Catalase-peroxidase from the cyanobacterium Synechocystis PCC 6803: cloning, overexpression in Escherichia coli, and kinetic characterization

Native SodB Overexpression of Synechocystis sp. PCC 6803 Improves Cell Growth Under Alcohol Stresses Whereas Its Gpx2 Overexpression Impacts on Growth Recovery from Alcohol Stressors

To overcome the limited resistance to alcohol stress, genetically engineered Synechocystis sp. PCC 6803 strains with overexpressions of genes related with the ROS detoxification system (sodB and gpx2, which encode superoxide dismutase and glutathione peroxidase, respectively) were developed. Three engineered strains including a sodB-overexpressing strain (OE + S), a gpx2-overexpressing strain (OE + G), and a sodB/gpx2-overexpressing strain (OE + SG) grew similarly as wild-type control under normal condition. When compared to wild-type control, OE + S and OE + SG strains grew faster for 4 days under 2.0% (v/v) ethanol and 0.3% (v/v) n-butanol conditions, as well as having higher chlorophyll a levels. On the other hand, the prominent growth recovery of OE + G and OE + SG was noted within 4 days in normal BG₁₁ medium after treating cells with high alcohol stresses for 1 h, in particular 15% ethanol and 2.5% n-butanol. Under 4 days of recovery from butanol stress, specific levels of intracellular pigments including chlorophyll a and carotenoids were dramatically increased in all modified strains. The overexpression of antioxidant genes then revealed a significant improvement of alcohol tolerance in Synechocystis sp. PCC 6803.

Regulation of antioxidant defense and glyoxalase systems in cyanobacteria

Oxidative stress is common consequence of abiotic stress in plants as well as cyanobacteria caused by generation of reactive oxygen species (ROS), an inevitable product of respiration and photosynthetic electron transport. ROS act as signalling molecule at low concentration however, when its production exceeds the endurance capacity of antioxidative defence system, the organisms suffer oxidative stress. A highly toxic metabolite, methylglyoxal (MG) is also produced in cyanobacteria in response to various abiotic stresses which consequently augment the ensuing oxidative damage. Taking recourse to the common lineage of eukaryotic plants and cyanobacteria, it would be worthwhile to explore the regulatory role of glyoxalase system and antioxidative defense mechanism in combating abiotic stress in cyanobacteria. This review provides comprehensive information on the complete glyoxalase system (GlyI, GlyII and GlyIII) in cyanobacteria. Furthermore, it elucidates the recent understanding regarding the production of ROS and MG, noteworthy link between intracellular MG and ROS and its detoxification via synchronization of antioxidants (enzymatic and non-enzymatic) and glyoxalase systems using glutathione (GSH) as common co-factor.

A role for catalase-peroxidase large loop 2 revealed by deletion mutagenesis: control of active site water and ferric enzyme reactivity

Catalase-peroxidases (KatGs), the only catalase-active members of their superfamily, all possess a 35-residue interhelical loop called large loop 2 (LL2). It is essential for catalase activity, but little is known about its contribution to KatG function. LL2 shows weak sequence conservation; however, its length is nearly identical across KatGs, and its apex invariably makes contact with the KatG-unique C-terminal domain. We used site-directed and deletion mutagenesis to interrogate the role of LL2 and its interaction with the C-terminal domain in KatG structure and catalysis. Single and double substitutions of the LL2 apex had little impact on the active site heme [by magnetic circular dichroism or electron paramagnetic resonance (EPR)] and activity (catalase or peroxidase). Conversely, deletion of a single amino acid from the LL2 apex reduced catalase activity by 80%. Deletion of two or more apex amino acids or all of LL2 diminished catalase activity by 300-fold. Peroxide-dependent but not electron donor-dependent kcat/KM values for deletion variant peroxidase activity were reduced 20-200-fold, and kon for cyanide binding diminished by 3 orders of magnitude. EPR spectra for deletion variants were all consistent with an increase in the level of pentacoordinate high-spin heme at the expense of hexacoordinate high-spin states. Together, these data suggest a shift in the distribution of active site waters, altering the reactivity of the ferric state, toward, among other things, compound I formation. These results identify the importance of LL2 length conservation for maintaining an intersubunit interaction that is essential for an active site water distribution that facilitates KatG catalytic activity.

Oxidative stress management in the filamentous, heterocystous, diazotrophic cyanobacterium, Anabaena PCC7120

Reactive oxygen species (ROS) are inevitably generated as by-products of respiratory/photosynthetic electron transport in oxygenic photoautotrophs. Unless effectively scavenged, these ROS can damage all cellular components. The filamentous, heterocystous, nitrogen-fixing strains of the cyanobacterium, Anabaena, serve as naturally abundant contributors of nitrogen biofertilizers in tropical rice paddy fields. Anabaena strains are known to tolerate several abiotic stresses, such as heat, UV, gamma radiation, desiccation, etc., that are known to generate ROS. ROS are detoxified by specific antioxidant enzymes like superoxide dismutases (SOD), catalases and peroxiredoxins. The genome of Anabaena PCC7120 encodes two SODs, two catalases and seven peroxiredoxins, indicating the presence of an elaborate antioxidant enzymatic machinery to defend its cellular components from ROS. This article summarizes recent findings and depicts important perspectives in oxidative stress management in Anabaena PCC7120.

Genome-wide analysis of putative peroxiredoxin in unicellular and filamentous cyanobacteria

BACKGROUND

Cyanobacteria are photoautotrophic prokaryotes with wide variations in genome sizes and ecological habitats. Peroxiredoxin (PRX) is an important protein that plays essential roles in protecting own cells against reactive oxygen species (ROS). PRXs have been identified from mammals, fungi and higher plants. However, knowledge on cyanobacterial PRXs still remains obscure. With the availability of 37 sequenced cyanobacterial genomes, we performed a comprehensive comparative analysis of PRXs and explored their diversity, distribution, domain structure and evolution.

RESULTS

Overall 244 putative prx genes were identified, which were abundant in filamentous diazotrophic cyanobacteria, Acaryochloris marina MBIC 11017, and unicellular cyanobacteria inhabiting freshwater and hot-springs, while poor in all Prochlorococcus and marine Synechococcus strains. Among these putative genes, 25 open reading frames (ORFs) encoding hypothetical proteins were identified as prx gene family members and the others were already annotated as prx genes. All 244 putative PRXs were classified into five major subfamilies (1-Cys, 2-Cys, BCP, PRX5_like, and PRX-like) according to their domain structures. The catalytic motifs of the cyanobacterial PRXs were similar to those of eukaryotic PRXs and highly conserved in all but the PRX-like subfamily. Classical motif (CXXC) of thioredoxin was detected in protein sequences from the PRX-like subfamily. Phylogenetic tree constructed of catalytic domains coincided well with the domain structures of PRXs and the phylogenies based on 16s rRNA.

CONCLUSIONS

The distribution of genes encoding PRXs in different unicellular and filamentous cyanobacteria especially those sub-families like PRX-like or 1-Cys PRX correlate with the genome size, eco-physiology, and physiological properties of the organisms. Cyanobacterial and eukaryotic PRXs share similar conserved motifs, indicating that cyanobacteria adopt similar catalytic mechanisms as eukaryotes. All cyanobacterial PRX proteins share highly similar structures, implying that these genes may originate from a common ancestor. In this study, a general framework of the sequence-structure-function connections of the PRXs was revealed, which may facilitate functional investigations of PRXs in various organisms.

Mn-catalase (Alr0998) protects the photosynthetic, nitrogen-fixing cyanobacterium Anabaena PCC7120 from oxidative stress

Role of the non-haem, manganese catalase (Mn-catalase) in oxidative stress tolerance is unknown in cyanobacteria. The ORF alr0998 from the Anabaena PCC7120, which encodes a putative Mn-catalase, was constitutively overexpressed in Anabaena PCC7120 to generate a recombinant strain, AnKat(+). The Alr0998 protein could be immunodetected in AnKat(+) cells and zymographic analysis showed a distinct thermostable catalase activity in the cytosol of AnKat(+) cells but not in the wild-type Anabaena PCC7120. The observed catalase activity was insensitive to inhibition by azide indicating that Alr0998 protein is indeed a Mn-catalase. In response to oxidative stress, the AnKat(+) showed reduced levels of intracellular ROS which was also corroborated by decreased production of an oxidative stress-inducible 2-Cys-Prx protein. Treatment of wild-type Anabaena PCC7120 with H(2)O(2) caused (i) RNA degradation in vivo, (ii) severe reduction of photosynthetic pigments and CO(2) fixation, (iii) fragmentation and lysis of filaments and (iv) loss of viability. In contrast, the AnKat(+) strain was protected from all the aforesaid deleterious effect under oxidative stress. This is the first report on protection of an organism from oxidative stress by overexpression of a Mn-catalase.

Eukaryotic extracellular catalase-peroxidase from Magnaporthe grisea - Biophysical/chemical characterization of the first representative from a novel phytopathogenic KatG group

All phytopathogenic fungi have two catalase-peroxidase paralogues located either intracellularly (KatG1) or extracellularly (KatG2). Here, for the first time a secreted bifunctional, homodimeric catalase-peroxidase (KatG2 from the rice blast fungus Magnaporthe grisea) has been produced heterologously with almost 100% heme occupancy and comprehensively investigated by using a broad set of methods including UV-Vis, ECD and resonance Raman spectroscopy (RR), thin-layer spectroelectrochemistry, mass spectrometry, steady-state & presteady-state spectroscopy. RR spectroscopy reveals that MagKatG2 shows a unique mixed-spin state, non-planar heme b, and a proximal histidine with pronounced imidazolate character. At pH 7.0 and 25 °C, the standard reduction potential E°' of the Fe(III)/Fe(II) couple for the high-spin native protein was found to fall in the range typical for the KatG family. Binding of cyanide was relatively slow at pH 7.0 and 25 °C and with a K(d) value significantly higher than for the intracellular counterpart. Demonstrated by mass spectrometry MagKatG2 has the typical Trp118-Tyr251-Met277 adduct that is essential for its predominantly catalase activity at the unique acidic pH optimum. In addition, MagKatG2 acts as a versatile peroxidase using both one- and two-electron donors. Based on these data, structure-function relationships of extracellular eukaryotic KatGs are discussed with respect to intracellular KatGs and possible role(s) in host-pathogen interaction.

Probing the two-domain structure of homodimeric prokaryotic and eukaryotic catalase-peroxidases

Catalase-peroxidases (KatGs) are ancestral bifunctional heme peroxidases found in archaeons, bacteria and lower eukaryotes. In contrast to homologous cytochrome c peroxidase (CcP) and ascorbate peroxidase (APx) homodimeric KatGs have a two-domain monomeric structure with a catalytic N-terminal heme domain and a C-terminal domain of high sequence and structural similarity but without obvious function. Nevertheless, without its C-terminal counterpart the N-terminal domain exhibits neither catalase nor peroxidase activity. Except some hybrid-type proteins all other members of the peroxidase-catalase superfamily lack this C-terminal domain. In order to probe the role of the two-domain monomeric structure for conformational and thermal stability urea and temperature-dependent unfolding experiments were performed by using UV-Vis-, electronic circular dichroism- and fluorescence spectroscopy, as well as differential scanning calorimetry. Recombinant prokaryotic (cyanobacterial KatG from Synechocystis sp. PCC6803) and eukaryotic (fungal KatG from Magnaporthe grisea) were investigated. The obtained data demonstrate that the conformational and thermal stability of bifunctional KatGs is significantly lower compared to homologous monofunctional peroxidases. The N- and C-terminal domains do not unfold independently. Differences between the cyanobacterial and the fungal enzyme are relatively small. Data will be discussed with respect to known structure and function of KatG, CcP and APx.

Intracellular catalase/peroxidase from the phytopathogenic rice blast fungus Magnaporthe grisea: expression analysis and biochemical characterization of the recombinant protein

Phytopathogenic fungi such as the rice blast fungus Magnaporthe grisea are unique in having two catalase/peroxidase (KatG) paralogues located either intracellularly (KatG1) or extracellularly (KatG2). The coding genes have recently been shown to derive from a lateral gene transfer from a (proteo)bacterial genome followed by gene duplication and diversification. Here we demonstrate that KatG1 is expressed constitutively in M. grisea. It is the first eukaryotic catalase/peroxidase to be expressed heterologously in Escherichia coli in high amounts, with high purity and with almost 100% haem occupancy. Recombinant MagKatG1 is an acidic, mainly homodimeric, oxidoreductase with a predominant five-co-ordinated high-spin haem b. At 25 °C and pH 7.0, the E0′ (standard reduction potential) of the Fe(III)/Fe(II) couple was found to be −186±10 mV. It bound cyanide monophasically with an apparent bimolecular rate constant of (9.0±0.4)×105 M−1·s−1 at pH 7.0 and at 25 °C and with a Kd value of 1.5 μM. Its predominantly catalase activity was characterized by a pH optimum at 6.0 and kcat and Km values of 7010 s−1 and 4.8 mM respectively. In addition, it acts as a versatile peroxidase with a pH optimum in the range 5.0–5.5 using both one-electron [2,2′-azinobis-(3-ethylbenzothiazoline-6-sulfonic acid) o-dianisidine, pyrogallol or guaiacol] and two-electron (Br−, I− or ethanol) donors. Structure–function relationships are discussed with respect to data reported for prokaryotic KatGs, as is the physiological role of MagKatG1. Phylogenetic analysis suggests that (intracellular) MagKatG1 can be regarded as a typical representative for catalase/peroxidase of both phytopathogenic and saprotrophic fungi.

Occurrence, phylogeny, structure, and function of catalases and peroxidases in cyanobacteria

Cyanobacteria have evolved approximately 3x10(9) years ago from ancient phototrophic microorganisms that already lived on our planet Earth. By opening the era of an aerobic, oxygen-containing biosphere, they are the true pacemakers of geological and biological evolution. Cyanobacteria must have been among the first organisms to elaborate mechanisms for the detoxification of partially reduced oxygen species including (hydrogen) peroxide. Since there is still an suprising lack of knowledge on the type, role, and mechanism(s) of peroxide-degrading enzymes in these bacteria, all 44 fully or partially sequenced genomes for haem and non-haem catalases and peroxidases have been critically analysed based on well known structure-function relationships of the corresponding oxidoreductases. It is demonstrated that H(2)O(2)-dismutating enzymes are mainly represented by bifunctional (haem) catalase-peroxidases and (binuclear) manganese catalases, with the latter being almost exclusively found in diazotrophic species. Several strains even lack a gene that encodes an enzyme with catalase activity. Two groups of peroxidases are found. Genes encoding putative (primordial) haem peroxidases (with homology to corresponding mammalian enzymes) and vanadium-containing iodoperoxidases are found only in a few species, whereas genes encoding peroxiredoxins (1-Cys, 2-Cys, type II, and Q-type) are ubiquitous in cyanobacteria. In addition, approximately 70% contain NADPH-dependent glutathione peroxidase-like proteins. The occurrence and phylogeny of these enzymes is discussed, as well as the present knowledge of their physiological role(s).

Hydrogen peroxide oxidation by catalase-peroxidase follows a non-scrambling mechanism

Despite catalyzing the same reaction (2 H2O2-->2 H2O+O2) heme-containing monofunctional catalases and bifunctional catalase-peroxidases (KatGs) do not share sequence or structural similarities raising the question of whether or not the reaction pathways are similar or different. The production of dioxygen from hydrogen peroxide by monofunctional catalases has been shown to be a two-step process involving the redox intermediate compound I which oxidizes H2O2 directly to O2. In order to investigate the origin of O2 released in KatG mediated H2O2 degradation we performed a gas chromatography-mass spectrometry investigation of the evolved O2 from a 50:50 mixture of H2(18)O2/H2(16)O2 solution containing KatGs from Mycobacterium tuberculosis and Synechocystis PCC 6803. The GC-MS analysis clearly demonstrated the formation of (18)O2 (m/e = 36) and (16)O2 (m/e = 32) but not (16)O(18)O (m/e = 34) in the pH range 5.6-8.5 implying that O2 is formed by two-electron oxidation without breaking the O-O bond. Also active site variants of Synechocystis KatG with very low catalase but normal or even enhanced peroxidase activity (D152S, H123E, W122F, Y249F and R439A) are shown to oxidize H2O2 by a non-scrambling mechanism. The results are discussed with respect to the catalatic mechanism of KatG.

Identification of Trp106 as the tryptophanyl radical intermediate in Synechocystis PCC6803 catalase-peroxidase by multifrequency Electron Paramagnetic Resonance spectroscopy

The reactive intermediates formed in the catalase-peroxidase from Synechocystis PCC6803 upon reaction with peroxyacetic acid, and in the absence of peroxidase substrates, are the oxoferryl-porphyrin radical and two subsequent protein-based radicals that we have previously assigned to a tyrosyl (Tyr()) and tryptophanyl (Trp()) radicals by using multifrequency Electron Paramagnetic Resonance (EPR) spectroscopy combined with deuterium labeling and site-directed mutagenesis. In this work, we have further investigated the Trp() in order to identify the site for the tryptophanyl radical formation, among the 26 Trp residues of the enzyme and to possibly understand the protein constraints that determine the selective formation of this radical. Based on our previous findings about the absence of the Trp() intermediate in four of the Synechocystis catalase-peroxidase variants on the heme distal side (W122F, W106A, H123Q, and R119A) we constructed new variants on Trp122 and Trp106 positions. Trp122 is very close to the iron on the heme distal side while Trp106 belongs to a short stretch (11 amino acid residues on the enzyme surface) that is highly conserved in catalase-peroxidases. We have used EPR spectroscopy to characterize the changes on the heme microenvironment induced by these mutations as well as the chemical nature of the radicals formed in each variant. Our findings identify Trp106 as the tryptophanyl radical site in Synechocystis catalase-peroxidase. The W122H and W106Y variants were specially designed to mimic the hydrogen-bond interactions of the naturally occurring Trp residues. These variants clearly demonstrated the important role of the extensive hydrogen-bonding network of the heme distal side, in the formation of the tryptophanyl radical. Moreover, the fact that W106Y is the only Synechocystis catalase-peroxidase variant of the distal heme side that recovers a catalase activity comparable to the WT enzyme, strongly indicates that the integrity of the extensive hydrogen-bonding network is also essential for the catalatic activity of the enzyme.

Role of the Main Access Channel of Catalase-Peroxidase in Catalysis

Catalase-peroxidases (KatG) are bifunctional heme peroxidases with an overwhelming catalatic activity. The structures show that the buried heme b is connected to the exterior of the enzyme by a main channel built up by KatG-specific loops named large loop LL1 and LL2, the former containing the highly conserved sequence Met-Gly-Leu-Ile-Tyr-Val-Asn-Pro-Glu-Gly. LL1 residues Ile248, Asn251, Pro252, and Glu253 of KatG from Synechocystis are the focus of this study because of their exposure to the solute matrix of the access channel. In particular, the I248F, N251L, P252A, E253Q, and E253D mutants have been analyzed by UV-visible and resonance Raman spectroscopies in combination with steady-state and presteady-state kinetic analyses. Exchange of these residues did not alter the kinetics of cyanide binding or the overall peroxidase activity. Moreover, the kinetics of compound I formation and reduction by one-electron donors was similar in the variants and the wild-type enzyme. However, the turnover numbers of the catalase activity of I248F, N251L, E253Q, and E253D were only 12.3, 32.6, 25, and 42% of the wild-type activity, respectively. These findings demonstrate that the oxidation reaction of hydrogen peroxide (not its reduction) was affected by these mutations. The altered kinetics allowed us to monitor the spectral features of the dominating redox intermediate of E253Q in the catalase cycle. Resonance Raman data and structural analysis demonstrated the existence of a very rigid and ordered structure built up by the interactions of these residues with distal side and also (via LL1) proximal side amino acids, with the heme itself, and with the solute matrix in the channel. The role of Glu253 and the other investigated channel residues in maintaining an ordered matrix of oriented water dipoles, which guides hydrogen peroxide to its site of oxidation, is discussed.

Single-site mutations on the catalase–peroxidase from Sinorhizobium meliloti: role of the distal Gly and the three amino acids of the putative intrinsic cofactor

KatB is the only catalase-peroxidase identified so far in Sinorhizobium meliloti. It plays a housekeeping role, as it is expressed throughout all the growth phases of the free-living bacterium and also during symbiosis. This paper describes the functional and structural characterization of the KatB mutants Gly303Ser, Trp95Ala, Trp95Phe, Tyr217Leu, Tyr217Phe and Met243Val carried out by optical and electron spin resonance spectroscopy. The aim of this work was to investigate the involvement of these residues in the catalatic and/or peroxidatic reaction and falls in the frame of the open dispute around the factors that influence the balance between catalatic and peroxidatic activity in heme enzymes. The Gly303 residue is not conserved in any other protein of this family, whereas the Trp95, Tyr217 and Met243 residues are thought to form an intrinsic cofactor that is likely to play a role in intramolecular electron transfer. Spectroscopic investigations show that the Gly303Ser mutant is almost similar to the wild-type KatB and should not be involved in substrate binding. Mutations on Trp95, Tyr217 and Met243 clear out the catalatic activity completely, whereas the peroxidatic activity is maintained or even increased with respect to that of the wild-type enzyme. The k (cat) values obtained for these mutants suggest that Trp95 and Tyr217 form a huge delocalized system that provides a pathway for electron transfer to the heme. Conversely, Met243 is likely to be placed close to the binding site of the organic molecules and plays a crucial role in substrate docking.

Light-dependent expression of superoxide dismutase from cyanobacterium Synechocystis sp. strain PCC 6803

The oxygenic phototrophic cyanobacterium Synechocystis sp. strain PCC 6803 inevitably evolves superoxide during photosynthesis. Synechocystis 6803 contains only one type of superoxide dismutase, designated as SodB; therefore, this protein plays an important role in preventing oxidative damages caused by light. Because there was no direct evidence that SodB in Synechocystis 6803 could be regulated by light, the relationship between SodB and light was investigated in the present study. The activity of SodB from the cells grown in continuous light culture was about 3.5-fold higher than that from the cells cultivated in continuous dark. Illumination maximally activated SodB within 12 h. The level of sodB mRNA increased 12-fold by light, and that of SodB protein proportionally. Therefore, the expression and activity of SodB from Synechocystis 6803 were dependent on the light.

Kinetics of Interconversion of Ferrous Enzymes, Compound II and Compound III, of Wild-type Synechocystis Catalase-peroxidase and Y249F

With the exception of catalase-peroxidases, heme peroxidases show no significant ability to oxidize hydrogen peroxide and are trapped and inactivated in the compound III form by H2O2 in the absence of one-electron donors. Interestingly, some KatG variants, which lost the catalatic activity, form compound III easily. Here, we compared the kinetics of interconversion of ferrous enzymes, compound II and compound III of wild-type Synechocystis KatG, the variant Y249F, and horseradish peroxidase (HRP). It is shown that dioxygen binding to ferrous KatG and Y249F is reversible and monophasic with apparent bimolecular rate constants of (1.2 +/- 0.3) x 10(5) M(-1) s(-1) and (1.6 +/- 0.2) x 10(5) M(-1) s(-1) (pH 7, 25 degrees C), similar to HRP. The dissociation constants (KD) of the ferrous-dioxygen were calculated to be 84 microm (wild-type KatG) and 129 microm (Y249F), higher than that in HRP (1.9 microm). Ferrous Y249F and HRP can also heterolytically cleave hydrogen peroxide, forming water and an oxoferryl-type compound II at similar rates ((2.4 +/- 0.3) x 10(5) M(-1) s(-1) and (1.1 +/- 0.2) x 10(5) M(-1) s(-1) (pH 7, 25 degrees C)). Significant differences were observed in the H2O2-mediated conversion of compound II to compound III as well as in the spectral features of compound II. When compared with HRP and other heme peroxidases, in Y249F, this reaction is significantly faster ((1.2 +/- 0.2) x 10(4) M(-1) s(-1))). Ferrous wild-type KatG was also rapidly converted by hydrogen peroxide in a two-phasic reaction via compound II to compound III (approximately 2.0 x 10(5) M(-1) s(-1)), the latter being also efficiently transformed to ferric KatG. These findings are discussed with respect to a proposed mechanism for the catalatic activity.

Probing the function of Mycobacterium tuberculosis catalase-peroxidase by site-directed mutagenesis

Catalase-peroxidase is a multi-functional heme-dependent enzyme which is well known for its ability to carry out both catalatic and peroxidatic reactions. Catalase-peroxidase from Mycobacterium tuberculosis(mtCP) is of particular interest because this enzyme activates the pro-antitubercular drug isoniazid. It is estimated that 2 billion people are infected with M. tuberculosis, the principal causative agent of tuberculosis, and that 2 million people die from the disease each year. The rise of drug-resistant strains continues to be of critical concern and it is well documented that mutations which reduce activity or inactivate mtCP lead to increased levels of isoniazid resistance in M. tuberculosis. The recent determination of the crystal structure for M. tuberculosis mtCP has aided the understanding of how the enzyme functions and provides a three-dimensional framework for testing hypotheses about the roles of various residues in the active site. Here we report site-directed mutagenesis studies of three conserved residues located near the heme of mtCP, His-108, Trp-107 and Trp-321 including the construction of the double mutant W107F-W321F. Resulting mutants have been purified and their catalatic and peroxidatic activities have been determined. Data are compared in the context of related studies aimed at dissecting the roles of these residues in the different activities of the enzyme. Analyses of single and double mutants studied here emphasise that the hydrogen bonding network surrounding the heme in the active site appears more important for maintenance of catalatic rather than peroxidatic activity in CP enzymes.

Crystal Structure of Mycobacterium tuberculosis Catalase-Peroxidase

The Mycobacterium tuberculosis catalase-peroxidase is a multifunctional heme-dependent enzyme that activates the core anti-tuberculosis drug isoniazid. Numerous studies have been undertaken to elucidate the enzyme-dependent mechanism of isoniazid activation, and it is well documented that mutations that reduce activity or inactivate the catalase-peroxidase lead to increased levels of isoniazid resistance in M. tuberculosis. Interpretation of the catalytic activities and the effects of mutations upon the action of the enzyme to date have been limited due to the lack of a three-dimensional structure for this enzyme. In order to provide a more accurate model of the three-dimensional structure of the M. tuberculosis catalase-peroxidase, we have crystallized the enzyme and now report its crystal structure refined to 2.4-A resolution. The structure reveals new information about dimer assembly and provides information about the location of residues that may play a role in catalysis including candidates for protein-based radical formation. Modeling and computational studies suggest that the binding site for isoniazid is located near the delta-meso heme edge rather than in a surface loop structure as currently proposed. The availability of a crystal structure for the M. tuberculosis catalase-peroxidase also permits structural and functional effects of mutations implicated in causing elevated levels of isoniazid resistance in clinical isolates to be interpreted with improved confidence.

Induction and functional analysis of two reduced nicotinamide adenine dinucleotide phosphate-dependent glutathione peroxidase-like proteins in Synechocystis PCC 6803 during the progression of oxidative stress

Synechocystis PCC 6803 contains two types of glutathione peroxidase-like proteins (GPX-1 and GPX-2) that utilize NADPH but not reduced glutathione and unsaturated fatty acid hydroperoxides or alkyl hydroperoxides. The steady-state transcript level of gpx-1 gradually increased under oxidative stress conditions imposed by high light intensity, high salinity, or application of methylviologen or t-butyl hydroperoxide in the wild-type and GPX-2 knock-out mutant (gpx-2Delta) cells. To examine the ability of GPX-1, GPX-2, and thioredoxin peroxidase to scavenge lipid hydroperoxide in vivo, we measured the photosynthetic evolution of O(2) and the level of lipid peroxidation in the wild-type and each type of mutant cell after the application of t-butyl hydroperoxide or H(2)O(2). The data reported here indicate that GPX-1 and GPX-2 are essential for the removal of lipid hydroperoxides under normal and stress conditions, leading to the protection of membrane integrity.

Kinetic study of the effects of calcium ions on cationic artichoke (Cynara scolymus L.) peroxidase: calcium binding, steady-state kinetics and reactions with hydrogen peroxide

The apparent catalytic constant (k(cat)) of artichoke (Cynara scolymus L.) peroxidase (AKPC) with 2,2'-azino-bis(3-ethylbenzthiazoline-6-sulphonic acid) (ABTS) increased 130-fold in the presence of calcium ions (Ca2+) but the affinity (K(m)) of the enzyme for ABTS was 500 times lower than for Ca2+-free AKPC. AKPC is known to exhibit an equilibrium between 6-aquo hexa-coordinate and penta-coordinate forms of the haem iron that is modulated by Ca2+ and affects compound I formation. Measurements of the Ca2+ dissociation constant (K(D)) were complicated by the water-association/dissociation equilibrium yielding a global value more than 1000 times too high. The value for the Ca2+ binding step alone has now been determined to be K(D) approximately 10 nM. AKPC-Ca2+ was more resistant to inactivation by hydrogen peroxide (H(2)O(2)) and exhibited increased catalase activity. An analysis of the complex H(2)O(2) concentration dependent kinetics of Ca2+-free AKPC is presented.

Influence of the unusual covalent adduct on the kinetics and formation of radical intermediates in synechocystis catalase peroxidase: a stopped-flow and EPR characterization of the MET275, TYR249, and ARG439 variants

Catalase-peroxidases (KatGs) are heme peroxidases with a catalatic activity comparable to monofunctional catalases. They contain an unusual covalent distal side adduct with the side chains of Trp(122), Tyr(249), and Met(275) (Synechocysis KatG numbering). The known crystal structures suggest that Tyr(249) and Met(275) could be within hydrogen-bonding distance to Arg(439). To investigate the role of this peculiar adduct, the variants Y249F, M275I, R439A, and R439N were investigated by electronic absorption, steady-state and transient-state kinetic techniques and EPR spectroscopy combined with deuterium labeling. Exchange of these conserved residues exhibited dramatic consequences on the bifunctional activity of this peroxidase. The turnover numbers of catalase activity of M275I, Y249F, R439A, and R439N are 0.6, 0.17, 4.9, and 3.14% of wild-type activity, respectively. By contrast, the peroxidase activity was unaffected or even enhanced, in particular for the M275I variant. As shown by mass spectrometry and EPR spectra, the KatG typical adduct is intact in both Arg(439) variants, as is the case of the wild-type enzyme, whereas in the M275I variant the covalent link exists only between Tyr(249) and Trp(122). In the Y249F variant, the link is absent. EPR studies showed that the radical species formed upon reaction of the Y249F and R439A/N variants with peroxoacetic acid are the oxoferryl-porphyrin radical, the tryptophanyl and the tyrosyl radicals, as in the wild-type enzyme. The dramatic loss in catalase activity of the Y249F variant allowed the comparison of the radical species formed with hydrogen peroxide and peroxoacetic acid. The EPR data strongly suggest that the sequence of intermediates formed in the absence of a one electron donor substrate, is por(.-)(+) --> Trp(.-) (or Trp(.-)(+)) --> Tyr(.-). The M275I variant did not form the Trp(.-) species because of the dramatic changes on the heme distal side, most probably induced by the repositioning of the remaining Trp(122)-Tyr(249) adduct. The results are discussed with respect to the bifunctional activity of catalase-peroxidases.

System for the expression of recombinant hemoproteins in Escherichia coli

Expression of recombinant hemoproteins in Escherichia coli is often limited because a vast majority of the protein produced lacks the heme necessary for function. This is compounded by the fact that standard laboratory strains of E. coli have a limited capacity to withdraw heme from the extracellular environment. We are developing a new tool designed to increase the heme content of our proteins of interest by simply supplementing the expression medium with low concentrations of hemin. This hemoprotein expression (HPEX) system is based on plasmids (pHPEX1-pHPEX3) that encode an outermembrane-bound heme receptor (ChuA) from E. coli O157:H7. This heme receptor, and others like it, confers on the host the ability to more effectively internalize exogenous heme. Transformation of a standard laboratory E. coli protein expression strain (BL-21 [DE3]) with the pHPEX plasmid led to the expression of a new protein with the appropriate molecular weight for ChuA. The receptor was functional as demonstrated by the ability of the transformant to grow on iron-deficient media supplemented with hemin, an ability that the unmodified expression strain lacked. Expression of our proteins of interest, catalase-peroxidases, using this system led to a dramatic and parallel increase in heme content and activity. On a per-heme basis, the spectral and kinetic properties of HPEX-derived catalase-peroxidase were the same as those observed for catalase-peroxidases expressed in standard E. coli-based systems. We suggest that the pHPEX plasmids may be a useful addition to other E. coli expression systems and may help address a broad range of problems in hemoprotein structure and function.

Protein-based radicals in the catalase-peroxidase of synechocystis PCC6803: a multifrequency EPR investigation of wild-type and variants on the environment of the heme active site

Catalase-peroxidases are bifunctional heme enzymes with a high structural homology to peroxidases from prokaryotic origin and a catalatic activity comparable to monofunctional catalases. These unique features of catalase-peroxidases make them good systems to study and understand the role of alternative electron pathways both in catalases and peroxidases. In particular, it is of interest to study the poorly understood role of tyrosyl and tryptophanyl radicals as alternative cofactors in the catalytic cycle of catalases and peroxidases. In this work, we have used a powerful combination of multifrequency EPR spectroscopy, isotopic labeling of tryptophan and tyrosine residues, and site-directed mutagenesis to unequivocally identify the reactive intermediates formed by the wild-type Synechocystis PCC6803 catalase-peroxidase. Selected variants of the heme distal and proximal sides of the Synechocystis enzyme were investigated. Variants on the aromatic residues of the short stretch located relatively close to the heme and spanning the distal and proximal sides were also investigated. In the wild-type enzyme, the EPR signal of the catalases and peroxidases (typical) Compound I intermediate [Fe(IV)=O por.+] was observed. Two protein-based radical intermediates were also detected and identified as a Tyr. and a Trp. . The site of Trp. is proposed to be Trp 106, a residue belonging to the conserved short stretch in catalase-peroxidases and located at a 7-8 A distance to the heme propionate groups. An extensive hydrogen-bonding network on the heme distal side, involving Trp122, His123, Arg119, seven structural waters, the heme 6-propionate group, and Trp106, is proposed to have a key role on the formation of the tryptophanyl radical. We used high-field EPR spectroscopy (95-285 GHz) to resolve the g-anisotropy of the protein-based radicals in Synechocystis catalase-peroxidase. The broad gx component of the HF EPR spectrum of the Tyr. in Synechocystis catalase-peroxidase was consistent with a distributed electropositive protein environment to the tyrosyl radical.

Properties of a novel periplasmic catalase–peroxidase from Escherichia coli O157:H7

A subset of catalase-peroxidases are distinguished by their periplasmic location and their expression by pathogens. Kinetic and spectral properties have not been reported for any of these enzymes. We report the cloning, expression, isolation, and characterization of KatP, a periplasmic catalase-peroxidase from Escherichia coli O157:H7. Absorption spectra indicated a mixture of heme states dominated by the pentacoordinate and hexacoordinate high-spin forms. Apparent k(cat) values for catalase (1.8x10(4) s(-1)) and peroxidase (77 s(-1)) activities were greater than those of other catalase-peroxidases. However, apparent K(M) values for H2O2 were also higher (27 mM for catalase and 3 mM for peroxidase). Ferric KatP reacted with peracetic acid to form compound I (8.8x10(3) M(-1) s(-1)) and with CN(-) to form a ferri-cyano complex (3.9x10(5) M(-1) s(-1)) consistent with other catalase-peroxidases. The isolation and characterization of KatP opens new avenues to explore mechanisms by which the periplasmic catalase-peroxidases may contribute to bacterial virulence.

Distal side tryptophan, tyrosine and methionine in catalase-peroxidases are covalently linked in solution

Distal side tryptophan and tyrosine have been shown to be essential in the catalase but not the peroxidase activity of bifunctional catalase-peroxidases (KatGs). Recently published crystal structures suggest that both residues could be part of a novel adduct including in addition a conserved methionine. A mass spectrometric analysis of the tryptic peptides from recombinant wild-type Synechocystis KatG and the variants Trp122Phe, Tyr249Phe and Met275Ile confirms that this novel adduct really exists in solution and thus may be common to all KatGs. Exchange of either Trp122 or Tyr249 prevents cross-linking, whereas exchange of Met275 still allowed bond formation between Trp122 and Tyr249. It is proposed that the covalent bond between Trp and Tyr may form before that between Tyr and Met. The findings are discussed with respect to the mechanism of cross-linking and its role in KatG catalysis.

Total conversion of bifunctional catalase-peroxidase (KatG) to monofunctional peroxidase by exchange of a conserved distal side tyrosine

Catalase-peroxidases (KatGs) are unique peroxidases exhibiting a high catalase activity and a peroxidase activity with a wide range of artificial electron donors. Exchange of tyrosine 249 in Synechocystis KatG, a distal side residue found in all as yet sequenced KatGs, had dramatic consequences on the bifunctional activity and the spectral features of the redox intermediate compound II. The Y249F variant lost catalase activity but retained a peroxidase activity (substrates o-dianisidine, pyrogallol, guaiacol, tyrosine, and ascorbate) similar to the wild-type protein. In contrast to wild-type KatG and similar to monofunctional peroxidases, the formation of the redox intermediate compound I could be followed spectroscopically even by addition of equimolar hydrogen peroxide to ferric Y249F. The corresponding bimolecular rate constant was determined to be (1.1 +/- 0.1) x 107 m-1 s-1 (pH 7 and 15 degrees C), which is typical for most peroxidases. Additionally, for the first time a clear transition of compound I to an oxoferryl-like compound II with peaks at 418, 530, and 558 nm was monitored when one-electron donors were added to compound I. Rate constants of reaction of compound I and compound II with tyrosine ((5.0 +/- 0.3) x 104 m-1 s-1 and (1.7 +/- 0.4) x 102 m-1 s-1) and ascorbate ((1.3 +/- 0.2) x 104 m-1 s-1 and (8.8 +/- 0.1) x 101 m-1 s-1 at pH 7 and 15 degrees C) were determined by using the sequential stopped-flow technique. The relevance of these findings is discussed with respect to the bifunctional activity of KatGs and the recently published first crystal structure.

The catalytic role of the distal site asparagine-histidine couple in catalase-peroxidases

Catalase-peroxidases (KatGs) are unique in exhibiting an overwhelming catalase activity and a peroxidase activity of broad specificity. Similar to other peroxidases the distal histidine in KatGs forms a hydrogen bond with an adjacent conserved asparagine. To investigate the catalytic role(s) of this potential hydrogen bond in the bifunctional activity of KatGs, Asn153 in Synechocystis KatG was replaced with either Ala (Asn153-->Ala) or Asp (Asn153-->Asp). Both variants exhibit an overall peroxidase activity similar with wild-type KatG. Cyanide binding is monophasic, however, the second-order binding rates are reduced to 5.4% (Asn153-->Ala) and 9.5% (Asn153-->Asp) of the value of native KatG [(4.8 +/- 0.4) x 105 m-1.s-1 at pH 7 and 15 degrees C]. The turnover number of catalase activity of Asn153-->Ala is 6% and that of Asn153-->Asp is 16.5% of wild-type activity. Stopped-flow analysis of the reaction of the ferric forms with H2O2 suggest that exchange of Asn did not shift significantly the ratio of rates of H2O2-mediated compound I formation and reduction. Both rates seem to be reduced most probably because (a) the lower basicity of His123 hampers its function as acid-base catalyst and (b) Asn153 is part of an extended KatG-typical H-bond network, the integrity of which seems to be essential to provide optimal conditions for binding and oxidation of the second H2O2 molecule necessary in the catalase reaction.

Engineering the proximal heme cavity of catalase-peroxidase

Catalase-peroxidases (KatGs) are prokaryotic heme peroxidases with homology to yeast cytochrome c peroxidase (CCP) and plant ascorbate peroxidases (APXs). KatGs, CCP and APXs contain identical amino acid triads in the heme pocket (distal Arg/Trp/His and proximal His/Trp/Asp), but differ dramatically in their reactivities towards hydrogen peroxide and various one-electron donors. Only KatGs have high catalase activity in addition to a peroxidase activity of broad specificity. Here, we investigated the effect of mutating the conserved proximal triad on KatG catalysis. With the exception of W341F, all variants (H290Q, W341A, D402N, D402E) exhibited a catalase activity <1% of wild-type KatG and spectral properties indicating alterations in heme coordination and spin states. Generally, the peroxidase activity was much less effected by these mutations. Compared with wild-type KatG the W341F variant had a catalase and halogenation activity of about 40% and an even increased overall peroxidase activity. This variant, for the first time, allowed to monitor the hydrogen peroxide mediated transitions of ferric KatG to compound I and back to the resting enzyme. Compound I reduction by aromatic one-electron donors (o-dianisidine, pyrogallol, aniline) was not influenced by exchanging Trp by Phe. The findings are discussed in comparison with the data known from CCP and APX and a reaction mechanism for the multifunctional activity of the W341F variant is suggested.

Effect of distal cavity mutations on the formation of compound I in catalase-peroxidases

Catalase-peroxidases have a predominant catalase activity but differ from monofunctional catalases in exhibiting a substantial peroxidase activity and in having different residues in the heme cavity. We present a kinetic study of the formation of the key intermediate compound I by probing the role of the conserved distal amino acid triad Arg-Trp-His of a recombinant catalase-peroxidase in its reaction with hydrogen peroxide, peroxoacetic acid, and m-chloroperbenzoic acid. Both the wild-type enzyme and six mutants (R119A, R119N, W122F, W122A, H123Q, H123E) have been investigated by steady-state and stopped-flow spectroscopy. The turnover number of catalase activity of R119A is 14.6%, R119N 0.5%, H123E 0.03%, and H123Q 0.02% of wild-type activity. Interestingly, W122F and W122A completely lost their catalase activity but retained their peroxidase activity. Bimolecular rate constants of compound I formation of the wild-type enzyme and the mutants have been determined. The Trp-122 mutants for the first time made it possible to follow the transition of the ferric enzyme to compound I by hydrogen peroxide spectroscopically underlining the important role of Trp-122 in catalase activity. The results demonstrate that the role of the distal His-Arg pair in catalase-peroxidases is important in the heterolytic cleavage of hydrogen peroxide (i.e. compound I formation), whereas the distal tryptophan is essential for compound I reduction by hydrogen peroxide.

Nucleotide sequence analysis, overexpression in Escherichia coli and kinetic characterization of Anacystis nidulans catalase-peroxidase

Bifunctional catalase-peroxidases are the least understood type of peroxidases. A high-level expression in Escherichia coli of a fully active recombinant form of a catalase-peroxidase (KatG) from the cyanobacterium Anacystis nidulans (Synechococcus PCC 6301) is reported. Since both physical and kinetic characterization revealed its identity with the wild-type protein, the large quantities of recombinant KatG allowed the examination of both the spectral characteristics and the reactivity of its redox intermediates by using the multi-mixing stopped-flow technique. The homodimeric acidic protein (pI = 4.6) contained high catalase activity (apparent K(m) = 4.8 mM and apparent k(cat) = 8850 s(-1)). Cyanide is shown to be an effective inhibitor of the catalase reaction. The second-order rate constant for cyanide binding to the ferric protein is (6.9 +/- 0.2) x 10(5) M(-1 )s(-1) at pH 7.0 and 15 degrees C and the dissociation constant of the cyanide complex is 17 microM. Because of the overwhelming catalase activity, peroxoacetic acid has been used for compound I formation. The apparent second-order rate constant for formation of compound I from the ferric enzyme and peroxoacetic acid is (1.3 +/- 0.3) x 10(4 )M(-1 )s(-1) at pH 7.0 and 15 degrees C. The spectrum of compound I is characterized by about 40% hypochromicity, a Soret region at 406 nm, and isosbestic points between the native enzyme and compound I at 355 and 428 nm. Rate constants for reduction of KatG compound I by o-dianisidine, pyrogallol, aniline and isoniazid are shown to be (7.3 +/- 0.4) x 10(6) M(-1 )s(-1), (5.4 +/- 0.3) x 10(5) M(-1 )s(-1), (1.6 +/- 0.3) x 10(5) M(-1 )s(-1) and (4.3 +/- 0.2) x 10(4) M(-1 )s(-1), respectively. The redox intermediate formed upon reduction of compound I did not exhibit the classical red-shifted peroxidase compound II spectrum which characterizes the presence of a ferryl oxygen species. Its spectral features indicate that the single oxidizing equivalent in KatG compound II is contained on an amino acid which is not electronically coupled to the heme.