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Antioxid Redox Signal. Author manuscript; available in PMC 2010 October 22.
Published in final edited form as:
PMCID: PMC2959186

Evolution of Catalases from Bacteria to Humans


Excessive hydrogen peroxide is harmful for almost all cell components, so its rapid and efficient removal is of essential importance for aerobically living organisms. Conversely, hydrogen peroxide acts as a second messenger in signal-transduction pathways. H2O2 is degraded by peroxidases and catalases, the latter being able both to reduce H2O2 to water and to oxidize it to molecular oxygen. Nature has evolved three protein families that are able to catalyze this dismutation at reasonable rates. Two of the protein families are heme enzymes: typical catalases and catalase–peroxidases. Typical catalases comprise the most abundant group found in Eubacteria, Archaeabacteria, Protista, Fungi, Plantae, and Animalia, whereas catalase–peroxidases are not found in plants and animals and exhibit both catalatic and peroxidatic activities. The third group is a minor bacterial protein family with a dimanganese active site called manganese catalases. Although catalyzing the same reaction (2 H2O2 → 2 H2O + O2), the three groups differ significantly in their overall and active-site architecture and the mechanism of reaction. Here, we present an overview of the distribution, phylogeny, structure, and function of these enzymes. Additionally, we report about their physiologic role, response to oxidative stress, and about diseases related to catalase deficiency in humans.


HYDROGEN PEROXIDE is one of the most frequently occurring reactive oxygen species in the biosphere. It emerges either in the environment or as a by-product of aerobic metabolism [i.e., by oxygen activation (e.g., superoxide formation and dismutation)] in respiratory and photosynthetic electron-transport chains and as product of enzymatic activity, mainly by oxidases. Both excessive hydrogen peroxide and its decomposition product hydroxyl radical, formed in a Fenton-type reaction, are harmful for almost all cell components. Thus, its rapid and efficient removal is of essential importance for all aerobically living prokaryotic (11) and eukaryotic cells (52). Conversely, hydrogen peroxide can act as a second messenger in signal-transduction pathways, mainly for immune cell activation, inflammation, cellular proliferation, and apoptosis (72, 76, 97). Evidence suggests that this signaling function of hydrogen peroxide was acquired rather late in evolution. In unicellular organisms, H2O2 mainly stimulates production of antioxidants and ROS-removing and repairing enzymes, whereas in multicellular organisms (both animals and plants), it is involved in activation of signaling pathways (83, 97).

Hydroperoxidases (catalases and peroxidases) are ubiquitous “housekeeping” oxidoreductases capable of the heterolytic cleavage of the peroxidic bond, predominantly in hydrogen peroxide (H-O-O-H), but also in some small organic peroxides (R-O-O-H; e.g., ethyl hydroperoxide or acetyl hydroperoxide). Catalases have the additional striking ability to evolve molecular oxygen (O2) by oxidation of hydrogen peroxide. Thus, enzymes with catalase activity degrade hydrogen peroxide by dismutation.

Several gene families evolved in the ancestral genomes capable of H2O2 dismutation. The most abundant are heme-containing enzymes that are spread among Bacteria, Archaea and Eukarya. They are divided in two main groups, typical or “monofunctional” catalases (E.C., hydrogen peroxide, hydrogen peroxide oxidoreductase) and catalase–peroxidases. Both types of heme enzymes exhibit high catalase activities, but have significant differences, including absence of any sequence similarity and very different active-site, tertiary, and quaternary structures. Enzymatic classification of bifunctional catalase–peroxidases is not clear because, besides their catalatic activity (E.C., they exhibit a peroxidase activity similar to that of conventional peroxidases (EC, hydrogen peroxide, donor oxidoreductase). Both protein families are present in prokaryotic and eukaryotic genomes. Nonheme manganese-containing catalases (Mn-catalases) constitute a third (minor) group of catalatically active enzymes. Mn-catalases (E.C., initially referred to as pseudo-catalases, are present only in bacteria. The known sequences of all enzymes from these three protein families are collected and annotated in PeroxiBase (68;

Investigation of heme enzymes with catalase activity has a very long documented history, going back to the 19th century (110). After the publication of the latter review, several new 3D structures of enzymes were solved, and new important mechanistic, functional, and physiologic aspects were published. It is, therefore, important to ask whether the data accumulated during the last decade led to a broadening of our knowledge concerning the diversity, enzymology, and physiologic role of these oxidoreductases in various organisms, most important, in humans. Here, we try to answer these most intriguing questions, including the role of catalases in symbiotic and host–pathogen interactions, as well as new aspects of catalase disorders in humans.

Catalase Versus Peroxidase Activity

Continuous and effective degradation of H2O2 is indispensable for aerobic life (110). This is essential both for removal of excessive H2O2 and for strict regulation of its concentration in signaling pathways (97). As mentioned earlier, nature has developed three protein families that can perform its dismutation at reasonable rates. The overall catalatic reaction includes the degradation of two molecules of hydrogen peroxide to water and molecular oxygen (reaction 1)

Reaction 1

Reaction 1 is the main reaction catalyzed by typical catalases, catalase–peroxidases, and Mn-containing catalases. Additionally, several heme-containing proteins, including most peroxidases, metmyoglobin, and methemoglobin, have been observed to exhibit a low level of catalatic activity (64). From these enzymes, multifunctional chloroperoxidase from the ascomycete Caldariomyces fumago has the greatest reactivity as a catalase (95). Because of its very limited distribution in nature, this protein is not discussed here in more detail.

In catalases and catalase–peroxidases, two distinct stages can be distinguished in the catalatic reaction pathway. The first stage involves oxidation of the heme iron by using hydrogen peroxide as substrate to form compound I (reaction 2) (28). The oxygen–oxygen bond in peroxides (R-O-O-H) is cleaved heterolytically, with one oxygen leaving as water and the other remaining at heme iron (reaction 2). Generally, compound I is a redox intermediate two oxidizing equivalents above the resting [i.e., ferric, Fe(III)] state of the enzyme. In monofunctional catalases, compound I is an oxoiron(IV) porphyrin π-cation radical species [•+Por Fe(IV) = O], which is reduced back to the ferric enzyme by a second molecule of hydrogen peroxide, with the release of molecular oxygen and water (reaction 3). Thus, in a catalase cycle, H2O2 acts as oxidant (reaction 2) and reductant (reaction 3). Compound I can also undergo an intramolecular one-electron reduction with or without a proton, resulting in the formation of an alternative compound I (reaction 4) that is catalatically inactive. Here, the protein moiety (AA) donates the electron that quenches the porphyryl radical. Reaction 4 is responsible for the decrease of catalase activity with time, because the formed intermediate returns to the resting enzyme very slowly. A role of NADPH, which binds to some clades of monofunctional catalases, as two-electron donor for the transition of •+AA Por Fe(IV)-OH back to ferric catalase, has been discussed (see later).

Reaction 2

Reaction 3

Reaction 4

A lively debate exists about the question whether catalase–peroxidases also follow reactions 2 and 3. As in monofunctional catalases, organic peroxides (e.g., acetyl hydroperoxide) can oxidize catalase–peroxidases to a compound I–like species according to reaction 2, which is rapidly transformed to a protein radical species (18, 42) with spectral UV-Vis signatures dissimilar to those described in monofunctional catalases (47). Thus, in the light of these data, a number of alternative reaction schemes for catalase–peroxidases have been proposed; these consider a rapid quenching of the porphyryl radical by an electron from the protein moiety. These alternative compound I species have been suggested to be •+MYW Fe(IV)-OH, or more generally, •+AA Fe(IV)-OH, with MYW being the catalase–peroxidase typical distal-side covalent adduct (see later), and AA being an alternative amino acid near the heme. In any case, also in catalase–peroxidases, the catalatic cycle has two distinct phases, with H2O2 acting as oxidant and reductant. In contrast to monofunctional catalases, this unique compound I species must be able to oxidize H2O2 (i.e., it participates in the catalase cycle).

The mechanism of H2O2 degradation by nonheme Mn-catalases follows a completely different scheme. The overall reaction (reaction 1) also holds for these binuclear metallo-proteins, and again, H2O2 acts as oxidant and reductant, but the mechanism of the two individual reaction steps is completely different and is described later.

Catalases and catalase–peroxidases can also catalyze a peroxidatic reaction (reaction 5) in which electron donors (AH2) are oxidized via one-electron transfers releasing radicals (AH). Thus, a peroxidase cycle includes three reactions {i.e., compound I formation [reaction 2], compound I reduction to compound II, an oxoiron(IV) species [PorFe(IV) = O; reaction 6], and compound II reduction back to the resting state (reaction 7)}. Generally, the peroxidatic reaction of monofunctional catalases is weak, but in bifunctional catalase–peroxidases, reaction 5 is important. Similar to the discussion of the catalase cycle, a dispute also exists about the electronic structure of redox intermediates of catalase–peroxidases in the peroxidase cycle (87).

Reaction 5

Reaction 6

Reaction 7

Peroxidatic reactions can also reduce compound I directly in a two-electron reaction back to the ferric enzyme (reaction 8). Especially mammalian and yeast catalases have been described to oxidize ethanol or other short-chain aliphatic alcohols to acetaldehyd or corresponding aldehydes (98, 110).

Reaction 8

In the following sections, we discuss the distribution and diversity of the three protein families (i.e., typical catalases, catalase–peroxidases, and Mn-catalases) that are known to catalyse reaction 1 efficiently.

Distribution, Phylogeny, Structure, and Function of Typical (“Monofunctional”) Catalases

Typical catalase is one of the most intensively studied heme enzymes in the last century (see 110 for details), with numerous publications on its occurrence and expression, structure, function, and modification. Analysis of prokaryotic and eukaryotic genomes reveals that genes coding for typical heme-containing catalases represent the largest group of hydrogen peroxide–degrading enzymes. Catalase has the Pfam accession number PF00199, and 943 DNA sequences (December 2007) currently contain this typical motif. Typical catalases are, strictly speaking, not monofunctional (as often designated in the literature) because besides H2O2, they can also perform two-electron oxidation reactions with other donors (see reaction 8). Moreover, recently these enzymes were described also to exhibit additional (e.g., oxidase) activities (98). Nevertheless, the originally synonym “typical” catalase (56, 110) seems still to be the proper name for this protein family.

PeroxiBase presently contains 151 homologous protein sequences of typical catalases (in December 2007), and further related sequences will be annotated in the near future. The output of previous phylogenetic analysis revealed the existence of three main clades that were segregated rather early in the evolution of this gene family through at least two gene-duplication events (Figs. (Figs.11 and and2)2) (54).

FIG. 1
Multiple sequence alignment of selected typical catalases
FIG. 2
Reconstructed phylogenetic tree of 70 typical catalases from all main living kingdoms

Clade 1 contains eubacterial, algal, and plant catalases of small-subunit size (55–69 kDa) and heme b as the prosthetic group. Clade 2 contains large-subunit (75–84 kDa) catalases from eubacteria and fungi, with mainly heme d as the prosthetic group and an additional “flavodoxin-like” domain.

Clade 3 is the most abundant subfamily, and many bacteria possessing catalase gene paralogues of the first two clades also have a representative in the third clade. Clade 3 catalases also are found in archaebacteria, fungi, protists, plants, and animals. The scientifically and medically important bovine liver and human erythrocyte catalases belong to clade 3. They are small-subunit (43–75 kDa) catalases containing heme b and NADPH as a second redox-active cofactor (see later).

As a particular example for this review, we analyzed the phylogenetic relationship of genes of catalatically active enzymes in cyanobacterial genomes. Cyanobacteria (blue–green algae) have evolved as the most primitive, oxygenic, plant-type photosynthetic organisms. They were the first that produced molecular oxygen as a by-product of photosynthetic activity. Today they live in habitats with potentially damaging photooxidative conditions due to high irradiation and oxygen concentrations. Because in evolutionary history, they were responsible for accumulation of molecular oxygen in a so-far reducing atmosphere, they must have been among the first organisms that developed strategies to cope with reactive oxygen species (ROS) produced by incomplete reduction of molecular oxygen via electron-transport processes. Interestingly, genes for typical catalases are very unusual in cyanobacteria (see for details and updates). At the moment, the only complete and nonfused gene in which all essential parts are conserved is found in Nostoc punctiforme [NopuKat01, entry 5534 in PeroxiBase; (68) and references therein] and, as is obvious from Fig. 2, it belongs to clade 3 of small-subunit catalases with a high bootstrap support. Another cyanobacterial protein of this family (NOspKatOx01 = entry 5629 in PeroxiBase) is part of a fusion protein between a typical catalase-related domain and a putative lipoxygenase domain, with significant similarity to the well-investigated fusion protein of allene oxide synthase from Plexaura homomalla and related corals (66). In NopuKat01, the phylogenetic analysis suggests a lateral gene transfer (LGT) from an ancestral proteobacterium to Nostoc punctiforme (Fig. 2). In the fusion protein, the corresponding cyanobacterial gene has a common ancestry with corresponding genes of marine metazoans. As is demonstrated later, the main catalatically active enzymes in cyanobacteria are catalase–peroxidases or Mn-catalases or both.

Until now, fourteen 3D structures of typical heme catalases in native and oxidized form have been solved, including bacterial (1), fungal (61) and mammalian (74) proteins. It should be mentioned that a heme catalase was among the first proteins crystallized by Sumner and Dounce in 1937 (110). Table 1 provides an overview of all available 3D structures of typical catalases. All known typical catalases are homotetrameric enzymes with 222 point-group symmetry (14, 37). Each subunit of clade 1 and 3 proteins contains a protoporphyrin IX containing Fe(III) (i.e., heme b). Only in clade 2 proteins with large subunits is heme b autocatalytically transformed to heme d through dihydroxylation at the C5-C6 double bond of pyrrole ring III, followed by cis-γ-spiro-lactone formation in position C6 (24). Figure 3 depicts two representative structures of both a small-subunit heme b and a large-subunit heme d catalase. Four invariant residues are found in all typical catalases sequenced so far: the distal residues histidine, asparagine (rare exception: PnoKat01 from the ascomycete Phaeosphaeria nodorum), and serine and the proximal heme iron ligand tyrosine. Although the architecture of the active center is nearly perfectly conserved throughout the evolution (Fig. 1A and C), a significant difference exists in the overall fold between small- and large-subunit catalases (Fig. 1B and D). The main differences are located on the N-terminal arm and in the presence of an extra C-terminal flavodoxin-like domain in large catalases.

FIG. 3
Structural comparison of catalase A from Saccharomyces cerevisiae and catalase-1 from Neurospora crassa
Overview of Known 3D-Structures of 14 Typical (Monofunctional) Catalases, 5 Catalase-Peroxidase (KatG) and 2 Mn-Catalases (as in December 2007)

In all catalatically active enzymes, the architecture of substrate channel(s) leading to the deeply buried active center has been shown to play a crucial role, especially in reaction 3 (i.e., H2O2 oxidation). In Saccharomyces cerevisiae catalase A (a model for all clade 3 catalases), classic molecular interaction potentials, molecular dynamics, and activated molecular dynamics calculations (51) clearly demonstrated that water can be a competitive inhibitor of catalase via blocking the access of hydrogen peroxide to the active site. The main channel, located perpendicular to the plain of the heme, was shown to be the preferred route for H2O2 access to the active site in a majority of known catalase structures (e.g., 16). The overall affinity of the main channel for binding of molecules of H2O2 is only slightly higher than that for H2O. Changes to the largely hydrophobic residues in the lower part of the channel just above the heme or generally enlarging the channel by exchanging bulky residues with smaller amino acids causes a decrease in the catalase and an increase in the peroxidase activity of typical catalases (60). This underlines the importance of evolutionary-optimized shape and size of substrate channels in catalatically active enzymes. Similar considerations also apply for catalase–peroxidases and manganese catalases (see later). The structures of typical catalases reveal two additional minor channels leading from the surface to the active site. Regarding the rapid turnover rate of catalases, they could function as separate inlet and outlet channels, allowing rapidly evolving oxygen to be removed without interfering with incoming H2O2 (17).

The reaction mechanism of typical catalases was investigated in detail by several groups (23, 28, 36, 37, 40). It is generally accepted that H2O2 dismutation occurs at the distal heme cavity, according to reactions 2 and 3. Hydroperoxide initially associates in the active site with heme Fe(III) and forms a hydrogen bond with the conserved distal histidine (Figs. (Figs.11 and and3),3), which has its imidazole residue coplanar with the heme. This stretches the O-H bond, allowing the second oxygen of the peroxide also to form a hydrogen bond with the imidazole. The proton of the oxygen bound to the iron is then transferred, via the imidazole, to the second oxygen, giving rise to water and compound I (64). The role of the conserved distal asparagine (Fig. 3) is to stabilize and polarize the peroxide. Compound I in typical catalases is an oxoiron(IV) porpyrin π-cation radical species (6).

In compound I reduction (i.e., H2O2 oxidation to O2), the second H2O2 binds to compound I, forming hydrogen bonds with the oxoiron(IV) oxygen, the distal histidine, and asparagine (28). This allows the transfer of one hydrogen to the oxoiron(IV) and a second to the imidazole, resulting in formation of molecular oxygen. The hydrogen and second reducing equivalent on the imidazole ring could then be transferred to the Fe(IV)=O to produce water and native ferric catalase (63).

In all typical catalases, the proximal heme iron ligand is a tyrosinate (Figs. (Figs.11 and and3)3) that modulates heme iron reactivity and stabilizes higher oxidation states (99). Its participation in redox reactions is also reflected by its unusual modifications in some catalases. In Escherichia coli HPII, the Nδ of the imidazole ring of a proximal histidine has been shown to be covalently linked with the Cβ of the essential tyrosine (8). Its role could be to enhance resistance to peroxide, thereby stabilizing the enzyme at high turnover rates at high peroxide concentrations and to avoid formation of catalatically inactive compound II. Similarly, in Neurospora crassa CAT-1, a Tyr-Cys covalent bond was detected (24). Many other unusual modifications in typical catalases have been described (64).

Among catalases, reaction rates and substrate affinities can differ significantly (90). Generally, catalatically active enzymes do not follow Michaelis–Menten kinetics except at very low substrate concentrations, and different enzymes are affected differently at higher substrate concentrations. Most small-subunit enzymes are inactivated by H2O2 concentrations >300–500 mM and never reach the Michaelis–Menten vmax predicted by extrapolation from rates at low substrate concentrations. Conversely, large-subunit enzymes are inhibited only at very high hydrogen peroxide concentrations (>500 mM; 90). Thus, presentation of Km and vmax values in the literature is problematic. Nevertheless, apparent values are reported to vary from 54,000 to 833,000 per second for kcat and 38 to 600 mM for Km (17). The activities of typical catalases are essentially pH independent from pH 5 to pH 10.

Some typical catalases are among the rare group of enzymes possessing two distinct prosthetic groups within one enzyme subunit: heme and NADPH, the latter being bound only to clade 3 catalases. The order of affinities is NADPH > NADH > NADP+ > NAD+, with one nucleotide bound per monomer. The nucleotide is not a compulsory cofactor of the catalase activity, and many small-subunit catalases lose NADPH during purification. Principally the structural adjustments required for nucleotide binding are minor. NADPH binds in a compact structure at a site ~20 Å from the heme iron in a highly conserved environment (Fig. 4).

FIG. 4
Structure of NADPH-binding pocket in Homo sapiens catalase

The role of NADPH is still under discussion. As was outlined earlier, typical catalases form an alternative compound I by intramolecular electron transfer (reaction 4), which is inactive in H2O2 oxidation. Inactivation could occur at very low H2O2 concentrations, where compound I exists for a longer time before another H2O2 converts it back to the resting state. It has been proposed that NADPH serves as an electron source by directly converting this intermediate back to the resting state, thereby circumventing inactivation (40). This fits with the observation that in large-subunit catalases, which lack NADPH binding, quenching of the porphyryl radical by intramolecular electron transport has never been found. It has been speculated that the compact NADPH structure in catalases may allow more-effective electron transfer donation from the nicotinamide while still allowing the adenine ring to serve as an anchor for the nucleotide on the enzyme surface (Fig. 4) (64).

Two recent findings should be mentioned. At low peroxide concentrations, bovine catalase has been reported to have the ability to consume hydrogen peroxide without generating O2 (22). In the presence of NADPH, reaction 4 was not observed. But in the absence of NADPH, both reaction 4 and H2O2 consumption occurred, suggesting that the catalase has reducing groups within its own protein moiety for recovering the native enzyme (22). Studies with 14C-labeled NADPH also demonstrated that bound NADPH is not displaced from catalase on oxidation. It was thus proposed that bovine liver catalase must have a reductase and transhydrogenase activity for regeneration of bound NADPH (30).

Recently, it was suggested that both purified mammalian catalase and cells expressing this enzyme could use released molecular oxygen for a unique oxidase activity (98). The authors found that the thermal stability of oxidase activity corresponded to that of catalase activity and that the oxidation reaction was distinct from a classic peroxidase reaction (98). Indole derivatives have been suggested as oxidase substrates, and a potential binding site has been addressed in the crystal structure of human catalase (74, 98). It was proven that eukaryotic typical catalases are involved not only in the degradation of hydrogen peroxide but also in inhibition of cellular redox signaling (80, 97).

Distribution, Phylogeny, Structure and Function of Catalase–Peroxidases

Bifunctional catalase–peroxidase (KatG) has raised considerable interest, because it represents the only peroxidase with a reasonably high catalatic activity around neutral pH (reaction 1), besides a usual peroxidase activity (reaction 6). Its overall fold and active-site architecture is typical peroxidase-like, which is underlined by their striking sequence homologies to other members of class I of one heme peroxidase superfamily (102). Together with these peroxidases (i.e., ascorbates peroxidases and cytochrome c peroxidase), KatGs have the Pfam accession number PF00141. The InterPro accession number IPR000763 is more specific only for catalase–peroxidase.

PeroxiBase currently (December 2007) contains 329 sequences of katG genes, and their evolution was analyzed recently (69). The most important output of this investigation is that katG genes are distributed in ~40% of bacterial genomes and sometimes even closely-related species differ in possessing katG genes of different origin or even do not possess any katG gene. Two different evolutionary lines of katG genes also exist in eukaryotes: one in algae, and one in fungi, and both are expected to originate from lateral gene transfer of corresponding bacterial genomes living in close relation with the eukaryotes (69, 109). Phylogenetic analysis reveals that the closest neighbor of all fungal katG genes is the katG gene of Flavobacterium johnsonii (Fig. 6). In fungal KatGs, two clades are found, one encoding cytosolic, and the other, secreted enzymes. The occurrence of extracellular fungal KatGs is underlined by the prediction of signal sequences.

FIG. 6
Reconstructed phylogenetic tree of bacterial and fungal KatGs obtained with the neighbor-joining method and 1,000 bootstrap replications

The four available crystal structures (Table 1) of the KatGs Haloarcula marismortui (1ITK) (106); Burkholderia pseudomallei (1MWV) (12), Mycobacterium tuberculosis (1SJ2) (7), and Synechococcus PCC 7942 (1UB2) (100) revealed that the homodimeric heme b enzymes have proximal and distal conserved amino acids at almost identical positions as in other class I peroxidases (Fig. 7). In particular, both the triads His/Trp/Asp (His279, Trp330, and Asp389; Burkholderia numbering) and His/Arg/Trp (His112, Arg108, Trp111) are conserved (Fig. 5). Moreover, the proximal His is hydrogen bonded to the carboxylate side chain of the nearby Asp residue, which, in turn, is hydrogen bonded to the nitrogen atom of the indole group of the nearby Trp residue (Fig. 7). However, the x-ray structures also revealed features unique to KatG. In the vicinity of the active site, novel covalent bonds are formed among the side chains of three distal residues, including the conserved Trp111 (Figs. (Figs.55 and and7).7). In particular, both x-ray crystallization data (Table 1) and mass spectrometric analysis (25, 45) confirmed the existence of a covalent adduct between Trp111, Tyr238, and Met264 (Fig. 7). Exchange of either Trp111 or Tyr238 prevents cross-linking, whereas exchange of Met264 still allowed the autocatalytic covalent bond formation between Trp111 and Tyr238 (31, 45).

FIG. 5FIG. 5
Selected parts of the multiple sequence alignment of 43 catalase–peroxidases (KatGs)
FIG. 7
Structure of catalase–peroxidase from Burkholderia pseudomallei

In addition to an extra C-terminal copy resulting from gene duplication (102), which lacks the prosthetic group but supports the architecture of the active site (3), other KatG-typical features are three large loops; two of them show highly conserved sequence patterns (111) and constrict the access channel of H2O2 to the prosthetic heme b group at the distal side. The channel is characterized by a pronounced funnel shape and a continuum of water molecules. At the narrowest part of the channel, which is similar but longer and more restricted than that in other monofunctional peroxidases, two highly conserved residues, Asp141 (Fig. 7) and Ser324 control the access to distal heme side. Together with a conserved glutamate residue at the entrance, both acidic residues seem to be critical for stabilizing the solute matrix and orienting the water dipoles in the channel. Exchange of both residues affects the catalase but not the peroxidase activity. It is interesting to note that in typical catalases, similar acidic residues participate in stabilization and orientation of the solute matrix in the main access channel for H2O2.

This extensive distal-site hydrogen-bonding network causes KatGs to differ from typical peroxidases. Typically, the catalatic but not the peroxidatic activity is very sensitive to mutations that disrupt this network (39, 44). Moreover, the integrity of this network is crucial for the formation of distinct protein radicals that are formed on incubation of KatG with peroxides (42, 43). Mutational analysis clearly underlined the importance of the KatG-typical covalent adduct Trp-Tyr-Met. Its disruption significantly decreased the catalase but not the peroxidase activity. For instance, on exchange of Tyr238 by Phe, the resulting KatG variant completely lost its capacity to oxidize H2O2 and was transformed to a typical peroxidase (43). Similarly, all other mutations so far performed in the heme cavity or substrate channel of a KatG affected the oxidation and not the reduction reaction of hydrogen peroxide (87), which is the initial reaction in all heme hydroperoxidases.

Hydrogen peroxide reduction (i.e., compound I formation) follows reaction 2, which—in contrast to typical catalases—is immediately followed by reaction 4, forming the catalatically active intermediate •+AA Por Fe(IV)-OH, with AA being the so-far unlocalized amino acid radical. Here, it is important to note that the exact electronic structure of this compound I species, which actually oxidizes H2O2, is still speculative (47). One role of the Trp-Tyr-Met adduct could be the (at least transient) radical site (•+MYW Fe(IV)-OH) that quenches the porphyryl radical. Similar to typical peroxidases, compound I formation (i.e., the heterolytic cleavage of H2O2) is clearly assisted by the conserved distal residues His112 and Arg108 (87), but the exact role of distal amino acids in H2O2 oxidation is unknown.

Both Km and kcat values of KatGs are significant lower compared with those of typical catalases. Apparent values range from 3,500 to 6,000 per second and 3.7 to 8 mM (87). In contrast to typical catalases and Mn-catalases, the catalase activity of KatG has a sharp maximum activity around pH 6.5.

KatG is a bifunctional enzyme. It oxidizes typical artificial peroxidase substrates like o-dianisidine, guaiacol, or ABTS. The pH profile of the peroxidase activity of KatG has its maximum around pH 5.5, independent of the nature of most donors. Compound I (formed with peroxoacetic acid) reduction by monosubstituted phenols and anilines has been shown to depend on the substitution effect on the benzene ring and follows the Hammet equation (77), similar to typical peroxidases. Additionally, KatGs have been reported also to have halogenation (46) and NADH oxidase (86) activity.

The naturally occurring peroxidase substrate is unknown. In physiologic conditions, it is well known that KatG from Mycobacterium tuberculosis can activate the antituberculosis drug isoniazid (112), but otherwise the physiologic function of these enzymes remains unknown. Because of the restricted access, only small peroxidase substrates can enter the main entrance channel. A second access route, found in monofunctional peroxidases, approximately in the plane of the heme, is blocked by the KatG-typical loops. However, another potential route that provides access to the core of the protein, between the two domains of the subunit, has been described (12). It has been speculated that this could be the binding site for substrates with extended, possibly even polymeric character. In any case, neither from prokaryotic nor from eukaryotic KatGs is the naturally occurring one-electron donor(s) known nor is the function of a high peroxidase activity in a catalatic enzyme. A reasonable role at low peroxide concentration could be the reduction of alternative and catalatically inactive compounds I and II species to ferric KatG (similar to NADPH in clade 3 catalases). The potential role of catalase–peroxidases in H2O2 signaling remains elusive and must be investigated on both transcriptional and translational levels. This is an actual topic mainly for eukaryotic catalase–peroxidases, as possible signaling pathways can exist mainly during the interaction between host and pathogen.

Distribution, Phylogeny, Structure, and Function of Manganese Catalases

Manganese catalases (Mn- or nonheme or dimanganase catalases) represent only a minor gene family. Currently (December 2007), in PeroxiBase, 30 representatives are known, and all of them originate in bacterial genomes. This protein family has the signatures PF05067 and IPR007760 belonging to the ferritin-like superfamily comprising also desaturases, ferritins, and phenol hydroxylases. Phylogenetic analysis reveals the presence of five distinct and well-segregated clades (with a high bootstrap support; Fig. 8). Lateral gene transfer occurred during the evolution of this protein family between various bacterial taxa. Clade 1 includes archaebacterial proteins and at least one Firmicutes representative. Clade 2 is distributed among the taxa of Actinomycetes and Firmicutes. Clades 1 and 2 share a common ancestor (Fig. 8). Clade 3 is spread among Firmicutes and Proteobacteria, whereas clade 4 was separated rather early from the common ancestor of all the mentioned clades. Its representatives occur mainly in genomes of the genus Bacteroides and in several cyanobacteria. In clade 5, proteobacterial sequences are found. It seems to be most distantly related to all other clades and diverted early in the history of this gene family.

FIG. 8
Inferred phylogenetic tree of 30 manganese catalases rooted with an outgroup

Multiple-sequence alignment reveals a rather high identity, mainly in the active-site residues. This is underlined by the so-far known three-dimensional structures of two distinct representatives [Table 1; i.e., Mn-catalase from Thermus thermophilus (2) and Lactobacillus plantarum (4)]. The enzymes contain six identical subunits (~30 kDa), each with a bridged binuclear Mn-center located within a conserved closely packed four-helix bundle domain (Fig. 9). The ligands for the dimanganese center are almost invariantly conserved among all known sequences (Fig. 10). A bridging glutamate (Glu66 in LplMnCat01) anchors the two ions in the binuclear cluster. Each Mn ion is further coordinated by one histidine (His69 to Mn1 and His181 to Mn2) and one glutamate (Glu35 to Mn1 and Glu148 to Mn2) bound to opposite faces of the cluster (Fig. 9) (104). These essential ligands are highly conserved, forming typical signatures of the MnCat sequence (Fig. 10), whereas the environments of these ligands differ slightly. The manganese core is completed by two solvent-derived oxygen atom bridges. The presented sequence alignment (Fig. 10) suggests that in the Bacterioides enzyme BcapMnCat, an aspartate could act as a ligand instead of a histidine (Fig. 10).

FIG. 9
Structure of manganese catalase of Lactobacillus plantarum
FIG. 10
Selected parts of multiple sequence alignment of 30 bacterial manganese catalases

Access to the binuclear center is via a central channel that extends the full width of the hexamer, with branches into each subunit. Each dimanganese center is embedded in a network of hydrogen bonds that radiate from the metal center toward the outer-sphere environment. The length (≈15 Å) and narrowness of these channels are strongly reminiscent of typical catalases and catalase–peroxidases. Again, a restricted access to the only substrate hydrogen peroxide is very important for the catalatic reaction. Because heterologous expression in E. coli was not successful, a homologous plasmid expression system in Lactobacillus plantarum was established (104). Mutational analyses of an essential outer-sphere tyrosine (Tyr 42), which is conserved in all available sequences clearly demonstrated the importance of an intact hydrogen-bonding network also in Mn-catalases (104). In the variant Tyr42Phe, a solvent bridge was broken, and an “open” form of the dimanganese cluster was generated, thereby considerably influencing the catalytic turnover, pH optimum, and interaction with H2O2.

The catalatic reaction in manganese catalases differs significantly from that in both typical catalases and catalase–peroxidases. The dimanganese cluster is equally stable in either Mn2+-Mn2+ or Mn3+-Mn3+ oxidation states. On isolation, the protein is frequently in a mixture of both oxidation states. Fully reduction to Mn2+-Mn2+ is mediated by hydroxylamine, whereas reoxidation to a homogeneous Mn3+-Mn3+ state can be achieved by molecular oxygen at pH >7.0 (103). The catalatic reaction can be written as a two-stage process. During catalytic turnover, the active site has to accommodate both the reduced and the oxidized state of the dimanganese core. The Mn2+-Mn2+ cluster is expected to polarize the O-O bond, favoring a heterolytic cleavage of the peroxidic bond and release of water (reaction 9) (104). Hydrogen peroxide oxidation and dioxygen release occurs by a simple electron transfer (reaction 10). Principally, no temporal order exists in the reduction and oxidation stages.

Reaction 9

Reaction 10

It is assumed that the change in the oxidation status is responsible for protonation or deprotonation of both bridging oxygens. Protonation is proposed for the reduced Mn2+-Mn2+ state (see reaction 9), which is in line with an observed drastic reduction of the magnetic exchange coupling between the two Mn2+ ions (94). In contrast to heme-containing catalases, no reactive intermediate is formed, and both product waters are formed in one reaction (reaction 9). No evidence exists for involvement of free radicals in manganese catalase turnover, and nothing is known about peroxidatic reactions with one- and two-electron donors. Recently, manganese catalases have been subjected to molecular dynamics calculations, allowing the detailed reconstruction of the reaction mechanism and comparison with a de novo biomimetic dimanganese protein analogue (88).

The H2O2 dismutation rates are lower compared with typical catalases and catalase–peroxidases, which could explain why Mn-catalases may not have become as widespread in nature. The activity varies only slightly over the pH range from 5 to 10, very similar to typical catalases. The apparent Km values are reported to be ~220 mM (104), suggesting a low catalytic efficiency at low H2O2 concentrations. Their exact physiologic role(s) and expression pattern are more or less unknown. This clearly implies that the only hypothetical role of manganese catalases in H2O2 signaling must be investigated on both transcriptional and translational levels.

Physiologic Role and Expression of Typical Catalases and Catalase–Peroxidases as a Response to Oxidative Stress

Of several definitions of oxidative stress, the most appropriate regarding the role of antioxidant enzymes is that oxidative stress is “a disruption of redox signaling and control” (50). Both typical catalases and catalase–peroxidases are known to belong to the constitutive enzymatic armory against oxidative stress or are expressed as a response to it or both. The role of both these groups in H2O2 signaling can be proposed but, mainly in the case of KatGs, it still must be comprehensively investigated. The whole aerobically living world from bacteria to higher eukaryotes has to maintain H2O2 concentrations at nonharmful levels. Many experimental data on this topic have been published. A few typical examples are presented here.

Contrary to earlier suggestions, even some obligate anaerobes are known to contain typical catalase(s). Certain butyric fermentation bacteria of the genus Clostridium express catalase, mainly in their spores. The addition of hemin significantly enhances the expression of catalases in C. acetobutyricum (9). Many species of the strict anaerobic genus Bacteroides are catalase positive. The catalase-free mutant of B. fragilis exhibits increased sensitivity to hydrogen peroxide, explaining the resistance of wild-type B. fragilis (among the most aerotolerant strict anaerobes) to oxygen (79). Catalase activity was also found in several Desulfovibrio strains. The typical catalase from D. gigas is constitutively expressed, but probably it is not fully saturated with the prosthetic heme group (26). The strict anaerobe Methanobrevibacter arboriphilus also exhibits catalase activity with a maximum in the stationary phase of growth. This catalase activity is significantly enhanced on addition of hemin in the growth medium, and the purified protein has many properties of typical catalases (84). Another example is the expression of KatA from Helicobacter hepaticus. This proteobacterial typical heme catalase was shown to play an important role in oxidative-stress resistance of this dangerous pathogen of mammalian liver. A catalase mutant deficient in corresponding chromosomal locus was not able to grow at elevated oxygen levels and exhibited a marked hypersensitivity in conditions of oxidative stress (41). Additionally, the catalase mutant exhibited severe DNA fragmentation after treatment with sublethal concentrations of hydrogen peroxide.

In most (eu)bacteria that possess catalase gene paralogues in their genomes, only one enzyme is specifically induced by oxidative stress. Bacillus subtilis contains two genes encoding typical catalases, with KatA being oxidative stress induced and KatE (and its homologues in other bacteria) being a general (σ-B dependent) stress protein (27). Regarding the physiology of catalase expression and its control in bacteria, Escherichia coli is well studied. E. coli contains two catalatically active enzymes, a catalase–peroxidase (HPI) and a typical catalase (HPII). KatG (HPI) is mainly expressed on induction of oxidative stress, whereas expression of the monofunctional catalase is increased as cells grow into stationary phase. HPI is controlled as part of the OxyR regulon, which senses active oxygen species. HPII is controlled as part of the σs regulon in stationary phase, and its role seems to be protection during periods of slow or no growth (62).

Generally, symbiotic and pathogenic organisms contain an increased number of catalase gene paralogues. In the completely sequenced genome of the soil bacterium Sinorhizobium meliloti, three genes encoding catalatically active heme proteins can be found (48). Two monofunctional catalases, KatA (small subunits) and KatC (large subunits), and a catalase–peroxidase (KatG) are expressed in S. meliloti. This soil bacterium is able to develop a symbiosis with some Medicago species, and such plant–bacterium interaction leads to the formation of root nodules (57). During this process, bacteria differentiate. The expression of the three heme enzymes is differentially regulated both during the development of the nodule and in the different phases of growth of the free-living cells. In the latter, only KatA is expressed by exposure to H2O2. Induction of KatC depends on heat stress, salt stress, and ethanol, whereas induction of KatG is not induced by any of the tested treatments (48, 85). Mutational studies clearly demonstrated the importance of this armory of H2O2-detoxifying enzymes in the symbiotic process with the plant (48).

As outlined earlier, genomes of oxygenic phototrophic cyanobacteria show the presence of KatGs or Mn-catalases or both, whereas typical catalases are uncommon. In phototrophic organisms, severe oxidative stress is induced by high-light illumination. Surprisingly, catalase–peroxidase of Synechococcus sp. strain PCC 7942 (and in similar way, KatG of Synechocystis strain PCC 6803) is dispensable for survival, as indicated by mutant growth experiments (70). The authors of this study suggest that a thioredoxin-dependent peroxidase is crucial in light-induced oxidative stress. In contrast, cyanobacterial KatG is essential for survival and the elimination of relatively high concentrations of externally added H2O2 (70), a fact that underlines the predominant catalatic function of KatG in vivo. The genomes of both mentioned cyanobacterial species do not contain genes encoding Mn-catalases.

In the fungus Neurospora crassa, catalase-1 (a typical catalase) plays–together with a diphosphate kinase (NDK-1)–an essential role for the survival of conidia under oxidative and light-induced stress, including singlet oxygen. ROS are reported to play an important role in light-mediated signal transduction (101), and N. crassa catalase-1 seems to be involved in this cascade.

In the yeast S. cerevisiae, two typical catalases, an organelle-located KatA and a cytosolic KatT, have been identified. Both are highly expressed under conditions that induce peroxisome formation. Recently, it was demonstrated that KatA is targeted to both peroxisomes and the mitochondrial matrix (71). This finding is surprising, because catalase A is well known to contain two distinct peroxisomal targeting signals (55), but it lacks a classic N-terminal mitochondrial import sequence. Nevertheless, mitochondrial and peroxisomal co-import was monitored in Saccharomyces cerevisiae, both qualitatively by fluorescence microscopy and functional complementation, and quantitatively by Western blot analysis of subcellular fractions and by activity assays. Increased import of catalase A in mitochondria was observed mainly when the culture was subjected to intensive respiration that favors oxygen activation and finally H2O2 accumulation. Generally, mitochondrial targeting was favored in cultures grown on nonfermentable carbon sources (71).

Catalases in fungi, which are pathogenic for humans, have been studied intensively in recent years. During the unspecific immune response, the so-called respiratory burst is induced, thereby releasing superoxide from special activated leukocytes. As a consequence, high concentrations of hydrogen peroxide are formed that must be overcome by the phytopathogenic fungi. Aspergillus fumigatus can express three catalatically active enzymes (67). One catalase is a clade-2 large-subunit, monofunctional, and heat-stable catalase and is located in conidia. In contrast to the two other mycelium-located enzymes, it is not a virulence factor. Both mycelial proteins (a KatG and a typical catalase) have been shown to be involved in the protection of this ascomycete against the oxidative burst in an experimental rat model (67). Recently, an inducible KatG from the dangerous human pathogen Penicillium marneffei was isolated and partially characterized (73). In two important phytopathogenic fungi, Gibberella zeae and Magnaporthe grisea, six and five distinct genes coding for typical catalases and catalase–peroxidases exist ( Their physiologic role and possible inducible expression is currently under investigation (109). But catalatically active enzymes also protect the host from pathogenic attack. Recent experiments in culture plants reveal that catalase activity of the host is directly associated in interactions leading to the resistance of maize to the infection caused by Aspergillus flavus (58).

The presence of typical catalases in peroxisomes is well known. Older studies in mammalian cultures revealed the presence of catalase activity in rat-heart mitochondria, where it was identified as a key antioxidant defense system of myocardium (75). Recent findings support the occurrence of typical catalases in mitochondria of insect (5) as well as mammalian cells (81). The constitutive presence of an endogenous heme catalase inside mitochondria was demonstrated by subcellular fractionation, proteinase K sensitivity, and immunogold electron microscopy on both isolated rat liver mitochondria and on the whole-rat liver tissue (81). Mitochondrial catalase has a preventive role mainly against membrane lipid alterations, inactivation of respiratory chain components, mutations and strand breaks in mitochondrial DNA, and opening of permeability transition pores by hydrogen peroxide. Recent physiological data from overexpression of catalase in transgenic mice reveal a protective function of heme catalase in cardiomyocytes against aging-induced contractile defects and protein damage (105).

Diseases Caused by Catalase Deficiency and Overexpression

Catalase deficiency can originate from too-low or too-high expression levels and improper folding, assembly, and function of the human enzyme and can be related to rapid changes in the physiologic H2O2 concentration, altered peroxide signaling pathways, and inefficient targeting to peroxisomes (92) and mitochondria (71). Even further cellular organelles like endoplasmic reticulum, nucleus, and plasma membrane have been reported to be affected by catalase deficiency (107).

Human erythrocytes are usually exposed to substantial amounts of oxygen. As a consequence of metabolic activity of various oxidases (mainly involved in the β-oxidation of fatty acid chains), hydrogen peroxide is generated in a rather high concentration. Hemoglobin is protected by catalase colocalized in the erythrocytes (29). In erythrocytes that lack catalase, ferrous hemoglobin is oxidized with hydrogen peroxide to methemoglobin [i.e., Por Fe(III)] and, finally, to an oxoiron(IV) species, Por Fe(IV)=O, with protein-based radical(s). As a consequence, hemolysis of erythrocytes, polymerization of haemoglobin, and aggregation of malfunctioned erythrocytes is observed (59).

Catalase was shown to prevent the redox-sensitive nuclear transcription factor NF-êB from activating a cascade leading to lung inflammation through rapid regulation of physiologic ROS levels (76). It was also observed that cancer cells generate abnormally high H2O2 levels because of substantially low catalase activities compared with normal cells (89). After the detection of the formation of a complex between Grb2–a growth factor–binding protein–and the motif 447Tyr-Val-Asn-Val in human catalase, it was proposed that catalase participates in integrin signaling (107). The binding of Grb2 occurs specifically on phosphorylation in tumor cells when stimulated with serum or ligands. On binding to catalase, the specific catalase activity was modified (108). These results propose indirect involvement of complexed catalase in the regulation of cell proliferation.

The human genetic disease related to loss of functional catalase is generally known as acatalasemia, or Takahara disease (65). Acatalasemia, spread in human lineages as inherited deficiency, has been detected in 113 cases in 59 families from 11 countries worldwide (33). In a strict sense, we have to distinguish between acatalasemia and hypocatalasemia. Whereas hypocatalasemia is heterozygous and leads to a ~50% reduction of catalase activity, acatalasemia is homozygous, and the overall catalase activity is decreased to <10% of normal level. Such a low value is critical for many physiologic processes. Patients from Japan, Switzerland, and Hungary were investigated intensively by using clinical, biochemical, and molecular genetic methods. The frequency of acatalasemia was determined to be 0.04:1,000 in Switzerland, 0.05:1,000 in Hungary, and 0.8:1,000 in Japan. The frequency of hypocatalasemia is 2.3:1,000 in Hungary and 2–4:1,000 in Japan (32). In other countries, the identification of acatalasemia types was only sporadic.

Obvious differences in detected mutations exist among Japanese, Swiss, and Hungarian types of acatalasemia. In the Swiss type, a truncated catalase protein is found (20). Comprehensive analysis of all detected mutations in the catalase gene was performed (33). Not all of them lead to a significant modification of the catalase protein structure; some are regulatory mutations resulting in a less-stable protein. The recently found single-base substitution in exon 9 leading to the exchange Arg354Cys in Hungarian type D acatalasemia is very likely to cause a drastic decrease in blood catalase activity (35). Inspection of Fig. 1B shows that this type of acatalasemia leads to an exchange of a highly conserved arginine involved in a characteristic motif around the proximal tyrosine. Arginine 354 favors the ionization of the hydroxyl group of proximal Tyr358, and its exchange by cysteine disturbs the proper folding of the active site, thereby leading to the dramatically decreased activity in type D acatalasemic patients (35).

The most frequently occurring clinical features of acatalasemia include oral gangrene; ulceration; altered lipid, saccharide, and homocysteine metabolism (homocysteinemia); and an increased risk of diabetes mellitus. All these features reveal a high incidence in childhood, and some of them lead to a shortened lifespan, also connected with various forms of cancer in adult persons (33). Progressive oral gangrene in acatalasemic patients was the first clinical trait detected in 1952 by Takahara (59), and obviously this phenomenon is the result of infection with H2O2-generating bacteria (mostly streptococci and pneumococci) or the action of phagocytic cells (neutrophils). In a similar way, all other traits are caused by abnormal levels of hydrogen peroxide not sufficiently removed by the mutated/altered catalase. A distinct polymorphism in the catalase gene sequence was described (33). Benign polymorphism in the catalase gene is represented by 22 detected base substitutions located in noncoding, flanking, and intron regions (35) as well as mutations of the third codon positions with no change in the amino acid sequence (53).

A relation between inherited catalase deficiency and diabetes mellitus in patients with type 1 diabetes was described in detail (19). Different deleterious mutations, formed mainly by single or double base insertions, missense mutations, and substitutions in the catalase gene, result in a dramatic decrease of blood catalase activity connected with increased hydrogen peroxide concentrations in serum and tissues. As a consequence, damage of the oxidation-sensitive insulin-producing pancreatic β cells has been reported. Other malign mutations in the catalase gene can affect the lipid, homocysteine, and erythrocyte metabolism (34) and give rise to hypertension (49) and vitiligo (15). Vitiligo is a complex genetic trait associated with genes responsible for melanin biosynthesis, response to oxidative stress, and regulation of autoimmunity. The epidermis of patients with vitiligo exhibits decreased catalase activity (33).

In last decade, a new aspect of catalase-related diseases and therapy has been investigated. Whereas most of the previous therapies dealt with catalase deficiency, now compelling evidence indicates that catalase overexpression can have adverse physiologic effects (see 83 for more details). Most of this knowledge comes from in vitro and in vivo experiments on mammalian cell lines, whereas in prokaryotes and lower eukaryotes, the direct evidence for this phenomenon is still missing. In human keratinocytes, hydrogen peroxide has been reported to support wound healing by inducing a specific vascular endothelial growth factor (82). Corresponding in vivo experiments on male mice revealed that adenoviral catalase gene delivery significantly impaired wound angiogenesis and closure. Moreover, catalase overexpression slowed tissue remodeling (80, 83). Obviously, this aspect of catalase overexpression also must be taken into consideration in the therapy for acatalasemic patients.

The mitochondrial–lysosomal axis theory of aging (93), a variant of the free radical theory of aging, proposes that accumulation of damage to mitochondria and especially to mitochondrial DNA leads to aging in mammals. This theory was supported by observations of increased accumulation of mutations in mitochondrial DNA, which is not protected by histones (as is the case for nuclear DNA). Such mutations occur mainly because of increased levels of activated oxygen species in aged mitochondria, resulting in loss of cellular energy, failures in cellular functions (10), and finally, phenoptosis. Recent results demonstrate an increase in health and lifespan after upregulation and overexpression of catalase imported in mouse mitochondria (21). This underlines the importance of future investigations on catalase that could lead to important new medical and biotechnologic applications.


This work was supported by the Austrian Science Foundation FWF (projects P19793 to M.Z. and P18751 to C.O.).


one-electron donor
gene coding for typical catalase
hydroperoxidase II (heme catalase)]
InterPro accession number identification at EBI database (U.K.)
typical (bacterial) catalase
lateral gene transfer
diphosphate kinase
positive regulator for oxidative stress–inducible genes
Pasteur culture collection
protein family
protein family
reactive oxygen species


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