Effect of anions and redox state on the activity of manganese containing catalase from Thermus thermophilus

Gazing into the remarkable world of non-heme catalases through the window of the cyanobacterial Mn-catalase 'KatB'

Catalases, enzymes that decompose H₂O₂, are broadly categorized as heme catalases or non-heme catalases. The non-heme catalases are also known as Mn-catalases as they have Mn atoms in their active sites. However, unlike the well characterized heme-catalases, the study of Mn-catalases has gained importance only in the last few years. The filamentous, heterocystous, N₂-fixing cyanobacterium Anabaena PCC 7120, shows the presence of two Mn-catalases, KatA and KatB, but lacks heme catalases. Of the two Mn-catalases, KatB, which is induced by salt/desiccation, plays a major role in overcoming salinity/oxidative stress. In this mini review, we have summarized the recent advances made in the field of Mn-catalases, particularly KatB, and have interpreted these results in the larger context of stress physiology. These aspects bring to the fore the distinctive biochemical/structural properties of Mn-catalases and furthermore highlight the in vivo importance of these enzymes in adapting to oxidative stresses.

Artificial Diiron Enzymes with a De Novo Designed Four-Helix Bundle Structure

A single polypeptide chain may provide an astronomical number of conformers. Nature selected only a trivial number of them through evolution, composing an alphabet of scaffolds, that can afford the complete set of chemical reactions needed to support life. These structural templates are so stable that they allow several mutations without disruption of the global folding, even having the ability to bind several exogenous cofactors. With this perspective, metal cofactors play a crucial role in the regulation and catalysis of several processes. Nature is able to modulate the chemistry of metals, adopting only a few ligands and slightly different geometries. Several scaffolds and metal-binding motifs are representing the focus of intense interest in the literature. This review discusses the widespread four-helix bundle fold, adopted as a scaffold for metal binding sites in the context of de novo protein design to obtain basic biochemical components for biosensing or catalysis. In particular, we describe the rational refinement of structure/function in diiron-oxo protein models from the due ferri (DF) family. The DF proteins were developed by us through an iterative process of design and rigorous characterization, which has allowed a shift from structural to functional models. The examples reported herein demonstrate the importance of the synergic application of de novo design methods as well as spectroscopic and structural characterization to optimize the catalytic performance of artificial enzymes.

Composition and catalase-like activity of Mn(II)-bicarbonate complexes

The composition and catalase-like activity of Mn2+ complexes with bicarbonate were investigated with voltammetry and kinetic methods (by the rate of O2 production from H2O2). Three linear sections were revealed on the dependence of the reduction potential of Mn2+ on logarithm of bicarbonate concentration (logC(NaHCO3)) having slopes equal to 0 mV/logC(NaHCO3), -14 mV/logC(NaHCO3), and -59 mV/logC(NaHCO3), corresponding to Mn2+ aqua complex (Mn2+(aq)) and to Mn2+-bicarbonate complexes of the composition [Mn2+(HCO3(-))]+ (at concentration of HCO3(-) 10-100 mM) and [Mn2+(HCO3(-))2]0 (at concentration of HCO3(-) 100-600 mM). Comparison of HCO3(-) concentration needed for the catalase-like activity of Mn2+ with the electrochemical data showed that only electroneutral complex Mn2+(HCO3(-))2 catalyzed decomposition of H2O2, whereas positively charged Mn2+(aq) complex and [Mn2+(HCO3(-))]+ were not active. The catalase-like activity of Mn2+ did not appear upon substitution of anions of carbonic acids (acetate and formate) for HCO3(-). The rate of O2 production in the system Mn2+-HCO3(-)-H2O2 (pH 7.4) is proportional to the second power of Mn2+ concentration and to the fourth power of HCO3(-) concentration that indicates simultaneous involvement of two Mn2+(HCO3(-))2 complexes in the reaction of H2O2 decomposition.

Studies of a Dinuclear Manganese Complex with Phenoxo and Bis-acetato Bridging in the Mn2(II,II) and Mn2(II,III) States: Coordination Structural Shifts and Oxidation State Control in Bridged Dinuclear Complexes

The dinucleating ligand, 2,6-bis{[(2-(2-pyridyl)ethyl)(2-pyridylmethyl)-amino]-methyl}-4-methylphenol) (L1OH) reacts with Mn(ClO4)2.6H2O to form the dinuclear complex [Mn2(II,II)(L1O)(mu-OOCCH3)2]ClO4 (1). The electrolytic oxidation of 1 at 0.7 V (vs Ag/AgCl) produces the mixed valent complex [Mn2(II,III)(L1O)(mu-OOCCH3)2](ClO4)2 (1ox) quantitatively, while electrolysis at 0.20 V converts 1ox back to 1. X-ray crystallographic structures show that both 1 and 1ox are dinuclear complexes in which the two manganese ions are each in distorted octahedral coordination environments bridged by the phenoxo oxygen and two acetate ions. The structural changes that occur upon the oxidation 1 to 1ox suggest an extended pi-bonding system involving the phenoxo ring C-O(phenoxo)-Mn(II)-N(pyridyl) chain. In addition, as 1 is oxidized to 1ox, the rearrangements in the coordination sphere resulting from the oxidation of one Mn(II) ion to Mn(III) are transmitted via the bridging Mn-O(phenoxo) bonds and cause structural changes that render the site of the second manganese ion unfit for the +3 state and hence unstable to reduction. Thus the electrolytic oxidation of 1ox in acetonitrile at 1.20 V takes up slightly greater than 1 F of charge/mol of 1ox, but the starting complex, 1ox, is recovered, showing the instability of the Mn2(III,III) state that is formed with respect to reduction to 1ox. Variable-temperature magnetic susceptibility measurements of 1 and 1ox over the temperature range from 1.8 to 300 K can be modeled with magnetic coupling constants J = -4.3 and -4.1 cm(-1), respectively showing the weak antiferromagnetic coupling between the two manganese ions in each dinuclear complex, which is commonly observed among similar phenoxo- and bis-1,3-carboxylato-bridged dinuclear Mn2(II,II) and Mn2(II,III) complexes.

Interaction of anions with rat liver arginase: specific inhibitory effects of fluoride

The inhibitory effects of anions, such as N(3)(-), NO(2)(-), BO(4)(3-), SCN(-), CH(3)COO(-), SO(4)(2-), ClO(4)(-), H(2)PO(4)(-), CN(-), I(-), Br(-), Cl(-) and F(-), on the hydrolysis of L-arginine (L-Arg) by rat liver arginase (RLA) have been studied. From all these anions, only F(-) exhibited a clear inhibitory effect at the mM level. Inhibition of RLA by F(-) is reversible and uncompetitive towards L-Arg binding with a K(i) value of 1.3+/-0.5 mM at pH 7.4. This effect is dependent on pH as the IC(50) value of F(-) towards RLA increases from 1.2 to 19 mM when increasing the pH from 7 to 10. Another specific inhibitor of RLA, N(omega)-hydroxy-L-nor-arginine (nor-NOHA), that has been recently shown to bind to RLA as a bridging ligand of its (Mn(II))(2) cluster, exhibits some similarities with F(-) in its inhibitory effects (identical pH dependence). It is thus tempting to propose that the inhibitory effects of F(-) could be due to its binding as a bridging ligand of the RLA (Mn(II))(2) cluster. However, further studies are required to determine the modes of interaction of F(-) with RLA.

Toward the de novo design of a catalytically active helix bundle: a substrate-accessible carboxylate-bridged dinuclear metal center

De novo design of proteins provides an attractive approach to uncover the essential features required for their functions. Previously, we described the design and crystal structure determination of a di-Zn(II) complex of "due-ferri-1" (DF1), a protein patterned after the diiron-dimanganese class of redox-active proteins [Lombardi, A.; Summa, C.; Geremia, S.; Randaccio, L.; Pavone, V.; DeGrado, W. F. Proc. Natl. Acad. Sci. U.S.A. 2000, 97, 6298-6305]. The overall structure of DF1, which contains a carboxylate-bridged dinuclear metal site, agrees well with the intended design. However, access to this dimetal site is blocked by a pair of hydrophobic leucine residues (L13 and L13'), which prevent facile entry of metal ions and small molecules. We have now taken the next step in the eventual construction of a catalytically active metalloenzyme by engineering an active site cavity into DF1 through the replacement of these two leucine residues with smaller residues. The crystal structure of the dimanganous form of L13A-DF1 indeed shows a substrate access channel to the dimetal center. In the crystal structure, water molecules and a ligating dimethyl sulfoxide molecule, which forms a monatomic bridge between the metal ions, occupy the cavity. Furthermore, the diferric form of a derivative of L13A-DF1, DF2, is shown to bind azide, acetate, and small aromatic molecules.

Mechanism of hydrogen peroxide dismutation by a dimanganese catalase mimic: dominant role of an intramolecular base on substrate binding affinity and rate acceleration

Several modifications of the manganese coordination environment and oxidation states of a family of synthetic dimanganese complexes have been introduced in search of the structural features that promote high rates of hydrogen peroxide dismutation (catalase activity). The X-ray structure of reduced catalase (T thermophilus) reveals a dimanganese(II,II) site linked by three bridges: mu 13-glutamate-, mu-OH-, and mu-OH2. The roles of a bridging hydroxide vs mu-aqua and the carboxylate have been examined in the reduced Mn2(II,II) complexes, [(L1,2)Mn2(mu-O2CCH3)(mu-X)]2+ for X- = OH- (7A) or X = H2O (1-4), and their oxidized Mn2(III,III) analogues, [(L1,2)Mn2(mu-O)(O2CCH3)(OH)]+ (6) (L1 is N,N,N',N'-tetrakis(2-methylenebenzamidazolyl)-1,3-diaminopropan- 2-ol, and L2 is the tetrakis-N-ethylated analogue of L1, which has all amine protons replaced by ethyl groups). The steady-state catalase rate is first-order in concentration of both substrate and reduced catalyst and saturates at high peroxide concentrations in all cases, confirming peroxide/catalyst complex formation. No catalyst decomposition is seen after > 2000 turnovers. Catalysis proceeds via a ping-pong mechanism between the Mn2(II,II/III,III) redox states, involving complexes 6 and 7A/7A'. The Mn2(III,IV) oxidation state was not active in catalase activity. Replacement of the mu-aqua bridge by mu-hydroxide eliminates a kinetic lag phase in production of the O2 product, increases the affinity for substrate peroxide in the rate-limiting step as seen by a 5-fold. decrease in the Michaelis constant (KM), and accelerates the maximum rate (kcat) by 65-fold The kinetic and spectroscopic data are consistent with substrate deprotonation by the hydroxide bridge, yielding a hydroperoxyl bridge coordinated between the Mn ions (mu, eta 2 geometry, "end-on") as the basis for catalysis: mu-OH- + H2O2-->mu-O2H- + H2O. Binding of a second hydroxide ion to 7A causes a further increase in kcat by 4-fold with no further change in substrate affinity (KM). By contrast, free (noncoordinating) bases in solution have no effect on catalysis, thus establishing intramolecular sites for both functional hydroxide anions. Solution structural studies indicate that the presence of 2-5 equiv of hydroxide in solution leads to formation of a bishydroxide species, [(L1,2)Mn2(mu 13-O2CCH3)(OH)2], which in the presence of air or oxygen auto-oxidizes to yield complex 6, a Mn2(III,III)(mu-O) species. Complex 6 oxidizes H2O2 to O2 without a kinetic lag phase and is implicated as the active form of the oxidized catalyst. A maximum increase by 240-fold in catalytic efficiency (kcat/KM = 700 s-1 M-1) is observed with the bishydroxide species versus the aquo complex 1, or only 800-fold less efficient than the enzyme. Deprotonation of the amine groups of the chelate ligand L was shown not to be involved in the hydroxide effects because identical results were obtained using the catalyst with tetrakis(N-ethylated)-L. Uncoupling of the Mn(II) spins by protonation of the alkoxyl bridge (LH) was observed to lower the catalase activity. Comparisons to other dimanganese complexes reveals that the Mn2(II,II)/Mn2(III,III) redox potential is not the determining factor in the catalase rate of these complexes. Rather, rate acceleration correlates with the availability of an intramolecular hydroxide for substrate deprotonation and with binding of the substrate at the bridging site between Mn ions in the reductive O-O bond cleavage step that forms water and complex 6.

The oxidized (3,3) state of manganese catalase. Comparison of enzymes from Thermus thermophilus and Lactobacillus plantarum

Manganese catalases contain a binuclear manganese cluster that catalyzes the redox dismutation of hydrogen peroxide, interconverting between dimanganese(II) [(2,2)] and dimanganese(III) [(3,3)] oxidation states during turnover. We have investigated the oxidized (3,3) states of the homologous enzymes from Thermus thermophilus and Lactobacillus plantarum using a combination of optical absorption, CD, MCD, and EPR spectroscopies as sensitive probes of the electronic structure and protein environment for the active site metal clusters. Comparison of results for these two enzymes allows the essential features of the active sites to be recognized and the differences identified. For both enzymes, preparations having the highest catalytic activity have diamagnetic ground states, consistent with the bis-mu-bridging dimanganese core structure that has been defined crystallographically. Oxidative damage and exogenous ligand binding perturb the core structure of LPC, converting the enzyme to a distinct form in which the cluster becomes paramagnetic as a result of altered exchange coupling mediated by the bridging ligands. The TTC cluster does not exhibit this sensitivity to ligand binding, implying a different reactivity for the bridges in that enzyme. A mechanism is proposed involving distinct coordination modes for peroxide substrate in each of the two half-reactions for enzyme turnover.

Purification and cloning of a thermostable manganese catalase from a thermophilic bacterium

We have purified a heat-stable catalase from a thermophilic bacterium, Thermus species strain YS 8-13. The enzyme was purified 160-fold from crude cellular extracts and possessed a specific activity of 8000 units/mg at 65 degrees C. The purified enzyme displayed the highest activity at pH 7 to 10 and temperatures around 85 degrees C. The catalase was determined to be a manganese catalase, based on results from atomic absorption spectra and inhibition experiments using sodium azide. The enzyme was composed of six identical subunits of molecular weight 36,000. Amino acid sequences determined from the purified protein were used to design oligonucleotide primers, which were in turn used to clone the coding gene. The nucleotide sequence of a 1.4-kb fragment of Thermus sp. YS 8-13 genomic DNA containing a 909-bp open reading frame was determined. The gene encoded a 302-residue polypeptide of deduced molecular weight 33,303. The deduced amino acid sequence displayed a region-specific homology with the sequences of the manganese catalase from a mesophilic organism, Lactobacillus plantarum.

EPR polarization studies on Mn catalase from Lactobacillus plantarum

The binuclear manganese active site of Mn catalase catalyzes redox disproportionation of hydrogen peroxide, forming dioxygen and water. We report here multifrequency EPR and microwave polarization studies of the catalytically active homovalent Mn2+ complex of Lactobacillus plantarum Mn catalase, resolving spectra from each of the thermally accessible multiplet states of the coupled complex by multivariate methods. The experimental spectra have been simulated using computational approaches for the binuclear cluster to predict both intensity and polarization for arbitrary values of the ground state parameters. These two spectroscopic properties define the nature of the ground state wavefunctions and so serve as a sensitive and quantitative measure of the inter-ion interactions in the reduced complex. Interpretation of the spectra in terms of a pair Hamiltonian that includes Heisenberg exchange, dipolar, single site zero field splitting, and Zeeman perturbations leads to the most complete ground state description of the active site metal centers. The results of this spectroscopic analysis support a picture of two high spin ions weakly coupled by exchange interactions (J = 40 cm-1) with relatively small dipole-dipole coupling and single site zero field splittings for the ligand-free reduced enzyme. The coupling between fluoride binding and protonation of the complex has been demonstrated by proton uptake studies. The binding of two fluoride ions in the active site dramatically changes the pair spectra, reflecting a substantially reduced J-coupling (J = 10.5 cm-1) that must be a consequence of perturbation of the bridging ligands. Anion binding to the binuclear Mn complex appears to result in poisoning of the active site by protons, possibly associated with insertion of fluoride into bridging positions of the dimanganese core.

The dimanganese(III,IV) oxidation state of catalase from Thermus thermophilus: electron nuclear double resonance analysis of water and protein ligands in the active site

The 1H hyperfine tensors of the dimanganese(III,IV) oxidation state of the non-heme-type catalase enzyme from the thermophilic bacterium Thermus thermophilus have been measured by electron nuclear double resonance (ENDOR) spectroscopy at pH 6.5-9. These were compared to model dimanganese(III,IV) complexes possessing six-coordinate N4O2, N3O3, and O6 atom donor sets to each Mn and mu-oxo and mu-carboxylato bridging ligands. The lack of 14N hyperfine couplings in the enzyme suggests either O6 or O5N ligand donors to each Mn. Moreover, the two sigma coordination sites on Mn(III) directed at the dz2 orbital cannot be occupied by N ligands. The 1H ENDOR spectrum revealed two types of anisotropic tensors, attributable to two D2O-exchangeable protons on the basis of the magnitude of the electron paramagnetic resonance (EPR) line narrowing in D2O. All six of the 1H hyperfine couplings are proposed to arise from a single displaceable water molecule in the active site, on the basis of their reversible disappearance, upon incubation in D2O or by precipitation from ammonium sulfate, and by simulation of the 1H ENDOR spectrum. The Mn ions are coordinated predominantly by nonmagnetic O atoms lacking covalently bound protons in both alpha and beta positions. This implicates predominantly carboxylato-type ligands (Asp and Glu) and possibly a di-mu-oxo bridge between Mn ions. The latter is supported also by the presence of strong antiferromagnetic coupling. Comparison to other dimetalloproteins also possessing the four-helix bundle structural motif shows that the polyoxo(carboxylato) coordination in catalase differs significantly from the polyhistidine coordination adopted by the diiron(II,II) site in the O2-binding protein myohemerythrin, but resembles the polycarboxylato ligation adopted by the diiron(III,III) site of ribonucleotide reductase. The catalase 1H ENDOR spectrum is essentially identical to that for the exchangeable protons in the active site of the diiron(II,III) state of uteroferrin, an acid phosphatase [Doi et al. (1988) J. Biol. Chem. 263, 5757-5763], and also for a polycarboxylato complex possessing the Mn2(mu-O)2 core with H-bonded water ligands. The 1H ENDOR line shape in catalase could be simulated using a theoretical model suitable for multispin clusters. It treats the two Mn spins as point dipoles which are exchange-coupled. It includes both dipolar and isotropic ligand hyperfine couplings. Using this model, the position of the proton with the largest interaction could be located with respect to the Mn-Mn vector because of the extreme sensitivity of line shape to position.(ABSTRACT TRUNCATED AT 400 WORDS)

Inactivation and reactivation of manganese catalase: oxidation-state assignments using X-ray absorption spectroscopy

The oxidation states of the Mn atoms in three derivatives of Mn catalase have been characterized using a combination of X-ray absorption near-edge structure (XANES) and EPR spectroscopies. The as-isolated enzyme has an average oxidation state of Mn(III) and contains a Mn(III) form, together with a reduced Mn(II) form and a variable amount (10-25%) of a Mn(III)/Mn(IV) mixed-valence derivative. Treatment with NH2OH rapidly reduces the majority of the enzyme to a Mn(II) derivative with no loss of activity. Inactivation by treatment with NH2OH + H2O2 converts all of the enzyme to a mixed-valence Mn(III)/Mn(IV) form. The inactive, mixed-valence derivative can be completely reactivated by long-term (greater than 1 h) anaerobic incubation with NH2OH, giving a reduced Mn(II)/Mn(II) derivative. These data suggest a catalytic model in which the enzyme cycles between a reduced Mn(II)/Mn(II) state and an oxidized Mn(III)/Mn(III) state.