Mechanisms of catalase activity of heme peroxidases

Ferryl-oxo species produced from Fenton's reagent via a two-step pathway: minimum free-energy path analysis

A mixture of ferrous ions and hydrogen peroxide, known as Fenton's reagent, is an effective oxidant and has been widely used in various industrial applications; however, there is still controversy about what the oxidizing agents are and how they are produced. In this study, we have determined minimum free-energy paths (MFEPs) from Fenton's reagent to possible oxidizing agents such as hydroxyl radicals and ferryl-oxo species by combining ab initio molecular dynamics simulations and an MFEP search method. Along the MFEPs, representative free-energy profiles of the Fenton reaction were elucidated. On the basis of the free-energy profiles, we revealed that the reaction producing ferryl-oxo species from Fenton's reagent is more energetically favorable than that yielding a free hydroxyl radical, by 24.4 kcal mol(-1), which indicates that the ferryl-oxo species is the primary oxidizing agent in reactions of Fenton's reagent. Moreover, we clarified that the ferryl-oxo species is favorably formed via a two-step reaction pathway, which reaches the product through a dihydroxyiron(IV) intermediate. The energetics charting the free-energy profiles provided valuable information for a comprehensive understanding of Fenton reactions. We concluded that a ferryl-oxo species produced from Fenton's reagent serves as the primary oxidizing agent in the Fenton reaction.

Molecular evolution of hydrogen peroxide degrading enzymes

For efficient removal of intra- and/or extracellular hydrogen peroxide by dismutation to harmless dioxygen and water (2H(2)O(2) → O(2) + 2H(2)O), nature designed three metalloenzyme families that differ in oligomeric organization, monomer architecture as well as active site geometry and catalytic residues. Here we report on the updated reconstruction of the molecular phylogeny of these three gene families. Ubiquitous typical (monofunctional) heme catalases are found in all domains of life showing a high structural conservation. Their evolution was directed from large subunit towards small subunit proteins and further to fused proteins where the catalase fold was retained but lost its original functionality. Bifunctional catalase-peroxidases were at the origin of one of the two main heme peroxidase superfamilies (i.e. peroxidase-catalase superfamily) and constitute a protein family predominantly present among eubacteria and archaea, but two evolutionary branches are also found in the eukaryotic world. Non-heme manganese catalases are a relatively small protein family with very old roots only present among bacteria and archaea. Phylogenetic analyses of the three protein families reveal features typical (i) for the evolution of whole genomes as well as (ii) for specific evolutionary events including horizontal gene transfer, paralog formation and gene fusion. As catalases have reached a striking diversity among prokaryotic and eukaryotic pathogens, understanding their phylogenetic and molecular relationship and function will contribute to drug design for prevention of diseases of humans, animals and plants.

High conformational stability of secreted eukaryotic catalase-peroxidases: answers from first crystal structure and unfolding studies

Catalase-peroxidases (KatGs) are bifunctional heme enzymes widely spread in archaea, bacteria, and lower eukaryotes. Here we present the first crystal structure (1.55 Å resolution) of an eukaryotic KatG, the extracellular or secreted enzyme from the phytopathogenic fungus Magnaporthe grisea. The heme cavity of the homodimeric enzyme is similar to prokaryotic KatGs including the unique distal (+)Met-Tyr-Trp adduct (where the Trp is further modified by peroxidation) and its associated mobile arginine. The structure also revealed several conspicuous peculiarities that are fully conserved in all secreted eukaryotic KatGs. Peculiarities include the wrapping at the dimer interface of the N-terminal elongations from the two subunits and cysteine residues that cross-link the two subunits. Differential scanning calorimetry and temperature- and urea-mediated unfolding followed by UV-visible, circular dichroism, and fluorescence spectroscopy combined with site-directed mutagenesis demonstrated that secreted eukaryotic KatGs have a significantly higher conformational stability as well as a different unfolding pattern when compared with intracellular eukaryotic and prokaryotic catalase-peroxidases. We discuss these properties with respect to the structure as well as the postulated roles of this metalloenzyme in host-pathogen interactions.

Enhancing the peroxidatic activity of KatG by deletion mutagenesis

Catalase-peroxidase (KatG) enzymes use a peroxidase active site to facilitate robust catalase activity, an ability all other members of its superfamily lack. KatG's have a Met-Tyr-Trp covalent adduct that is essential for catalatic but not peroxidatic turnover. The tyrosine (Y226 in E. coli KatG) is supplied by a large loop (LL1) that is absent from all other plant peroxidases. Elimination of Y226 from the KatG structure, either by site directed mutagenesis (i.e., Y226F KatG) or by deletion of larger portions of LL1 invariably eliminates catalase activity, but deletion variants were substantially more active as peroxidases, up to an order of magnitude. Moreover, the deletion variants were more resistant to H(2)O(2)-dependent inactivation than Y226F KatG. Stopped-flow evaluation of reactions of H(2)O(2) with Y226F KatG and the most peroxidase active deletion variant (KatG[Δ209-228]) produced highly similar rate constants for formation of compounds I and II, and about a four-fold faster formation of compound III for the deletion variant as opposed to Y226F. Conversely, single turnover experiments showed a 60-fold slower return of Y226F KatG to its ferric state in the presence of the exogenous electron donor 2,2'-azino-bis(3-ethylbenzthiazoline-6-sulfonic acid) (ABTS) than was determined for KatG(Δ209-228). Our data suggest that the peroxidatic output of KatG cannot be optimized simply by elimination of catalase activity alone, but also requires modifications that increase electron transfer between exogenous electron donors and the heme prosthetic group.

Catalases versus peroxidases: DFT investigation of H₂O₂ oxidation in models systems and implications for heme protein engineering

Catalases and peroxidases are ubiquitous heme enzymes that catalyze the removal of hydrogen peroxide (H(2)O(2)). Both enzymes use one molecule of hydrogen peroxide to form a high valent iron intermediate named Compound I (Cpd I). However, whereas catalase Cpd I oxidizes a second H(2)O(2) molecule to oxygen, peroxidases use this intermediate to oxidize other substrates rather than H(2)O(2). The origin of the different reactivity of peroxidases and catalases is not known, but it is likely to be related to structural differences between the two heme active sites. Recent modeling studies suggest that the oxidation of H(2)O(2) by catalase Cpd I may take place by two hydrogen atom transfer steps. In this work, we investigate how catalases and peroxidases compare along the same hydrogen transfer steps to give hints into the question why peroxidases cannot efficiently oxidize H(2)O(2). The use of simplified models allows us to probe the direct effect of the proximal ligand (tyrosinate in catalases and histidine in peroxidases) without masking from the protein environment. We show that the nature of the fifth ligand (His in peroxidase and Tyr in catalase) has little effect on the energy barriers of the hydrogen transfer steps. On the contrary, the Cpd I-hydrogen peroxide (O(Fe)-O(peroxide)) distance affects significantly the reaction barriers. We propose that the distal side architecture of peroxidases do not allow to attain short O(Cpd I)-O(peroxide) distances, thus resulting in a lower efficiency towards H(2)O(2) oxidation.

Comparative phenotypic and molecular characterization of porcine mesenchymal stem cells from different sources for translational studies in a large animal model

Mesenchymal stem cells have demonstrated their potentiality for therapeutic use in treating diseases or repairing damaged tissues. However, in some cases, the results of clinical trials have been disappointing or have not worked out as well as hoped. These disappointing results can be attributed to an inadequate or insufficient preclinical study. For medical and surgical purposes, the similarities between the anatomy of pig and human make this animal an attractive preclinical model. In this sense, for mesenchymal stem cell-based therapy, it is strongly necessary to have well characterized animal-derived mesenchymal stem cell lines to validate preclinical effectiveness of these cells. In this work, porcine mesenchymal stem cells (pMSCs) were isolated from bone marrow, adipose tissue and peripheral blood and compared in terms of differentiation potential, cell surface markers and gene expression. Our results demonstrated that the isolation and in vitro expansion protocols were feasible and effective. The data presented in this work are relevant because they provide an extensive phenotypic characterization; genetic study and differentiation behavior of the most commonly used stem cell lines for clinical practices. These pMSCs are widely available to scientists and could be a valuable tool to evaluate the safety and efficacy of adoptively transferred cells.

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.

Oxidation of phenolic compounds by the bifunctional catalase-phenol oxidase (CATPO) from Scytalidium thermophilum

The thermophilic fungus Scytalidium thermophilum produces a novel bifunctional catalase with an additional phenol oxidase activity (CATPO); however, its phenol oxidation spectrum is not known. Here, 14 phenolic compounds were selected as substrates, among which (+)-catechin, catechol, caffeic acid, and chlorogenic acid yielded distinct oxidation products examined by reversed-phase HPLC chromatography method. Characterization of the products by LC-ESI/MS and UV-vis spectroscopy suggests the formation of dimers of dehydrocatechin type B (hydrophilic) and type A (hydrophobic), as well as oligomers, namely, a trimer and tetramer from (+)-catechin, the formation of a dimer and oligomer of catechol, a dimer from caffeic acid with a caffeicin-like structure, as well as trimeric and tetrameric derivatives, and a single major product from chlorogenic acid suggested to be a dimer. Based on the results, CATPO oxidizes phenolic compounds ranging from simple phenols to polyphenols but all having an ortho-diphenolic structure in common. The enzyme also appears to have stereoselectivity due to the oxidation of (+)-catechin, but not that of epicatechin. It is suggested that CATPO may contribute to the antioxidant mechanism of the fungus and may be of value for future food and biotechnology applications where such a bifunctional activity would be desirable.

Role of proximal methionine residues in Leishmania major peroxidase

The active site architecture of Leishmania major peroxidase (LmP) is very similar with both cytochrome c peroxidase and ascorbate peroxidase. We utilized point mutagenesis to investigate if the conserved proximal methionine residues (Met248 and Met249) in LmP help in controlling catalysis. Steady-state kinetics of methionine mutants shows that ferrocytochrome c oxidation is <2% of wild type levels without affecting the second order rate constant of first phase of Compound I formation, while the activity toward a small molecule substrate, guaiacol or iodide, increases. Our diode array stopped-flow spectral studies show that the porphyrin π-cation radical of Compound I in mutant LmP is more stable than wild type enzyme. These results suggest that the electronegative sulfur atoms of the proximal pocket are critical factors for controlling the location of a stable Compound I radical in heme peroxidases and are important in the oxidation of ferrocytochrome c.

Iron content differs between Francisella tularensis subspecies tularensis and subspecies holarctica strains and correlates to their susceptibility to H(2)O(2)-induced killing

Francisella tularensis, the causative agent of tularemia, is one of the most infectious bacterial pathogens known and is classified as a category A select agent and a facultative intracellular bacterium. Why F. tularensis subsp. tularensis causes a more severe form of tularemia than F. tularensis subsp. holarctica does is not known. In this study, we have identified prominent phenotypic differences between the subspecies, since we found that F. tularensis subsp. tularensis strains contained less iron than F. tularensis subsp. holarctica strains. Moreover, strain SCHU S4 of F. tularensis subsp. tularensis was less susceptible than FSC200 and the live vaccine strain (LVS) of F. tularensis subsp. holarctica to H(2)O(2)-induced killing. The activity of the H(2)O(2)-degrading enzyme catalase was similar between the strains, whereas the iron content affected their susceptibility to H(2)O(2), since iron starvation rendered F. tularensis subsp. holarctica strains more resistant to H(2)O(2). Complementing LVS with fupA, which encodes an important virulence factor that regulates iron uptake, reduced its iron content and increased the resistance to H(2)O(2)-mediated killing. By real-time PCR, it was demonstrated that FSC200 and LVS expressed higher levels of gene transcripts related to iron uptake and storage than SCHU S4 did, and this likely explained their high iron content. Together, the results suggest that F. tularensis subsp. tularensis strains have restricted iron uptake and storage, which is beneficial for their resistance to H(2)O(2)-induced killing. This may be an important factor for the higher virulence of this subspecies of F. tularensis, as reactive oxygen species, such as H(2)O(2), are important bactericidal components during tularemia.

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.