Corynebacterium glutamicum superoxide dismutase is a manganese-strict non-cambialistic enzyme in vitro

Superoxide dismutase (SOD) of Corynebacterium glutamicum was purified and characterized. The enzyme had a native molecular weight of about 80kDa, whereas a monomer with molecular weight of 24kDa was found on SDS-PAGE suggesting it to be homotetramer. The native SOD activity stained gel revealed a unique cytosolic enzyme. Supplementing growth media with manganese increased the specific activity significantly, while adding iron did not result in significant difference. No growth perturbation was observed with the supplemented media. In vitro metal removal and replacement studies revealed conservation of about 85% of the specific activity by substitution with manganese, while substitution with copper, iron, nickel or zinc did not restore any significant specific activity. Manganese was identified by atomic absorption spectrometer, while no signals corresponding to fixing other metallic elements were detected. Thus, C. glutamicum SOD could be considered a strict (non-cambialistic) manganese superoxide dismutase (MnSOD).     

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

The superoxide dismutase produced by Streptococcus mutans OMZ176 during aerobic growth in a chemically defined medium (modified FMC) that was treated with Chelex 100 (to lower trace metal contamination) and supplemented with high purity manganese was purified (162-fold) by heat treatment, ammonium sulfate precipitation, and chromatofocusing chromatography. The superoxide dismutase produced during aerobic growth in the same medium, but without manganese and supplemented with high purity iron, was similarly purified (220-fold). The molecular masses of each holoenzyme were approximately 43,000 with a subunit mass of 20,700, indicating that the enzymes were dimers of two equally sized subunits. The superoxide dismutase from manganese-grown cells was a manganese enzyme (MnSOD) containing 1.2 atoms of manganese and 0.25 atoms of iron/subunit. The superoxide dismutase from iron-grown cells was an iron enzyme (FeSOD) containing 0.07 atoms of manganese and 0.78 atoms of iron/subunit. The amino acid compositions of the MnSOD and the FeSOD were virtually identical, and their amino-terminal sequences were identical through the first 22 amino acids. Dialysis of the FeSOD with o-phenanthroline and sodium ascorbate generated aposuperoxide dismutase with 94% loss of activity; subsequent dialysis of apoenzyme with either manganese sulfate or ferrous sulfate reconstituted activity (recoveries of 37 and 30%, respectively). Electrophoretic determination of cytoplasmic radioiron distribution indicated that (during aerobic growth) manganese prevented insertion of iron into superoxide dismutase, although the iron levels of at least two other cytoplasmic fractions were not altered by manganese. Therefore, S. mutans used the same aposuperoxide dismutase to form either FeSOD or MnSOD, depending upon which metal was available in the culture medium. Such "cambialistic" enzymes (those capable of making a cofactor substitution) may represent a previously unrecognized family of superoxide dismutases.     

Identity of the metal ligands in the manganese- and iron-containing superoxide dismutases

Alignment of the amino acid sequence of peptides obtained following digestion of Photobacterium leiognathi iron superoxide dismutase with the known sequence of Bacillus stearothermophilus manganese superoxide dismutase shows that the residues found to form ligands to the manganese are conserved in the iron enzyme. This indicates that the metal ligands in both proteins are identical.     

Metallation state of human manganese superoxide dismutase expressed in Saccharomyces cerevisiae

Human manganese superoxide dismutase (Sod2p) has been expressed in yeast and the protein purified from isolated yeast mitochondria, yielding both the metallated protein and the less stable apoprotein in a single chromatographic step. At 30 °C growth temperature, more than half of the purified enzyme is apoprotein that can be fully activated following reconstitution, while the remainder contains a mixture of manganese and iron. In contrast, only fully metallated enzyme was isolated from a similarly constructed yeast strain expressing the homologous yeast manganese superoxide dismutase. Both the manganese content and superoxide dismutase activity of the recombinant human enzyme increased with increasing growth temperatures. The dependence of in vivo metallation state on growth temperature resembles the in vitro thermal activation behavior of human manganese superoxide dismutase observed in previous studies. Partially metallated human superoxide dismutase is fully active in protecting yeast against superoxide stress produced by addition of paraquat to the growth medium. However, a splice variant of human manganese superoxide dismutase (isoform B) is expressed as insoluble protein in both Escherichia coli and yeast mitochondria and did not protect yeast against superoxide stress.     

Engineering and characterization of human manganese superoxide dismutase mutants with high activity and low product inhibition

Human manganese superoxide dismutase is a mitochondrial metalloenzyme that is involved in protecting aerobic organisms against superoxide toxicity, and has been implicated in slowing tumor growth. Unfortunately, this enzyme exhibits strong product inhibition, which limits its potential biomedical applications. Previous efforts to alleviate human manganese superoxide dismutase product inhibition utilized rational protein design and site-directed mutagenesis. These efforts led to variants of human manganese superoxide dismutase at residue 143 with dramatically reduced product inhibition, but also reduced catalytic activity and efficiency. Here, we report the use of a directed evolution approach to engineer two variants of the Q143A human manganese superoxide dismutase mutant enzyme with improved catalytic activity and efficiency. Two separate activity-restoring mutations were found--C140S and N73S--that increase the catalytic efficiency of the parent Q143A human manganese superoxide dismutase enzyme by up to five-fold while maintaining low product inhibition. Interestingly, C140S is a context-dependent mutation, and the C140S-Q143A human manganese superoxide dismutase did not follow Michaelis-Menten kinetics. The re-engineered human manganese superoxide dismutase mutants should be useful for biomedical applications, and our kinetic and structural studies also provide new insights into the structure-function relationships of human manganese superoxide dismutase.     

Manganese superoxide dismutase from a higher plant

A manganese-containing superoxide dismutase (EC 1.15.1.1) was purified to homogeneity from a higher plant for the first time. The enzyme was isolated fromPisum sativum leaf extracts by thermal fractionation, ammonium sulfate salting out, ion-exchange and gel-filtration column chromatography, and preparative polyacrylamide gel electrophoresis. Pure manganese superoxide dismutase had a specific activity of about 3,000 U mg^-1 and was purified 215-fold, with a yield of 1.2 mg enzyme per kg whole leaf. The manganese superoxide dismutase had a molecular weight of 94,000 and contained one g-atom of Mn per mol of enzyme. No iron and copper were detected. Activity reconstitution experiments with the pure enzyme ruled out the possibility of a manganese loss during the purification procedure. The stability of manganese superoxide dismutase at-20°C, 4°C, 25°C, 50°C, and 60°C was studied, and the enzyme was found more labile at high temperatures than bacterial manganese superoxide dismutases and iron superoxide dismutases from an algal and bacterial origin.Abbreviations NBT nitro blue tetrazolium - SOD superoxide dismutase (EC 1.15.1.1)     

Protection of Methanosarcina barkeri against oxidative stress: identification and characterization of an iron superoxide dismutase

Methanosarcina barkeri is a methanogenic archaeon that can only grow under strictly anoxic conditions but which can survive oxidative stress. We have recently reported that the organism contains a monofunctional catalase. We describe here that it also possesses an active iron superoxide dismutase. The enzyme was purified in three steps over 130-fold in a 14% yield to a specific activity of 1500 U/mg. SDS-PAGE revealed the presence of only one band, at an apparent molecular mass of 25 kDa. The primary structure determined from the cloned and sequenced gene revealed similarity to iron- and manganese superoxide dismutases. The highest similarity was to the iron superoxide dismutase from Methanobacterium thermoautotrophicum. The enzyme from M. barkeri was found to contain, per mol, 1 mol iron, but no manganese in agreement with the general observation that anaerobically growing organisms only contain iron superoxide dismutase. The enzyme was not inhibited by cyanide (10 mM), which is a property shared by all iron- and manganese superoxide dismutases. The presence of superoxide dismutase in M. barkeri is noteworthy since a gene encoding superoxide dismutase (sod) has not been found in Archaeoglobus fulgidus, a sulfate-reducing archaeon most closely related to the Methanosarcinaceae.     

The single superoxide dismutase of Rhodobacter capsulatus is a cambialistic,manganese-containing enzyme

The phototrophic bacterium Rhodobacter capsulatus contains a single, oxygen-responsive superoxide dismutase (SOD(Rc)) homologous to iron-containing superoxide dismutase enzymes. Recombinant SOD(Rc), however, displayed higher activity after refolding with Mn(2+), especially when the pH of the assay mixture was raised. SOD(Rc) isolated from Rhodobacter cells also preferentially contains manganese, but metal discrimination depends on the culture conditions, with iron fractions increasing from 7% in aerobic cultures up to 40% in photosynthetic cultures. Therefore, SOD(Rc) behaves as a Mn-containing dismutase with cambialistic properties.     

Manganese and iron superoxide dismutases are structural homologs

The crystal structure of a tetrameric manganese superoxide dismutase from a thermophilic bacterium, Thermus thermophilus HB8, has been determined at 4.4-A resolution by local averaging of electron density maps calculated by isomorphous replacement. The spatial arrangement of the principal secondary structural features of iron superoxide dismutase is conserved in manganese dismutase. The structural homology is displayed by orienting the polypeptide chain of Escherichia coli Fe dismutase in the electron density map of Mn dismutase. Densities corresponding to bound Mn3+ occur at locations equivalent to the Fe3+ positions in iron dismutase, indicating one metal binding site per chain, or four sites per tetramer. The Mn tetramer, with 222 symmetry, is approximately rectangular in shape and appears to be constructed with only two unique interfaces. One set of interchain contacts closely resembles the dimer interface of Fe dismutase, but the other interface utilizes an inserted polypeptide segment that has no equivalent in Fe dismutase.     

Effects of iron and manganese availability on growth and production of superoxide dismutase by Streptococcus suis

A complex medium supported good growth of the type strain of Streptococcus suis irrespective of the presence or absence of a high concentration (1 microM) of the iron chelating agent, ethylenediamine di-o-hydroxyphenylacetic acid. Good growth was also obtained using a complex medium that had been treated with Chelex-100 to reduce the iron content, but only if this medium was supplemented with manganese; supplementation with iron had little effect. Collectively, these results indicate that S. suis requires manganese, but not iron, for growth. Polyacrylamide gel electrophoresis of cell extracts followed by activity staining revealed the presence of a single manganese-cofactored superoxide dismutase; activity staining and enzyme assays revealed that manganese availability during growth affected the activity of the superoxide dismutase in cell extracts. The results are discussed with respect to the capacity of S. suis to avoid damage by reactive oxygen species.     

Effect of iron on expression of superoxide dismutase by Aeromonas salmonicida and associated resistance to superoxide anion

Abstract Superoxide dismutase activity was detected in Aeromonas salmonicida under iron-replete and iron-limited culture conditions. Under iron-replete conditions an iron superoxide dismutase, molecular mass 50,400 Da, was identified based on inhibition by hydrogen peroxide but not by millimolar concentrations of cyanide. When the available iron in the culture medium was limited by addition of the non-assimilable iron chelator 2,2-dipyridyl, a manganese superoxide dismutase, molecular mass 45,600 Da, was identified, which was resistant to inhibition by either hydrogen peroxide or cyanide. The change in enzyme production would appear to be iron dependent, as addition of FeCl₃ in excess to iron-limited broths resulted in only the iron superoxide dismutase being synthesised. Examination of the location of the superoxide dismutase enzymes revealed that the manganese superoxide dismutase expressed under iron limitation is located in the periplasm, while the iron superoxide dismutase has a cytoplasmic location. The periplasmic manganese superoxide dismutase was able to protect A. salmonicida against extracellular riboflavin-generated superoxide, with A. salmonicida grown under iron-limited conditions exhibiting a 32-fold increase in minimum bactericidal concentration of riboflavin compared to cells cultured under iron-replete conditions. Furthermore, in a time-course study of bactericidal activity of exogenously generated superoxide against A. salmonicida , bacteria grown under iron-replete conditions and expressing cytoplasmic iron superoxide dismutase were rapidly killed, whilst those grown under iron limitation expressing periplasmic manganese superoxide dismutase survived for the duration of the experiment.     

Immunocytochemical evidence for a peroxisomal localization of manganese superoxide dismutase in leaf protoplasts from a higher plant

The controversial question of the intracellular location of manganese-containing superoxide dismutase in higher plants was examined under a new experimental approach by applying the more rigorous and specific methods of immunocytochemistry to protoplasts isolated fromPisum sativum L. leaves. Manganese superoxide dismutase (EC 1.15.1.1) was purified to homogeneity from 15 kg of leaves ofPisum sativum L. Rabbits were immunized with the mangano enzyme and the antibody specific for pea manganese superoxide dismutase was purified and found not to contain antigenic sites in common with (i) human manganese superoxide dismutase, (ii) iron superoxide dismutase from eitherEscherichia coli or higher plants, or (iii) plant or animal cuprozinc-superoxide dismutase.Pisum sativum L. manganese superoxide dismutase only appears to have antigenic determinants similar to other manganese superoxide dismutases from higher land plants. The antibody to pea Mn-superoxide dismutase was used to locate the enzyme in protoplasts isolated from young pea leaves by indirect immunofluorescence, and by electron microscopy using the unlabelled antibody peroxidase-antiperoxidase method. Results from immunofluorescence showed that chloroplasts were devoid of specific fluorescence which appeared scattered over the cytosolic spaces among chloroplasts, and demonstrate the absence of manganese superoxide dismutase inside chloroplasts. The metalloenzyme was found to be localized only in peroxisomes, whereas mitochondria, the traditionally accepted site for this enzyme in many eukaryotic organisms, did not show any specific staining. The possible subcellular roles of manganese superoxide dismutase inPisum sativum L. leaves are discussed in the light of its peroxisomal location.     

Frataxin depletion in yeast triggers up-regulation of iron transport systems before affecting iron-sulfur enzyme activities

The primary function of frataxin, a mitochondrial protein involved in iron homeostasis, remains controversial. Using a yeast model of conditional expression of the frataxin homologue YFH1, we analyzed the primary effects of YFH1 depletion. The main conclusion unambiguously points to the up-regulation of iron transport systems as a primary effect of YFH1 down-regulation. We observed that inactivation of aconitase, an iron-sulfur enzyme, occurs long after the iron uptake system has been activated. Decreased aconitase activity should be considered part of a group of secondary events promoted by iron overloading, which includes decreased superoxide dismutase activity and increased protein carbonyl formation. Impaired manganese uptake, which contributes to superoxide dismutase deficiency, has also been observed in YFH1-deficient cells. This low manganese content can be attributed to the down-regulation of the metal ion transporter Smf2. Low Smf2 levels were not observed in AFT1/YFH1 double mutants, indicating that high iron levels could be responsible for the Smf2 decline. In summary, the results presented here indicate that decreased iron-sulfur enzyme activities in YFH1-deficient cells are the consequence of the oxidative stress conditions suffered by these cells.     

Effect of transition metals on stress, lipid peroxidation and antioxidant enzyme activities in tobacco cell cultures

to-baccoBright Yellow 2 (BY-2) suspension culture to understand the mechanisms of metal resistance in plant cells.We have analysed superoxide dismutase, catalase, and ascorbate peroxidase enzyme activities and superoxidedismutase-isoforms by isoelectric focusing gels in tobacco cells grown at two different toxic concentrations ofeach of the transition metals: copper, iron, manganese and zinc. Exposure of tobacco cells to these metals causedchanges in total superoxide dismutase activity in a different manner, depending on the metal assayed: after cop-perand manganese treatments, total superoxide dismutase activity was enhanced, while it was reduced after ironand zinc exposure. Superoxide dismutase-isoforms were affected by the metal used, and a Fe-SOD band with thesame isoelectric point as a Cu, Zn-SOD from non-treated cells, was induced after iron and zinc treatments. Cu,Zn-SODs were present in all metal-treatments whereas Mn-SOD was not detected in any case. Concerning otherantioxidant enzymes tested, such as catalase and ascorbate peroxidase, the latter showed a remarkable increase inactivity in response to copper treatments and catalase activity was enhanced after iron and with the lowest man-ganeseconcentration. Lipid peroxidation was increased after each metal treatment, as an indication of the oxi-dativedamage caused by metal concentration assayed in tobacco cells. These results suggest that an activation ofsome antioxidant enzymes in response to oxidative stress induced by transition metals is not enough to confertolerance to metal accumulation.     

Characterization of a Manganese Superoxide Dismutase from the Higher Plant Pisum sativum

A manganese-containing superoxide dismutase (EC 1.15.1.1) was fully characterized from leaves of the higher plant Pisum sativum L., var. Lincoln. The amino acid composition determined for the enzyme was compared with that of a wide spectrum of superoxide dismutases and found to have a highest degree of homology with the mitochondrial manganese superoxide dismutases from rat liver and yeast. The enzyme showed an apparent pH optimum of 8.6 and at 25°C had a maximum stability at alkaline pH values. By kinetic competition experiments, the rate constant for the disproportionation of superoxide radicals by pea leaf manganese superoxide dismutase was found to be 1.61 × 10⁹ molar⁻¹·second⁻¹ at pH 7.8 and 25°C. The enzyme was not sensitive to NaCN or to H₂O₂, but was inhibited by N₃⁻. The sulfhydryl reagent p-hydroxymercuribenzoate at 1 mm concentration produced a nearly complete inhibition of the manganese superoxide dismutase activity. The metal chelators o-phenanthroline, EDTA, and diethyldithiocarbamate all inhibited activity slightly in decreasing order of intensity. A comparative study between this higher plant manganese superoxide dismutase and other dismutases from different origins is presented.     

Exogenous manganous ion at millimolar levels rescues all known dioxygen-sensitive phenotypes of yeast lacking CuZnSOD

Yeasts lacking copper-zinc superoxide dismutase (sod1Delta) exhibit a broad range of phenotypes, many of which can be rescued by growth in the presence of high levels of ionic manganese. We undertook a comprehensive survey of the effects of manganese on wild-type and sod1Delta yeasts and found that 5 mM Mn2+ rescued all known growth-related phenotypes, such as slow growth in air, temperature sensitivity, specific amino acid auxotrophies, no growth in high oxygen, poor growth in nonfermentable carbon sources, and decreased stationary-phase survival. Iron-related phenotypes-elevated electron paramagnetic resonance detectable ("free") iron, decreased aconitase activity, and fragmenting vacuoles-as well as zinc sensitivity were also rescued. The activity of manganese superoxide dismutase remained constant or was reduced when the yeasts were grown in the presence of MnCl2, indicating that induction of this alternative superoxide dismutase is not the explanation. In contrast to MnCl2 treatment, addition of two manganese-containing superoxide dismutase mimetic compounds to the growth medium did not provide any rescue of sod1Delta yeast growth but rather had an sod1Delta-selective inhibitory effect at micromolar concentrations. Mechanisms by which ionic manganese can effect this rescue, while the mimetic compounds do not, are discussed.     

A superoxide dismutase from the archaeon Sulfolobus solfataricus is an extracellular enzyme and prevents the deactivation by superoxide of cell-bound proteins.

An oxygen-induced iron superoxide dismutase was found in the culture fluid of the thermoacidophilic crenarchaeon Sulfolobus solfataricus during growth on glucose-rich media. This protein was also identified as being associated with the cell-surface, with the amount of the released and cell-bound protein fractions depending on the growth phase of the cells. The steady decrease in cell-associated superoxide dismutase during continued growth correlated with the increase of free superoxide dismutase in the medium. Both enzyme fractions were purified to homogeneity and found to be active with different catalytic efficiency, with the released superoxide dismutase showing a fourfold lower specific activity. Characterization in comparison with the cytosolic superoxide dismutase revealed identical N-terminal sequences, electrophoretic mobility, isoelectric point, and molecular mass for all three differently located enzymes. In order to clarify the physiological role of the cell-associated superoxide dismutase, the prevention of cell-bound protein deactivation by oxyradicals was also investigated. Glucose dehydrogenase, which was chosen as a model enzyme, was demonstrated to be located on the cell surface and to be inactivated by potassium superoxide by in vivo assays. The direct protective effect of superoxide dismutase on glucose dehydrogenase was demonstrated by in vitro assays on the free released enzyme. Similarly, the prevention of deactivation by potassium superoxide was also demonstrated for the integral membrane protein succinate dehydrogenase by intact cell assay. Superoxide dismutase added to cells was shown to moderately reduce the critical damaging peroxidation and hence play a major role in maintaining the integrity of the outer cell envelope components.     

Battles with iron: manganese in oxidative stress protection

The redox-active metal manganese plays a key role in cellular adaptation to oxidative stress. As a cofactor for manganese superoxide dismutase or through formation of non-proteinaceous manganese antioxidants, this metal can combat oxidative damage without deleterious side effects of Fenton chemistry. In either case, the antioxidant properties of manganese are vulnerable to iron. Cellular pools of iron can outcompete manganese for binding to manganese superoxide dismutase, and through Fenton chemistry, iron may counteract the benefits of non-proteinaceous manganese antioxidants. In this minireview, we highlight ways in which cells maximize the efficacy of manganese as an antioxidant in the midst of pro-oxidant iron.     

The effects of mitochondrial iron homeostasis on cofactor specificity of superoxide dismutase 2

Many metalloproteins have the capacity to bind diverse metals, but in living cells connect only with their cognate metal cofactor. In eukaryotes, this metal specificity can be achieved through metal-specific metallochaperone proteins. Herein, we describe a mechanism whereby Saccharomyces cerevisiae manganese superoxide dismutase (SOD2) preferentially binds manganese over iron based on the differential bioavailability of these ions within mitochondria. The bulk of mitochondrial iron is normally unavailable to SOD2, but when mitochondrial iron homeostasis is disrupted, for example, by mutations in S. cerevisiae mtm1, ssq1 and grx5, iron accumulates in a reactive form that potently competes with manganese for binding to SOD2, inactivating the enzyme. Studies in mtm1 mutants indicate that iron inactivation of SOD2 involves the Mrs3p/Mrs4p mitochondrial carriers and iron-binding frataxin (Yfh1p). A small pool of SOD2-reactive iron also exists under normal iron homeostasis conditions and binds SOD2 when mitochondrial manganese is low. The ability to control this reactive pool of iron is critical to maintaining SOD2 activity and has important potential implications for oxidative stress in disorders of iron overload.     

Thermally triggered metal binding by recombinant Thermus thermophilus manganese superoxide dismutase, expressed as the apo-enzyme

Manganese superoxide dismutase from the extremely thermophilic eubacterium Thermus thermophilus has been cloned and expressed at high levels in a mesophilic host (Escherichia coli) as a soluble tetrameric protein mainly present as the metal-free apo-enzyme. Incubation of the purified apo-enzyme with manganese salts at ambient temperature did not restore superoxide dismutase activity, but reactivation could be achieved by heating the protein with Mn(II) at higher temperatures, approaching the physiological growth temperature for T. thermophilus. Heat annealing followed by incubation with manganese at lower temperature fails to reactivate the enzyme, demonstrating that a simple misfolding of the protein is not responsible for the observed behavior. The in vitro metal uptake is nonspecific, and manganese, iron, and vanadium all bind, but only manganese restores catalytic activity. Bound metal ions do not exchange during heat treatment, indicating that the formation of the metal complex is effectively irreversible under these conditions. The metallation process is strongly temperature-dependent, suggesting that substantial activation barriers to metal uptake at ambient temperature are overcome by a thermal transition in the apo-protein structure. A mechanism for SOD metallation is proposed, focusing on interactions at the domain interface.