Truncated hemoglobin GlbO from Mycobacterium leprae alleviates nitric oxide toxicity

Intracellular peroxynitrite perturbs redox balance, bioenergetics, and Fe-S cluster homeostasis in Mycobacterium tuberculosis

The ability of Mycobacterium tuberculosis (Mtb) to tolerate nitric oxide (•NO) and superoxide (O2•-) produced by phagocytes contributes to its success as a human pathogen. Recombination of •NO and O2•- generates peroxynitrite (ONOO-), a potent oxidant produced inside activated macrophages causing lethality in diverse organisms. While the response of Mtb toward •NO and O2•- is well established, how Mtb responds to ONOO- remains unclear. Filling this knowledge gap is important to understand the persistence mechanisms of Mtb during infection. We synthesized a series of compounds that generate both •NO and O2•-, which should combine to produce ONOO-. From this library, we identified CJ067 that permeates Mtb to reliably enhance intracellular ONOO- levels. CJ067-exposed Mtb strains, including multidrug-resistant (MDR) and extensively drug-resistant (XDR) clinical isolates, exhibited dose-dependent, long-lasting oxidative stress and growth inhibition. In contrast, Mycobacterium smegmatis (Msm), a fast-growing, non-pathogenic mycobacterial species, maintained redox balance and growth in response to intracellular ONOO-. RNA-sequencing with Mtb revealed that CJ067 induces antioxidant machinery, sulphur metabolism, metal homeostasis, and a 4Fe-4S cluster repair pathway (suf operon). CJ067 impaired the activity of the 4Fe-4S cluster-containing TCA cycle enzyme, aconitase, and diminished bioenergetics of Mtb. Work with Mtb strains defective in SUF and IscS involved in Fe-S cluster biogenesis pathways showed that both systems cooperatively protect Mtb from intracellular ONOO- in vitro and inducible nitric oxide synthase (iNOS)-dependent growth inhibition during macrophage infection. Thus, Mtb is uniquely sensitive to intracellular ONOO- and targeting Fe-S cluster homeostasis is expected to promote iNOS-dependent host immunity against tuberculosis (TB).

Strategies of Pathogens to Escape from NO-Based Host Defense

Nitric oxide (NO) is an essential signaling molecule present in most living organisms including bacteria, fungi, plants, and animals. NO participates in a wide range of biological processes including vasomotor tone, neurotransmission, and immune response. However, NO is highly reactive and can give rise to reactive nitrogen and oxygen species that, in turn, can modify a broad range of biomolecules. Much evidence supports the critical role of NO in the virulence and replication of viruses, bacteria, protozoan, metazoan, and fungi, thus representing a general mechanism of host defense. However, pathogens have developed different mechanisms to elude the host NO and to protect themselves against oxidative and nitrosative stress. Here, the strategies evolved by viruses, bacteria, protozoan, metazoan, and fungi to escape from the NO-based host defense are overviewed.

Truncated (2/2) hemoglobin: Unconventional structures and functional roles in vivo and in human pathogenesis

Truncated hemoglobins (trHbs) build a sub-class of the globin family, found in eubacteria, cyanobacteria, unicellular eukaryotes, and in higher plants; among these, selected human pathogens are found. The trHb fold is based on a 2/2 α-helical sandwich, consisting of a simplified and reduced-size version of the classical 3/3 α-helical sandwich of vertebrate and invertebrate globins. Phylogenetic analysis indicates that trHbs further branch into three groups: group I (or trHbN), group II (or trHbO), and group III (or trHbP), each group being characterized by specific structural features. Among these, a protein matrix tunnel, or a cavity system implicated in diatomic ligand diffusion through the protein matrix, is typical of group I and group II, respectively. In general, a highly intertwined network of hydrogen bonds stabilizes the heme bound ligand, despite variability of the heme distal residues in the different trHb groups. Notably, some organisms display genes from more than one trHb group, suggesting that trHbN, trHbO, and trHbP may support different functions in vivo, such as detoxification of reactive nitrogen and oxygen species, respiration, oxygen storage/sensoring, thus aiding survival of an invading microorganism. Here, structural features and proposed functions of trHbs from human pathogens are reviewed.

Acanthamoeba castellanii as a Screening Tool for Mycobacterium avium Subspecies paratuberculosis Virulence Factors with Relevance in Macrophage Infection

The high prevalence of Johne's disease has driven a continuous effort to more readily understand the pathogenesis of the etiological causative bacterium, Mycobacterium avium subsp. paratuberculosis (MAP), and to develop effective preventative measures for infection spread. In this study, we aimed to create an in vivo MAP infection model employing an environmental protozoan host and used it as a tool for selection of bacterial virulence determinants potentially contributing to MAP survival in mammalian host macrophages. We utilized Acanthamoeba castellanii (amoeba) to explore metabolic consequences of the MAP-host interaction and established a correlation between metabolic changes of this phagocytic host and MAP virulence. Using the library of gene knockout mutants, we identified MAP clones that can either enhance or inhibit amoeba metabolism and we discovered that, for most part, it mirrors the pattern of MAP attenuation or survival during infection of macrophages. It was found that MAP mutants that induced an increase in amoeba metabolism were defective in intracellular growth in macrophages. However, MAP clones that exhibited low metabolic alteration in amoeba were able to survive at a greater rate within mammalian cells, highlighting importance of both category of genes in bacterial pathogenesis. Sequencing of MAP mutants has identified several virulence factors previously shown to have a biological relevance in mycobacterial survival and intracellular growth in phagocytic cells. In addition, we uncovered new genetic determinants potentially contributing to MAP pathogenicity. Results of this study support the use of the amoeba model system as a quick initial screening tool for selection of virulence factors of extremely slow-grower MAP that is challenging to study.

Discovery and dissection of metabolic oscillations in the microaerobic nitric oxide response network of Escherichia coli

The virulence of many pathogens depends upon their ability to cope with immune-generated nitric oxide (NO·). In Escherichia coli, the major NO· detoxification systems are Hmp, an NO· dioxygenase (NOD), and NorV, an NO· reductase (NOR). It is well established that Hmp is the dominant system under aerobic conditions, whereas NorV dominates anaerobic conditions; however, the quantitative contributions of these systems under the physiologically relevant microaerobic regime remain ill defined. Here, we investigated NO· detoxification in environments ranging from 0 to 50 μM O2, and discovered a regime in which E. coli NO· defenses were severely compromised, as well as conditions that exhibited oscillations in the concentration of NO·. Using an integrated computational and experimental approach, E. coli NO· detoxification was found to be extremely impaired at low O2 due to a combination of its inhibitory effects on NorV, Hmp, and translational activities, whereas oscillations were found to result from a kinetic competition for O2 between Hmp and respiratory cytochromes. Because at least 777 different bacterial species contain the genetic requirements of this stress response oscillator, we hypothesize that such oscillatory behavior could be a widespread phenomenon. In support of this hypothesis,Pseudomonas aeruginosa, whose respiratory and NO· response networks differ considerably from those of E. coli, was found to exhibit analogous oscillations in low O2 environments. This work provides insight into how bacterial NO· defenses function under the low O2 conditions that are likely to be encountered within host environments.

Ligand uptake in Mycobacterium tuberculosis truncated hemoglobins is controlled by both internal tunnels and active site water molecules

Mycobacterium tuberculosis, the causative agent of human tuberculosis, has two proteins belonging to the truncated hemoglobin (trHb) family. Mt-trHbN presents well-defined internal hydrophobic tunnels that allow O 2 and •NO to migrate easily from the solvent to the active site, whereas Mt-trHbO possesses tunnels interrupted by a few bulky residues, particularly a tryptophan at position G8. Differential ligand migration rates allow Mt-trHbN to detoxify •NO, a crucial step for pathogen survival once under attack by the immune system, much more efficiently than Mt-trHbO. In order to investigate the differences between these proteins, we performed experimental kinetic measurements, •NO decomposition, as well as molecular dynamics simulations of the wild type Mt-trHbN and two mutants, VG8F and VG8W. These mutations affect both the tunnels accessibility as well as the affinity of distal site water molecules, thus modifying the ligand access to the iron. We found that a single mutation allows Mt-trHbN to acquire ligand migration rates comparable to those observed for Mt-trHbO, confirming that ligand migration is regulated by the internal tunnel architecture as well as by water molecules stabilized in the active site.

Ligand uptake in Mycobacterium tuberculosis truncated hemoglobins is controlled by both internal tunnels and active site water molecules

Mycobacterium tuberculosis, the causative agent of human tuberculosis, has two proteins belonging to the truncated hemoglobin (trHb) family. Mt-trHbN presents well-defined internal hydrophobic tunnels that allow O 2 and (•)NO to migrate easily from the solvent to the active site, whereas Mt-trHbO possesses tunnels that are partially blocked by a few bulky residues, particularly a tryptophan at position G8. Differential ligand migration rates allow Mt-trHbN to detoxify (•)NO, a crucial step for pathogen survival once under attack by the immune system, much more efficiently than Mt-trHbO. In order to investigate the differences between these proteins, we performed experimental kinetic measurements, (•)NO decomposition, as well as molecular dynamics simulations of the wild type Mt-trHbN and two mutants, VG8F and VG8W. These mutations introduce modifications in both tunnel topologies and affect the incoming ligand capacity to displace retained water molecules at the active site. We found that a single mutation allows Mt-trHbN to acquire ligand migration rates comparable to those observed for Mt-trHbO, confirming that ligand migration is regulated by the internal tunnel architecture as well as by water molecules stabilized in the active site.

The globins of cold-adapted Pseudoalteromonas haloplanktis TAC125: from the structure to the physiological functions

Evolution allowed Antarctic microorganisms to grow successfully under extreme conditions (low temperature and high O2 content), through a variety of structural and physiological adjustments in their genomes and development of programmed responses to strong oxidative and nitrosative stress. The availability of genomic sequences from an increasing number of cold-adapted species is providing insights to understand the molecular mechanisms underlying crucial physiological processes in polar organisms. The genome of Pseudoalteromonas haloplanktis TAC125 contains multiple genes encoding three distinct truncated globins exhibiting the 2/2 α-helical fold. One of these globins has been extensively characterised by spectroscopic analysis, kinetic measurements and computer simulation. The results indicate unique adaptive structural properties that enhance the overall flexibility of the protein, so that the structure appears to be resistant to pressure-induced stress. Recent results on a genomic mutant strain highlight the involvement of the cold-adapted globin in the protection against the stress induced by high O2 concentration. Moreover, the protein was shown to catalyse peroxynitrite isomerisation in vitro. In this review, we first summarise how cold temperatures affect the physiology of microorganisms and focus on the molecular mechanisms of cold adaptation revealed by recent biochemical and genetic studies. Next, since only in a very few cases the physiological role of truncated globins has been demonstrated, we also discuss the structural and functional features of the cold-adapted globin in an attempt to put into perspective what has been learnt about these proteins and their potential role in the biology of cold-adapted microorganisms.

The diversity of 2/2 (truncated) globins

Small size globins that have been defined as 'truncated haemoglobins' or as '2/2 haemoglobins' have increasingly been discovered in microorganisms since the early 1990s. Analysis of amino acid sequences allowed to distinguish three groups that collect proteins with specific and common structural properties. All three groups display 3D structures that are based on four main α-helices, which are a subset of the conventional eight-helices globin fold. Specific features, such as the presence of protein matrix tunnels that are held to promote diffusion of functional ligands to/from the haem, distinguish members of the three groups. Haem distal sites vary for their accessibility, local structures, polarity, and ligand stabilization mechanisms, suggesting functional roles that are related to O2/NO chemistry. In a few cases, such activities have been proven in vitro and in vivo through deletion mutants. The issue of 2/2 haemoglobin varied biological functions throughout the three groups remains however fully open.

Haemoglobins of Mycobacteria: structural features and biological functions

The genus Mycobacterium is comprised of Gram-positive bacteria occupying a wide range of natural habitats and includes species that range from severe intracellular pathogens to economically useful and harmless microbes. The recent upsurge in the availability of microbial genome data has shown that genes encoding haemoglobin-like proteins are ubiquitous among Mycobacteria and that multiple haemoglobins (Hbs) of different classes may be present in pathogenic and non-pathogenic species. The occurrence of truncated haemoglobins (trHbs) and flavohaemoglobins (flavoHbs) showing distinct haem active site structures and ligand-binding properties suggests that these Hbs may be playing diverse functions in the cellular metabolism of Mycobacteria. TrHbs and flavoHbs from some of the severe human pathogens such as Mycobacterium tuberculosis and Mycobacterium leprae have been studied recently and their roles in effective detoxification of reactive nitrogen and oxygen species, electron cycling, modulation of redox state of the cell and facilitation of aerobic respiration have been proposed. This multiplicity in the function of Hbs may aid these pathogens to cope with various environmental stresses and survive during their intracellular regime. This chapter provides recent updates on genomic, structural and functional aspects of Mycobacterial Hbs to address their role in Mycobacteria.

Peroxynitrite, a potent macrophage-derived oxidizing cytotoxin to combat invading pathogens

Macrophages are among the first cellular actors facing the invasion of microorganisms. These cells are able to internalize pathogens and destroy them by means of toxic mediators, many of which are produced enzymatically and have strong oxidizing capacity. Indeed, macrophages count on the NADPH oxidase complex activity, which is triggered during pathogen invasion and leads to the production of superoxide radical inside the phagosome. At the same time, the induction of nitric oxide synthase results in the production of nitric oxide in the cytosol which is able to readily diffuse to the phagocytic vacuole. Superoxide radical and nitric oxide react at diffusion controlled rates with each other inside the phagosome to yield peroxynitrite, a powerful oxidant capable to kill micro-organisms. Peroxynitrite toxicity resides on oxidations and nitrations of biomolecules in the target cell. The central role of peroxynitrite as a key effector molecule in the control of infections has been proven in a wide number of models. However, some microorganisms and virulent strains adapt to survive inside the potentially hostile oxidizing microenvironment of the phagosome by either impeding peroxynitrite formation or rapidly detoxifying it once formed. In this context, the outcome of the infection process is a result of the interplay between the macrophage-derived oxidizing cytotoxins such as peroxynitrite and the antioxidant defense machinery of the invading pathogens.

Antarctic bacterial haemoglobin and its role in the protection against nitrogen reactive species

In a cold and oxygen-rich environment such as Antarctica, mechanisms for the defence against reactive oxygen and nitrogen species are needed and represent important components in the evolutionary adaptations. In the Antarctic bacterium Pseudoalteromonas haloplanktis TAC125, the presence of multiple genes encoding 2/2 haemoglobins and a flavohaemoglobin strongly suggests that these proteins fulfil important physiological roles, perhaps associated to the peculiar features of the Antarctic habitat. In this work, the putative role of Ph-2/2HbO, encoded by the PSHAa0030 gene, was investigated by in vivo and in vitro experiments in order to highlight its involvement in NO detoxification mechanisms. The PSHAa0030 gene was cloned and then over-expressed in a flavohaemoglobin-deficient mutant of Escherichia coli, unable to metabolise NO, and the resulting strain was studied analysing its growth properties and oxygen uptake in the presence of NO. We here demonstrate that Ph-2/2HbO protects growth and cellular respiration of the heterologous host from the toxic effect of NO-donors. Unlike in Mycobacterium tuberculosis 2/2 HbN, the deletion of the N-terminal extension of Ph-2/2HbO does not seem to reduce the NO scavenging activity, showing that the N-terminal extension is not a requirement for efficient NO detoxification. Moreover, the ferric form of Ph-2/2HbO was shown to catalyse peroxynitrite isomerisation in vitro, confirming its potential role in the scavenging of reactive nitrogen species. This article is part of a Special Issue entitled: Oxygen Binding and Sensing Proteins.

Hemoglobin: a nitric-oxide dioxygenase

Members of the hemoglobin superfamily efficiently catalyze nitric-oxide dioxygenation, and when paired with native electron donors, function as NO dioxygenases (NODs). Indeed, the NOD function has emerged as a more common and ancient function than the well-known role in O2 transport-storage. Novel hemoglobins possessing a NOD function continue to be discovered in diverse life forms. Unique hemoglobin structures evolved, in part, for catalysis with different electron donors. The mechanism of NOD catalysis by representative single domain hemoglobins and multidomain flavohemoglobin occurs through a multistep mechanism involving O2 migration to the heme pocket, O2 binding-reduction, NO migration, radical-radical coupling, O-atom rearrangement, nitrate release, and heme iron re-reduction. Unraveling the physiological functions of multiple NODs with varying expression in organisms and the complexity of NO as both a poison and signaling molecule remain grand challenges for the NO field. NOD knockout organisms and cells expressing recombinant NODs are helping to advance our understanding of NO actions in microbial infection, plant senescence, cancer, mitochondrial function, iron metabolism, and tissue O2 homeostasis. NOD inhibitors are being pursued for therapeutic applications as antibiotics and antitumor agents. Transgenic NOD-expressing plants, fish, algae, and microbes are being developed for agriculture, aquaculture, and industry.

Structural determinants of ligand migration in Mycobacterium tuberculosis truncated hemoglobin O

Mycobacterium tuberculosis is the causative agent of human tuberculosis, one of the most prevalent infectious diseases in the world. Its genome hosts the glbN and glbO genes coding for two proteins, truncated hemoglobin N (trHbN) and truncated hemoglobin O (trHbO), that belong to different groups (I and II, respectively) of the recently discovered trHb family of hemeproteins. The different expression pattern and kinetics rates constants for ligand association and NO oxidation rate suggest different functions for these proteins. Previous experimental and theoretical studies showed that, in trHbs, ligand migration along the internal tunnel cavity system is a key issue in determining the ligand-binding characteristics. The X-ray structure of trHbO has been solved and shows several internal cavities and secondary-docking sites. In this work, we present an extensive investigation of the tunnel/cavity system ofM. tuberculosis trHbO by means of computer-simulation techniques. We have computed the free-energy profiles for ligand migration along three found tunnels in the oxy and deoxy w.t. and mutant trHbO proteins. Our results show that multiple-ligand migration paths are possible and that several conserved residues such as TrpG8 play a key role in the ligand-migration regulation.

Responses of Mycobacterium tuberculosis hemoglobin promoters to in vitro and in vivo growth conditions

The success of Mycobacterium tuberculosis as one of the dreaded human pathogens lies in its ability to utilize different defense mechanisms in response to the varied environmental challenges during the course of its intracellular infection, latency, and reactivation cycle. Truncated hemoglobins trHbN and trHbO are thought to play pivotal roles in the cellular metabolism of this organism during stress and hypoxia. To delineate the genetic regulation of the M. tuberculosis hemoglobins, transcriptional fusions of the promoters of the glbN and glbO genes with green fluorescent protein were constructed, and their responses were monitored in Mycobacterium smegmatis and M. tuberculosis H37Ra exposed to environmental stresses in vitro and in M. tuberculosis H37Ra after in vivo growth inside macrophages. The glbN promoter activity increased substantially during stationary phase and was nearly 3- to 3.5-fold higher than the activity of the glbO promoter, which remained more or less constant during different growth phases in M. smegmatis, as well as in M. tuberculosis H37Ra. In both mycobacterial hosts, the glbN promoter activity was induced 1.5- to 2-fold by the general nitrosative stress inducer, nitrite, as well as the NO releaser, sodium nitroprusside (SNP). The glbO promoter was more responsive to nitrite than to SNP, although the overall increase in its activity was much less than that of the glbN promoter. Additionally, the glbN promoter remained insensitive to the oxidative stress generated by H(2)O(2), but the glbO promoter activity increased nearly 1.5-fold under similar conditions, suggesting that the trHb gene promoters are regulated differently under nitrosative and oxidative stress conditions. In contrast, transition metal-induced hypoxia enhanced the activity of both the glbN and glbO promoters at all growth phases; the glbO promoter was induced approximately 2.3-fold, which was found to be the highest value for this promoter under all the conditions evaluated. Addition of iron along with nickel reversed the induction in both cases. Interestingly, a concentration-dependent decrease in the activity of both trHb gene promoters was observed when the levels of iron in the growth media were depleted by addition of an iron chelator. These results suggested that an iron/heme-containing oxygen sensor is involved in the modulation of the trHb gene promoter activities directly or indirectly in conjunction with other cellular factors. The modes of promoter regulation under different physiological conditions were found to be similar for the trHbs in both M. smegmatis and M. tuberculosis H37Ra, indicating that the promoters might be regulated by components that are common to the two systems. Confocal microscopy of THP-1 macrophages infected with M. tuberculosis carrying the trHb gene promoter fusions showed that there was a significant level of promoter activity during intracellular growth in macrophages. Time course evaluation of the promoter activity after various times up to 48 h by fluorescence-activated cell sorting analysis of the intracellular M. tuberculosis cells indicated that the glbN promoter was active at all time points assessed, whereas the activity of the glbO promoter remained at a steady-state level up to 24 h postinfection and increased approximately 2-fold after 48 h of infection. Thus, the overall regulation pattern of the M. tuberculosis trHb gene promoters correlates not only with the stresses that the tubercle bacillus is likely to encounter once it is in the macrophage environment but also with our current knowledge of their functions. The in vivo studies that demonstrated for the first time expression of trHbs during macrophage infection of M. tuberculosis strongly indicate that the hemoglobins are required, and thus important, during the intracellular phase of the bacterial cycle. The present study of transcriptional regulation of M. tuberculosis hemoglobins in vitro under various stress conditions and in vivo after macrophage infection supports the hypothesis that biosynthesis of both trHbs (trHbN and trHbO) in the native host is regulated via the environmental signals that the tubercle bacillus receives during macrophage infection and growth in its human host.

Interactions of NO with hemoglobin: from microbes to man

Hemoglobins are found in organisms from every major phylum and subserve life-sustaining respiratory functions across a broad continuum. Sustainable aerobic respiration in mammals and birds relies on the regulated delivery of oxygen (O2) and nitric oxide (NO) bioactivity by hemoglobin, through reversible binding of NO and O2 to hemes as well as S-nitrosylation of cysteine thiols (SNO synthase activity). In contrast, bacterial and yeast flavohemoglobins function in vivo as denitrosylases (O2 nitroxylases), and some multimeric, invertebrate hemoglobins function as deoxygenases (Cys-dependent NO dioxygenases), which efficiently consume rather than deliver NO and O2, respectively. Analogous mechanisms may operate in plants. Bacteria and fungi deficient in flavohemoglobin show compromised virulence in animals that results from impaired resistance to NO, whereas animals and humans deficient in S-nitrosylated Hb exhibit altered vasoactivity. NO-related functions of hemoglobins center on reactions with ferric (FeIII) heme iron, which is exploited in enzymatic reactions that address organismal requirements for delivery or detoxification of NO and O2. Delivery versus detoxification of NO/O2 is largely achieved through structural changes and amino acid rearrangements within the heme pockets, thereby influencing the propensity for heme/cysteine thiol redox coupling. Additionally, the behavior exhibited by hemoglobin in vivo may be profoundly dependent both on the abundance of NO and O2 and on the allosteric effects of heterotropic ligands. Here we review well-documented examples of redox interactions between NO and hemoglobin, with an emphasis on biochemical mechanisms and physiological significance.

Truncated hemoglobins in actinorhizal nodules of Datisca glomerata

Three types of hemoglobins exist in higher plants, symbiotic, non-symbiotic, and truncated hemoglobins. Symbiotic (class II) hemoglobins play a role in oxygen supply to intracellular nitrogen-fixing symbionts in legume root nodules, and in one case ( Parasponia Sp.), a non-symbiotic (class I) hemoglobin has been recruited for this function. Here we report the induction of a host gene, dgtrHB1, encoding a truncated hemoglobin in Frankia-induced nodules of the actinorhizal plant Datisca glomerata. Induction takes place specifically in cells infected by the microsymbiont, prior to the onset of bacterial nitrogen fixation. A bacterial gene (Frankia trHBO) encoding a truncated hemoglobin with O (2)-binding kinetics suitable for the facilitation of O (2) diffusion ( ) is also expressed in symbiosis. Nodule oximetry confirms the presence of a molecule that binds oxygen reversibly in D. glomerata nodules, but indicates a low overall hemoglobin concentration suggesting a local function. Frankia trHbO is likely to be responsible for this activity. The function of the D. glomerata truncated hemoglobin is unknown; a possible role in nitric oxide detoxification is suggested.

NO dissociation represents the rate limiting step for O2-mediated oxidation of ferrous nitrosylated Mycobacterium leprae truncated hemoglobin O

Mycobacterium leprae truncated hemoglobin O (trHbO) protects from nitrosative stress and sustains mycobacterial respiration. Here, kinetics of M. leprae trHbO(II)-NO denitrosylation and of O(2)-mediated oxidation of M. leprae trHbO(II)-NO are reported. Values of the first-order rate constant for *NO dissociation from M. leprae trHbO(II)-NO (k(off)) and of the first-order rate constant for O(2)-mediated oxidation of M. leprae trHbO(II)-NO (h) are 1.3 x 10(-4) s(-1) and 1.2 x 10(-4) s(-1), respectively. The coincidence of values of k(off) and h suggests that O(2)-mediated oxidation of M. leprae trHbO(II)-NO occurs with a reaction mechanism in which *NO, that is initially bound to heme(II), is displaced by O(2) but may stay trapped in a protein cavity(ies) close to heme(II). Next, M. leprae trHbO(II)-O(2) reacts with *NO giving the transient Fe(III)-OONO species preceding the formation of the final product M. leprae trHbO(III). *NO dissociation from heme(II)-NO represents the rate limiting step for O(2)-mediated oxidation of M. leprae trHbO(II)-NO.

Mycobacterial truncated hemoglobins: from genes to functions

Infections caused by bacteria belonging to genus Mycobacterium are among the most challenging threats for human health. The ability of mycobacteria to persist in vivo in the presence of reactive nitrogen and oxygen species implies the presence in these bacteria of effective detoxification mechanisms. Mycobacterial truncated hemoglobins (trHbs) have recently been implicated in scavenging of reactive nitrogen species. Individual members from each trHb family (N, O, and P) can be present in the same mycobacterial species. The distinct features of the heme active site structure combined with different ligand binding properties and in vivo expression patterns of mycobacterial trHbs suggest that these globins may accomplish diverse functions. Here, recent genomic, structural and biochemical information on mycobacterial trHbs is reviewed, with the aim of providing further insights into the role of these globins in mycobacterial physiology.

Intracellular growth and survival of Salmonella enterica serovar Typhimurium carrying truncated hemoglobins of Mycobacterium tuberculosis

Two distantly related truncated hemoglobins (trHbs), HbN and HbO, are produced at different growth stages of Mycobacterium tuberculosis. Oxygen and nitric oxide (NO) binding properties of these trHbs suggest their vital role(s) in adaptation of tubercle bacillus under hypoxic and nitrosative stress conditions. Here, we have demonstrated that HbN of M. tuberculosis provides distinct advantage over HbO in supporting intracellular growth and survival of the heterologous host, Salmonella enterica serovar Typhimurium, during macrophage infection specifically against toxicity of NO. HbN and HbO encoding genes of M. tuberculosis have been expressed in a NO-sensitive hmp mutant of S. enterica serovar Typhimurium that exhibits attenuated growth within the macrophages. Presence of HbN and HbO conferred distinct oxygen dependent NO metabolizing activity to the mutant S. enterica serovar Typhimurium. However, the HbN carrying cells exhibited nearly 2-3-fold higher NO metabolizing activity than the isogenic strain having HbO under aerobic condition. More than half of the NO uptake activity of HbN carrying cells was retained when oxygen level dropped to microaerobic condition. In comparison, NO uptake activity of HbO carrying cells of mutant S. enterica dropped drastically (90%) under similar hypoxic conditions. When internalized by mice peritoneal macrophages, HbN carrying cells exhibited 3- and 4-fold higher survival compared to similarly bound and internalized HbO carrying and control cells, respectively. The protective effect of HbN persisted even after activation of macrophages in the presence of IFN-gamma, whereas, HbO did not show any significant effect on survival of the NO-sensitive hmp mutant of Salmonella. These results provide strong experimental evidence in support of the protective role of HbN against nitrosative stress inside macrophages and suggest that intracellular protection conferred by HbN of M. tuberculosis might not be restricted to its native host only.

Peroxynitrite scavenging by ferrous truncated hemoglobin GlbO from Mycobacterium leprae

Mycobacterium leprae GlbO has been proposed to represent merging of both O(2) uptake/transport and scavenging of nitrogen reactive species. Peroxynitrite reacts with M. leprae GlbO(II)-NO leading to GlbO(III) via the GlbO(III)-NO species. The value of the second order rate constant for GlbO(III)-NO formation is >1x10(8)M(-1)s(-1) in the absence and presence of CO(2) (1.2x10(-3)M). The CO(2)-independent value of the first order rate constant for GlbO(III)-NO denitrosylation is (2.5+/-0.4)x10(1)s(-1). Furthermore, peroxynitrite reacts with GlbO(II)-O(2) leading to GlbO(III) via the GlbO(IV)O species. Values of the second order rate constant for GlbO(IV)O formation are (4.8+/-0.5)x10(4) and (6.3+/-0.7)x10(5)M(-1)s(-1) in the absence and presence of CO(2) (=1.2x10(-3)M), respectively. The value of the second order rate constant for the peroxynitrite-mediated GlbO(IV)O reduction (= (1.5+/-0.2)x10(4)M(-1)s(-1)) is CO(2)-independent. These data argue for a role of GlbO in the defense of M. leprae against nitrosative stress.