococcus sanguinis colonizes teeth and is an important cause of infective endocarditis. Our prior work showed that the lipoprotein SsaB is critical for S. sanguinis virulence for endocarditis and belongs to the LraI family of conserved metal transporters. In this study, we demonstrated that an ssaB mutant accumulates less manganese and iron than its parent. A mutant lacking the manganese-dependent superoxide dismutase, SodA, was significantly less virulent than wild-type in a rabbit model of endocarditis, but significantly more virulent than the ssaB mutant. Neither the ssaB nor the sodA mutation affected sensitivity to phagocytic killing or efficiency of heart valve colonization. Animal virulence results for all strains could be reproduced by growing bacteria in serum under physiological levels of O(2). SodA activity was reduced, but not eliminated in the ssaB mutant in serum and in rabbits. Growth of the ssaB mutant in serum was restored upon addition of Mn(2+) or removal of O(2). Antioxidant supplementation experiments suggested that superoxide and hydroxyl radicals were together responsible for the ssaB mutant's growth defect. We conclude that manganese accumulation mediated by the SsaB transport system imparts virulence by enabling cell growth in oxygen through SodA-dependent and independent mechanisms.

Structural, spectroscopic and functional investigation into Fe-substituted MnSOD from human pathogen Clostridium difficile

SOD_cd could modulate the Fe and Mn dependent activity through its active site microenvironment.

Identification and characterization of superoxide dismutase in Phytophthora cinnamomi

Superoxide dismutase (SOD) activities of the oomycete Phytophthora cinnamomi were examined. Five polypeptides with manganese superoxide dismutase (MnSOD) activity were found in mycelium growing in liquid culture with relative molecular weights ranging from approximately 25 to 100 kDa. Comparison with characterized avocado SODs showed no evidence for the presence of either iron or copper/zinc SODs in P. cinnamomi. The level of activity of the MnSOD polypeptides decreased in the presence of avocado root or cell wall components. Growth of P. cinnamomi, measured as dry weight, increased when the mycelium was grown in the presence of superoxide anion (O(2) (-)), which was added exogenously. Our results suggest that the metabolism of O(2) (-) has an important role in the development of P. cinnamomi.

Borrelia burgdorferi, a pathogen that lacks iron, encodes manganese-dependent superoxide dismutase essential for resistance to streptonigrin

Borrelia burgdorferi, the causative agent of Lyme disease, exists in nature through a complex life cycle involving ticks of the Ixodes genus and mammalian hosts. During its life cycle, B. burgdorferi experiences fluctuations in oxygen tension and may encounter reactive oxygen species (ROS). The key metalloenzyme to degrade ROS in B. burgdorferi is SodA. Although previous work suggests that B. burgdorferi SodA is an iron-dependent superoxide dismutase (SOD), later work demonstrates that B. burgdorferi is unable to transport iron and contains an extremely low intracellular concentration of iron. Consequently, the metal cofactor for SodA has been postulated to be manganese. However, experimental evidence to support this hypothesis remains lacking. In this study, we provide biochemical and genetic data showing that SodA is a manganese-dependent enzyme. First, B. burgdorferi contained SOD activity that is resistant to H(2)O(2) and NaCN, characteristics associated with Mn-SODs. Second, the addition of manganese to the Chelex-treated BSK-II enhanced SodA expression. Third, disruption of the manganese transporter gene bmtA, which significantly lowers the intracellular manganese, greatly reduced SOD activity and SodA expression, suggesting that manganese regulates the level of SodA. In addition, we show that B. burgdorferi is resistant to streptonigrin, a metal-dependent redox cycling compound that produces ROS, and that SodA plays a protective role against the streptonigrin. Taken together, our data demonstrate the Lyme disease spirochete encodes a manganese-dependent SOD that contributes to B. burgdorferi defense against intracellular superoxide.

Cellular iron distribution in Bacillus anthracis

Although successful iron acquisition by pathogens within a host is a prerequisite for the establishment of infection, surprisingly little is known about the intracellular distribution of iron within bacterial pathogens. We have used a combination of anaerobic native liquid chromatography, inductively coupled plasma mass spectrometry, principal-component analysis, and peptide mass fingerprinting to investigate the cytosolic iron distribution in the pathogen Bacillus anthracis. Our studies identified three of the major iron pools as being associated with the electron transfer protein ferredoxin, the miniferritin Dps2, and the superoxide dismutase (SOD) enzymes SodA1 and SodA2. Although both SOD isozymes were predicted to utilize manganese cofactors, quantification of the metal ions associated with SodA1 and SodA2 in cell extracts established that SodA1 is associated with both manganese and iron, whereas SodA2 is bound exclusively to iron in vivo. These data were confirmed by in vitro assays using recombinant protein preparations, showing that SodA2 is active with an iron cofactor, while SodA1 is cambialistic, i.e., active with manganese or iron. Furthermore, we observe that B. anthracis cells exposed to superoxide stress increase their total iron content more than 2-fold over 60 min, while the manganese and zinc contents are unaffected. Notably, the acquired iron is not localized to the three identified cytosolic iron pools.

The superoxide dismutase SodA is targeted to the periplasm in a SecA-dependent manner by a novel mechanism

The manganese/iron-type superoxide dismutase (SodA) of Rhizobium leguminosarum bv. viciae 3841 is exported to the periplasm of R. l. bv. viciae and Escherichia coli. However, it does not possess a hydrophobic cleaved N-terminal signal peptide typically present in soluble proteins exported by the Sec-dependent (Sec) pathway or the twin-arginine translocation (TAT) pathway. A tatC mutant of R. l. bv. viciae exported SodA to the periplasm, ruling out export of SodA as a complex with a TAT substrate as a chaperone. The export of SodA was unaffected in a secB mutant of E. coli, but its export from R. l. bv. viciae was inhibited by azide, an inhibitor of SecA ATPase activity. A temperature-sensitive secA mutant of E. coli was strongly reduced for SodA export. The 10 N-terminal amino acid residues of SodA were sufficient to target the reporter protein alkaline phosphatase to the periplasm. Our results demonstrate the export of a protein lacking a classical signal peptide to the periplasm by a SecA-dependent, but SecB-independent targeting mechanism. Export of the R. l. bv. viciae SodA to the periplasm was not limited to the genus Rhizobium, but was also observed in other proteobacteria.

A tandem duplication of manganese superoxide dismutase in Nosema bombycis and its evolutionary origins

Microsporidia are a group of obligate intracellular eukaryotic parasites with small genomes. They infect animals from a wide variety of phyla, including humans. Two manganese superoxide dismutase (MnSOD) genes, designated NbMnSOD1 and NbMnSOD2, were found to be organized in a tandem array within the Nosema bombycis genome. The genes, both 678 bp in length, were found to be more similar to each other than they are to homologous genes of other Microsporidia, suggesting that the tandem duplication occurred subsequent to the development of this lineage. Reverse transcript PCR shows that mRNA for both genes is present in the spores. Analysis of the primary structure, hydrophobic cluster analysis, target signal analysis, and phylogenetic analysis all indicate that NbMnSOD1 is dimeric and targeted to the cytosol. NbMnSOD2 seems to have changed more rapidly and is under less evolutionary constraint than NbMnSOD1 suggesting that NbMnSOD2 may function under different conditions or in different tissues of its host rather than simply resulting in an increase in expression. A phylogenetic analysis of MnSOD sequences from eukaryotes, Archaea, and bacteria shows the microsporidial MnSODs to be grouped with the bacteria suggesting a possible horizontal gene transfer.

Superoxide dismutase (SOD) genes in Streptomyces peucetius: effects of SODs on secondary metabolites production

Two superoxide dismutase (SOD) genes; sod1 and sod2, from Streptomyces peucetius ATCC 27952 show high similarity to other known SODs from Streptomyces coelicolor A3(2) and Streptomyces avermitilis MA-4680. These sod1 and sod2 were cloned into pIBR25 expression vector under a strong ermE* promoter to enhance secondary metabolites from Streptomyces strains. The recombinant expression plasmids; pIBR25SD1 and pIBR25SD2, were constructed to overexpress sod1 and sod2 respectively to enhance production of doxorubicin (DXR) in S. peucetius, clavulanic acid (CA) in Streptomyces clavuligerus NRRL 3585 and actinorhodin (ACT) and undecylprodigiosin (Red) in Streptomyces lividans TK24. Biomass variation, antibiotics production and transcriptional analysis of regulatory genes in recombinant strains have been studied to understand the effect of sod1 and sod2. The cell growth analysis shows that life span of all recombinant strains was found to be elevated as compared to wild type cells. In S. peucetius, overexpression of sod1 and sod2 was not effective in DXR production but in case of S. clavuligerus, CA production was increased by 2.5 and 1.5 times in sod1 and sod2 overexpression, respectively while in case of S. lividans, ACT production was increased by 1.4 and 1.6 times and Red production by 1.5 and 1.2 times upon sod1 and sod2 overexpressions, respectively as compared to the corresponding wild type strains.

Regulation of the properties of superoxide dismutase from the dental pathogenic microorganism Streptococcus mutans by iron- and manganese-bound co-factor

Streptococcus mutans, the main pathogen involved in the development of dental caries, is an aerotolerant microorganism. The bacterium lacks cytochromes and catalase, but possesses other antioxidant enzymes, such as superoxide dismutase (SmSOD). Previous researches suggested that SmSOD belongs to the 'cambialistic' group, functioning with Fe or Mn in the active site. A recombinant SmSOD (rSmSOD) with a His-tail has been produced and characterised. Studies on metal uptake and exchange proved that rSmSOD binds either Fe or Mn as a metal co-factor, even though with a consistent preference for Fe accommodation. The analysis of several enzyme samples with different values of the Mn/Fe ratio in the active site proved that the type of metal is crucial for the regulation of the activity of rSmSOD. Indeed, differently from the significant preference for Fe displayed by the enzyme in the binding reaction, its Mn-form was 71-fold more active compared to the Fe-form. The rSmSOD was endowed with a significant thermostability, its half-inactivation occurring after 10 min exposure at 71 or 73 degrees C, depending on the bound metal. Moreover, the enthalpic and entropic contribution to the heat inactivation process of rSmSOD were strongly regulated by the Mn content of the enzyme. The effect of typical inhibitors/inactivators has been investigated. rSmSOD was inhibited by sodium azide, and its sensitivity increased in the presence of higher Mn levels. Concerning two physiological inactivators, the enzyme displayed a different behaviour, being quite resistant to hydrogen peroxide and significantly sensitive to sodium peroxynitrite. Furthermore, the Mn co-factor had an amplifying role in the regulation of this different sensitivity. These results confirm the cambialistic nature of SmSOD and prove that its properties are regulated by the different metal content. The adaptative response of S. mutans during its aerobic exposure in the oral cavity could involve a different metal uptake by SmSOD.

Genomic island TnSmu2 of Streptococcus mutans harbors a nonribosomal peptide synthetase-polyketide synthase gene cluster responsible for the biosynthesis of pigments involved in oxygen and H2O2 tolerance

The oral biofilm community consists of >800 microbial species, among which Streptococcus mutans is considered a primary pathogen for dental caries. The genomic island TnSmu2 of S. mutans comprises >2% of the genome. In this study, we demonstrate that TnSmu2 harbors a gene cluster encoding nonribosomal peptide synthetases (NRPS), polyketide synthases (PKS), and accessory proteins and regulators involved in nonribosomal peptide (NRP) and polyketide (PK) biosynthesis. Interestingly, the sequences of these genes and their genomic organizations and locations are highly divergent among different S. mutans strains, yet each TnSmu2 region encodes NRPS/PKS and accessory proteins. Mutagenesis of the structural genes and putative regulatory genes in strains UA159, UA140, and MT4653 resulted in colonies that were devoid of their yellow pigmentation (for strains UA140 and MT4653). In addition, these mutant strains also displayed retarded growth under aerobic conditions and in the presence of H(2)O(2). High-performance liquid chromatography profiling of cell surface extracts identified unique peaks that were missing in the mutant strains, and partial characterization of the purified product from UA159 demonstrated that it is indeed a hybrid NRP/PK, as predicted. A genomic survey of 94 clinical S. mutans isolates suggests that the TnSmu2 gene cluster may be more prevalent than previously recognized.

Conversion of the metal-specific activity of Escherichia coli Mn-SOD by site-directed mutagenesis of Gly165Thr

Glycine 165, which is located near the active site metal, is mostly conserved in aligned amino acid sequences of manganese-containing superoxide dismutase (Mn-SOD) proteins, but is substituted to threonine in most iron-containing SODs (Fe-SODs). Because threonine 165 is located between Trp128 and Trp130, and Trp128 is one of the metal-surrounding aromatic amino acids, the conversion of this amino acid may affect the metal-specific activity of Escherichia coli Mn-SOD. In order to clarify this possibility, we prepared a mutant of E. coli Mn-SOD with the replacement of Gly165 by Thr. The ratio of the specific activities of Mn- to Fe-reconstituted enzyme increased from 0.006 in the wild-type to 0.044 in the mutant SOD; therefore, the metal-specific SOD was converted to a metal-tolerant SOD. The visible absorption spectra of the Fe- and Mn-reconstituted mutant SODs indicated the loss of Mn-SOD character. It was concluded that Gly at position 165 plays a catalytic role in maintaining the integrity of the metal specificity of Mn-SOD.

Activation of Escherichia coli UDP-3-O-[(R)-3-hydroxymyristoyl]-N-acetylglucosamine deacetylase by Fe2+ yields a more efficient enzyme with altered ligand affinity

The metal-dependent deacetylase UDP-3-O-[(R)-3-hydroxymyristoyl]-N-acetylglucosamine deacetylase (LpxC) catalyzes the first committed step in lipid A biosynthesis, the hydrolysis of UDP-3-O-myristoyl-N-acetylglucosamine to form UDP-3-O-myristoylglucosamine and acetate. Consequently, LpxC is a target for the development of antibiotics, nearly all of which coordinate the active site metal ion. Here we examine the ability of Fe(2+) to serve as a cofactor for wild-type Escherichia coli LpxC and a mutant enzyme (EcC63A), in which one of the ligands for the inhibitory metal binding site has been removed. LpxC exhibits higher activity (6-8-fold) with a single bound Fe(2+) as the cofactor compared to Zn(2+)-LpxC; both metalloenzymes have a bell-shaped dependence on pH with similar pK(a) values, indicating that at least two ionizations are important for maximal activity. X-ray absorption spectroscopy experiments suggest that the catalytic metal ion bound to Fe(2+)-EcLpxC is five-coordinate, suggesting that catalytic activity may correlate with coordination number. Furthermore, the ligand affinity of Fe(2+)-LpxC compared to the Zn(2+) enzyme is altered by up to 6-fold. In contrast to Zn(2+)-LpxC, the activity of Fe(2+)-LpxC is redox-sensitive, and a time-dependent decrease in activity is observed under aerobic conditions. The LpxC activity of crude E. coli cell lysates is also aerobically sensitive, consistent with the presence of Fe(2+)-LpxC. These data indicate that EcLpxC can use either Fe(2+) or Zn(2+) to activate catalysis in vitro and possibly in vivo, which may allow LpxC to function in E. coli grown under different environmental conditions.

The structural biochemistry of the superoxide dismutases

The discovery of superoxide dismutases (SODs), which convert superoxide radicals to molecular oxygen and hydrogen peroxide, has been termed the most important discovery of modern biology never to win a Nobel Prize. Here, we review the reasons this discovery has been underappreciated, as well as discuss the robust results supporting its premier biological importance and utility for current research. We highlight our understanding of SOD function gained through structural biology analyses, which reveal important hydrogen-bonding schemes and metal-binding motifs. These structural features create remarkable enzymes that promote catalysis at faster than diffusion-limited rates by using electrostatic guidance. These architectures additionally alter the redox potential of the active site metal center to a range suitable for the superoxide disproportionation reaction and protect against inhibition of catalysis by molecules such as phosphate. SOD structures may also control their enzymatic activity through product inhibition; manipulation of these product inhibition levels has the potential to generate therapeutic forms of SOD. Markedly, structural destabilization of the SOD architecture can lead to disease, as mutations in Cu,ZnSOD may result in familial amyotrophic lateral sclerosis, a relatively common, rapidly progressing and fatal neurodegenerative disorder. We describe our current understanding of how these Cu,ZnSOD mutations may lead to aggregation/fibril formation, as a detailed understanding of these mechanisms provides new avenues for the development of therapeutics against this so far untreatable neurodegenerative pathology.

Understanding the influence of the protein environment on the Mn(II) centers in Superoxide Dismutases using High-Field Electron Paramagnetic Resonance

One of the most puzzling questions of manganese and iron superoxide dismutases (SODs) is what is the basis for their metal-specificity. This review summarizes our findings on the Mn(II) electronic structure of SODs and related synthetic models using high-field high-frequency electron paramagnetic resonance (HFEPR), a technique that is able to achieve a very detailed and quantitative information about the electronic structure of the Mn(II) ions. We have used HFEPR to compare eight different SODs, including iron, manganese and cambialistic proteins. This comparative approach has shown that in spite of their high structural homology each of these groups have specific spectroscopic and biochemical characteristics. This has allowed us to develop a model about how protein and metal interactions influence protein pK, inhibitor binding and the electronic structure of the manganese center. To better appreciate the thermodynamic prerequisites required for metal discriminatory SOD activity and their relationship to HFEPR spectroscopy, we review the work on synthetic model systems that functionally mimic Mn-and FeSOD. Using a single ligand framework, it was possible to obtain metal-discriminatory "activity" as well as variations in the HFEPR spectra that parallel those found in the proteins. Our results give new insights into protein-metal interactions from the perspective of the Mn(II) and new steps towards solving the puzzle of metal-specificity in SODs.

A highly stable cambialistic-superoxide dismutase from Antrodia camphorata: Expression in yeast and enzyme properties

A cDNA encoding a putative superoxide dismutase (SOD) was identified in expressed sequence tags of Antrodia camphorata, a medicinal mushroom found only in Taiwan. The deduced protein was aligned with Mn-SODs and Fe-SODs from other organisms, this SOD showed greater homology to Mn-SOD. Functional A. camphorata SOD protein was overexpressed in yeast and purified. The purified enzyme showed two active forms on a 12.5% native PAGE, a dimer and a monomer. The dimeric protein's half-life of deactivation at 80 degrees C was 7 min, and its thermal inactivation rate constant K(d) was 9.87 x 10(-2)min(-1). The enzyme was stable in a broad pH range from 5-11; in the presence of 0.4M imidazole and 2% SDS. The atomic absorption spectrometric assay showed that 1.0 atom of manganese/iron (9:1) was present in each SOD subunit. The high stability of the enzyme make it better suited than other cambialistic-SODs for use in cosmetics. The SOD also documents its future utility in developing anti-inflammatory agent and in the treatment of chronic diseases.

Superoxide dismutases of heavy metal resistant streptomycetes

Heavy metal tolerant and resistant strains of streptomycetes isolated from a former uranium mining site were screened for their superoxide dismutase expression. From the strains tolerating high concentrations of different heavy metals, one was selected for its tolerance of concentrations of heavy metals (Ni, Cu, Cd, Cr, Mn, Zn, Fe). This strain, Streptomyces acidiscabies E13, was chosen for the purpose of superoxide dismutase analysis. Gel electrophoresis and activity staining revealed only one each of a nickel (NiSOD) and an iron (FeZnSOD) containing superoxide dismutase as shown by differential enzymatic repression studies. The gene for nickel containing superoxide dismutase, sodN, was cloned and sequenced from this strain. The genomic sequence shows 92.7% nucleotide identity and 96.1% amino acid identity to sodN of S. coelicolor. Expression can be activated by nickel as well as other heavy metals and active enzyme is produced in media lacking nickel but containing copper, iron or zinc. Thus, the selected strain is well suited for further characterization of the enzyme encoded by sodN.

Kinetic analysis of the metal binding mechanism of Escherichia coli manganese superoxide dismutase

The acquisition of a catalytic metal cofactor is an essential step in the maturation of every metalloenzyme, including manganese superoxide dismutase (MnSOD). In this study, we have taken advantage of the quenching of intrinsic protein fluorescence by bound metal ions to continuously monitor the metallation reaction of Escherichia coli MnSOD in vitro, permitting a detailed kinetic characterization of the uptake mechanism. Apo-MnSOD metallation kinetics are "gated", zero order in metal ion for both the native Mn2+ and a nonnative metal ion (Co2+) used as a spectroscopic probe to provide greater sensitivity to metal binding. Cobalt-binding time courses measured over a range of temperatures (35-50 degrees C) reveal two exponential kinetic processes (fast and slow phases) associated with metal binding. The amplitude of the fast phase increases rapidly as the temperature is raised, reflecting the fraction of Apo-MnSOD in an "open" conformation, and its temperature dependence allows thermodynamic parameters to be estimated for the "closed" to "open" conformational transition. The sensitivity of the metallated protein to exogenously added chelator decreases progressively with time, consistent with annealing of an initially formed metalloprotein complex (k anneal = 0.4 min(-1)). A domain-separation mechanism is proposed for metal uptake by apo-MnSOD.

Inhibition of Family II Pyrophosphatases by Analogs of Pyrophosphate and Phosphate

Imidodiphosphate (the pyrophosphate analog containing a nitrogen atom in the bridge position instead of oxygen) is a potent inhibitor of family II pyrophosphatases from Streptococcus mutans and Streptococcus gordonii (inhibition constant Ki approximately 10 microM), which is slowly hydrolyzed by these enzymes with a catalytic constant of approximately 1 min(-1). Diphosphonates with different substituents at the bridge carbon atom are much less effective (Ki = 1-6 mM). The value of Ki for sulfate (a phosphate analog) is only 12 mM. The inhibitory effect of the pyrophosphate analogs exhibits only a weak dependence on the nature of the metal ion (Mn, Mg, or Co) bound in the active site.