Crystal structure of manganese catalase from Lactobacillus plantarum

Multifrequency Pulsed EPR Studies of Biologically Relevant Manganese(II) Complexes

Electron paramagnetic resonance studies at multiple frequencies (MF EPR) can provide detailed electronic structure descriptions of unpaired electrons in organic radicals, inorganic complexes, and metalloenzymes. Analysis of these properties aids in the assignment of the chemical environment surrounding the paramagnet and provides mechanistic insight into the chemical reactions in which these systems take part. Herein, we present results from pulsed EPR studies performed at three different frequencies (9, 31, and 130 GHz) on [Mn(II)(H(2)O)(6)](2+), Mn(II) adducts with the nucleotides ATP and GMP, and the Mn(II)-bound form of the hammerhead ribozyme (MnHH). Through line shape analysis and interpretation of the zero-field splitting values derived from successful simulations of the corresponding continuous-wave and field-swept echo-detected spectra, these data are used to exemplify the ability of the MF EPR approach in distinguishing the nature of the first ligand sphere. A survey of recent results from pulsed EPR, as well as pulsed electron-nuclear double resonance and electron spin echo envelope modulation spectroscopic studies applied to Mn(II)-dependent systems, is also presented.

Disruption of sitA compromises Sinorhizobium meliloti for manganese uptake required for protection against oxidative stress

During the initial stages of symbiosis with the host plant Medicago sativa, Sinorhizobium meliloti must overcome an oxidative burst produced by the plant in order for proper symbiotic development to continue. While identifying mutants defective in symbiosis and oxidative stress defense, we isolated a mutant with a transposon insertion mutation of sitA, which encodes the periplasmic binding protein of the putative iron/manganese ABC transporter SitABCD. Disruption of sitA causes elevated sensitivity to the reactive oxygen species hydrogen peroxide and superoxide. Disruption of sitA leads to elevated catalase activity and a severe decrease in superoxide dismutase B (SodB) activity and protein level. The decrease in SodB level strongly correlates with the superoxide sensitivity of the sitA mutant. We demonstrate that all free-living phenotypes of the sitA mutant can be rescued by the addition of exogenous manganese but not iron, a result that strongly implies that SitABCD plays an important role in manganese uptake in S. meliloti.

Production of a heterologous nonheme catalase by Lactobacillus casei: an efficient tool for removal of H2O2 and protection of Lactobacillus bulgaricus from oxidative stress in milk

Lactic acid bacteria (LAB) are generally sensitive to H2O2, a compound that they can paradoxically produce themselves, as is the case for Lactobacillus bulgaricus. Lactobacillus plantarum ATCC 14431 is one of the very few LAB strains able to degrade H2O2 through the action of a nonheme, manganese-dependent catalase (hereafter called MnKat). The MnKat gene was expressed in three catalase-deficient LAB species: L. bulgaricus ATCC 11842, Lactobacillus casei BL23, and Lactococcus lactis MG1363. While the protein could be detected in all heterologous hosts, enzyme activity was observed only in L. casei. This is probably due to the differences in the Mn contents of the cells, which are reportedly similar in L. plantarum and L. casei but at least 10- and 100-fold lower in Lactococcus lactis and L. bulgaricus, respectively. The expression of the MnKat gene in L. casei conferred enhanced oxidative stress resistance, as measured by an increase in the survival rate after exposure to H2O2, and improved long-term survival in aerated cultures. In mixtures of L. casei producing MnKat and L. bulgaricus, L. casei can eliminate H2O2 from the culture medium, thereby protecting both L. casei and L. bulgaricus from its deleterious effects.

Thein vitroeffect of hydrogen peroxide on vaginal microbial communities

This study presents a series of experiments carried out in order to elucidate the role of H2O2 in antimicrobial activity of lactobacilli. Vaginal swabs were collected from 60 premenopausal women and checked for pH and Nugent score, and Lactobacillus species were cultured, phenotyped and genotyped. The main outcome measures involved: (1) species of vaginal lactobacilli most effective in liberating H2O2, (2) minimal microbicidal concentrations of added H2O2, (3) kinetics of H2O2 liberation in relation to oxygen tension, (4) antimicrobial activity of pure H2O2 versus one produced by selected vaginal lactobacilli and the total activity of their culture supernatants. Results showed that H2O2 was liberated especially by: Lactobacillus delbrueckii, Lactobacillus acidophilus, Lactobacillus crispatus, Lactobacillus johnsonii and L. gasseri. Hydrogen peroxide reached concentrations from 0.05 to 1.0 mM, which under intensive aeration increased even up to 1.8 mM. Microorganisms related to vaginal pathologies show varied resistance to the action of pure H2O2. Most potent inhibitory activity against bacteria and yeasts was presented by Lactobacillus culture supernate producing H2O2, followed by the nonproducing strain and pure H2O2. To conclude - the antimicrobial activity of lactobacilli is a summation of various inhibitory mechanisms in which H2O2 plays some but not a crucial role, in addition to other substances.

Synthesis, structure and catalase-like activity of dimanganese(III) complexes of 1,5-bis(X-salicylidenamino)pentan-3-ol (X = 3- and 5-methyl). Influence of phenyl-ring substituents on catalytic activity

The diMn(III) complexes [Mn2(5-Me-salpentO)(mu-MeO)(mu-AcO)(H2O)Br] (1) and [Mn2(3-Me-salpentO)(mu-MeO)(mu-AcO)(MeOH)2]Br (2), where salpentOH = 1,5-bis(salicylidenamino)pentan-3-ol, were synthesised and structurally characterized. The two complexes include a bis(micro-alkoxo)(micro-acetato) triply-bridged diMn(III) core with an Mn...Mn separation of 2.93-2.94 A, the structure of which is retained upon dissolution. Complexes 1 and 2 show catalytic activity toward disproportionation of H2O2, with first-order dependence on the catalyst, and saturation kinetics on [H2O2], in methanol and DMF. In DMF, the two complexes are able to disproportionate at least 1500 eq. of H2O2 without significant decomposition, while in methanol, they rapidly lose activity with formation of a non-coupled Mn(II) species. Electrospray ionisation mass spectrometry, EPR and UV/vis spectroscopy used to monitor the reaction suggest that the major active form of the catalyst occurs in the Mn2(III) oxidation state during cycling. The correlation between log(k(cat)) and the redox potentials of 1, 2 and analogous complexes of other X-salpentOH derivatives indicates that, in this series, the oxidation of the catalyst is probably the rate-limiting step in the catalytic cycle. It is also noted that formation of the catalyst-peroxide adduct is more sensitive to steric effects in DMF than in methanol. Overall, kinetics and spectroscopic studies of H2O2 dismutation by these complexes converge at a catalytic cycle that involves the Mn2(III) and Mn2(IV) oxidation states.

Structure of the DPS-like protein from Sulfolobus solfataricus reveals a bacterioferritin-like dimetal binding site within a DPS-like dodecameric assembly

The superfamily of ferritin-like proteins has recently expanded to include a phylogenetically distinct class of proteins termed DPS-like (DPSL) proteins. Despite their distinct genetic signatures, members of this subclass share considerable similarity to previously recognized DPS proteins. Like DPS, these proteins are expressed in response to oxidative stress, form dodecameric cage-like particles, preferentially utilize H(2)O(2) in the controlled oxidation of Fe(2+), and possess a short N-terminal extension implicated in stabilizing cellular DNA. Given these extensive similarities, the functional properties responsible for the preservation of the DPSL signature in the genomes of diverse prokaryotes have been unclear. Here, we describe the crystal structure of a DPSL protein from the thermoacidophilic archaeon Sulfolobus solfataricus. Although the overall fold of the polypeptide chain and the oligomeric state of this protein are indistinguishable from those of authentic DPS proteins, several important differences are observed. First, rather than a ferroxidase site at the subunit interface, as is observed in all other DPS proteins, the ferroxidase site in SsDPSL is buried within the four-helix bundle, similar to bacterioferritin. Second, the structure reveals a channel leading from the exterior surface of SsDPSL to the bacterioferritin-like dimetal binding site, possibly allowing divalent cations and/or H(2)O(2) to access the active site. Third, a pair of cysteine residues unique to DPSL proteins is found adjacent to the dimetal binding site juxtaposed between the exterior surface of the protein and the active site channel. The cysteine residues in this thioferritin motif may play a redox active role, possibly serving to recycle iron at the ferroxidase center.

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.

Mimicking catalase and catecholase enzymes by copper(II)-containing complexes

An imidazolate-bridged copper(II)-zinc(II) complex (Cu(II)-diethylenetriamino-μ-imidazolato-Zn(II)-tris(2-aminoethyl)amine perchlorate (denoted as “Cu,Zn complex”) and a simple copper(II) complex (Cu(II)-tris(2-aminoethyl) amine chloride (“Cu-tren”) were prepared and immobilised on silica gel (by hydrogen or covalent bonds) and montmorillonite (by ion exchange). The immobilised substances were characterised by FT-IR spectroscopy and their thermal characteristics were also studied. The obtained materials were tested in two probe reactions: catalytic oxidation of 3,5-di-tert-butyl catechol (DTBC) (catecholase activity) and the decomposition of hydrogen peroxide (catalase activity). It was found that the catecholase activity of the Cu,Zn complex increased considerably upon immobilization on silica gel via hydrogen bonds and intercalation by ion exchange among the layers of montmorillonite. The imidazolate-bridged copper(II)-zinc(II) complex and its immobilised versions were inactive in hydrogen peroxide decomposition. The Cu(II)-tris(2-aminoethyl)amine chloride complex displayed good catalase activity; however, immobilisation could not improve it.

Response of a Designed Metalloprotein to Changes in Metal Ion Coordination, Exogenous Ligands, and Active Site Volume Determined by X-ray Crystallography

The de novo protein DF1 is a minimal model for diiron and dimanganese metalloproteins, such as soluble methane monooxygenase. DF1 is a homodimeric four-helix bundle whose dinuclear center is formed by two bridging Glu side chains, two chelating Glu side chains, and two monodentate His ligands. Here, we report the di-Mn(II) and di-Co(II) derivatives of variants of this protein. Together with previously solved structures, 23 crystallographically independent four-helix bundle structures of DF1 variants have been determined, which differ in the bound metal ions and size of the active site cavity. For the di-Mn(II) derivatives, as the size of the cavity increases, the number and polarity of exogenous ligands increases. This collection of structures was analyzed to determine the relationship between protein conformation and the geometry of the active site. The primary mode of backbone movement involves a coordinated tilting and sliding of the first helix in the helix-loop-helix motif. Sliding depends on crystal-packing forces, the steric bulk of a critical residue that determines the dimensions of the active site access cavity, and the intermetal distance. Additionally, a torsional motion of the bridging carboxylates modulates the intermetal distance. This analysis provides a critical evaluation of how conformation, flexibility, and active site accessibility affect the geometry and ligand-binding properties of a metal center. The geometric parameters defining the DF structures were compared to natural diiron proteins; DF proteins have a restricted active site cavity, which may have implications for substrate recognition and chemical stability.

High-field EPR investigations of Mn(III)Mn(IV) and Mn(II)Mn(III) states of dimanganese catalase and related model systems

Multi-frequency EPR experiments at 9, 34 and 94 GHz are reported on the antiferromagnetically coupled mixed valence Mn(II)Mn(III) complex of manganese catalase and on several dinuclear manganese model systems. They are compared with similar experiments obtained earlier for the Mn(III)Mn(IV) states. It is demonstrated how accurate information on the G- and 55Mn hyperfine tensors can be derived from this approach. Furthermore, the effect of oxidation state, planarity of the manganese-oxygen core and the type of ligands bridging the manganese ions on the magnetic resonance parameters and the related electronic structure is investigated. 'Broken-symmetry' density functional calculations on two Mn(III)Mn(IV) complexes, including the superoxidized state of the catalase, are presented. The agreement between calculated and experimental EPR parameters and complex geometries is remarkably good. Implications of these results for the structure and function of the dimanganese catalase are discussed.

Artificial di-iron proteins: solution characterization of four helix bundles containing two distinct types of inter-helical loops

Peptide-based models have an enormous impact for the development of metalloprotein models, as they seem appropriate candidates to mimic both the structural characteristics and reactivity of the natural systems. Through the de novo design of four-helix bundles, we developed the DF (Due Ferri) family of artificial proteins, as models of di-iron and di-manganese metalloproteins. The goal of our research is to elucidate how the electrostatic environment, polarity and solvent accessibility of the metal-binding site, influence the functional properties of di-iron proteins. The first two subsets of the DF protein family, DF1 and DF2, consist of two non-covalently associated helix-loop-helix motifs, which bind the di-metal cofactor near the center of the structure. The DF2 subset was designed to improve the properties of DF1: DF2 and DF2t have several changes in their sequences to improve solubility and metal ion access, as well as a change in the loop connecting the two helices. In order to evaluate how these changes affect the overall structure of the model proteins, we solved the NMR structures of the di-Zn(II) complexes of DF2 and DF2t, and compared these structures with those recently obtained from X-ray crystallography. Further, we examined the thermodynamic consequences associated with the mutations, by measuring the stability of DF2t in the presence of different metal ions, and comparing the results with the data already obtained for DF2. Taken together, analysis of all the data showed the importance of the turn conformation in the design and stability of four-helix bundle.

Catalase: A repertoire of unusual features

Catalases are antioxidant enzymes which catalyze the breakdown of hydrogen peroxide to water and oxygen, and are one of the oldest enzymes to be studied biochemically. The first crystal structure of a catalase appeared in the year 1980 and it revealed the tetrameric nature of the enzyme and presence of channels accessing the deeply buried active site heme. An interesting feature of the tetrameric structure is the characteristic interweaving or arm exchange of the subunits. The recent elucidation of the crystal structure of transport proteins (porins, aquaporins) showed that these proteins are also tetrameric in nature and posses channels. However, recent specific investigations focusing on the roles for these channels, in the mechanism of enzyme action of catalases, revealed significant similarities with that observed for the transport of water and/or glycerol, in aquaporins.

Artificial diiron proteins: from structure to function

De novo protein design provides an attractive approach for the construction of models to probe the features required for the function of complex metalloproteins. These minimal models contain the essential elements believed necessary for activity of the protein. In this article, we summarize the design, structure determination, and functional properties of a family of artificial diiron proteins.

Dimanganese catalase--spectroscopic parameters from broken-symmetry density functional theory of the superoxidized Mn(III)/Mn(IV) state

Broken-symmetry density functional theory was used to study the catalytic center of manganese catalase in the superoxidized Mn(III)/Mn(IV) state. Heisenberg exchange coupling constants, 55Mn and 14N hyperfine coupling constants (hfcs) and nuclear quadrupole splittings, as well as the electronic g tensors were evaluated for different model systems of the active site after complete geometry optimizations in the high-spin and broken-symmetry states. A comparison of the experimental data with the spectroscopic parameters computed for the models with unprotonated and protonated mu-oxo bridges shows best agreement between theory and experiment for a Mn2(mu-O)2(mu-OAc) core. The calculated Mn-Mn distances and 55Mn hfcs clearly support a dimanganese cluster with unprotonated mu-oxo bridges in the superoxidized state. Furthermore, it is shown that an interchange of the Mn(III) and Mn(IV) oxidation states in this trapped valence system leads to specific changes in the molecular and electronic structure of the manganese clusters.

Synthesis, Structure, and Characterization of New Mononuclear Mn(II) Complexes. Electrochemical Conversion into New Oxo-Bridged Mn2(III,IV) Complexes. Role of Chloride Ions

Two Mn(II) complexes are isolated and X-ray characterized, namely, cis-[(L(2))Mn(II)(Cl)(2)] (1) and [(L(3))Mn(II)Cl(OH(2))](ClO(4)) (2(ClO(4))), where L(2) and L(3) are the well-known tetradentate N,N'-dimethyl-N,N'-bis(2-pyridylmethyl)ethane-1,2-diamine and N,N'-dimethyl-N,N'-bis(2-pyridylmethyl)propane-1,3-diamine ligands, respectively. The crystal structure reveals that whereas the ligand L(2) is in the cis-alpha conformation in complex 1, the ligand L(3) is in the more unusual cis-beta conformation in 2. EPR spectra are recorded on frozen solutions for both complexes and are characteristic of Mn(II) species. Electrochemical behaviors are investigated on acetonitrile solution for both complexes and show that cation 2 exists as closely related Mn(II) species in equilibrium. For both complexes exhaustive bulk electrolyses of acetonitrile solution are performed at oxidative potential in various experimental conditions. In the presence of 2,6-lutidine and after elimination of chloride ligands, the formation of the di-mu-oxo mixed-valent complexes [(L(2))Mn(III)(mu-O)(2)Mn(IV)(L(2))](3+) (3a) and [(L(3))Mn(III)(mu-O)(2)Mn(IV)(L(3))](3+) (4) is confirmed by UV-vis and EPR spectroscopies and cyclic voltammetry. In addition crystals of 4(ClO(4))(3) were isolated, and the X-ray structure reveals the cis-alphaconformation of L(3). In the absence of 2,6-lutidine and without elimination of the exogenous chloride ions, the electrochemical oxidation of 1 leads to the formation of the mononuclear Mn(III) complex, namely, [(L(2))Mn(III)(Cl)(2)](+) (5), as confirmed by UV-vis as well as parallel mode EPR spectroscopy and cyclic voltammetry. In the same conditions, the electrochemical oxidation of complex 2 is more intricate, and a thorough analysis of EPR spectra establishes the formation of the binuclear mono-mu-oxo mixed-valent [(L(3))ClMn(III)(mu-O)Mn(IV)Cl(L(3))](3+) (6) complexes. Electrochemical conversion of Mn(II) complexes into mixed-valent Mn(2)(III,IV) oxo-bridged complexes in the presence of 2,6-lutidine is discussed. The role of the chloride ligands as well as that of L(3) in the building of oxo bridges is discussed. Differences in behavior between L(2) and L(3) are commented on.

Dynamical Flexibility and Proton Transfer in the Arginase Active Site Probed by ab Initio Molecular Dynamics

We have used ab initio molecular dynamics (AIMD) to investigate the dynamical flexibility of the bridged binuclear structural motif in the active site of arginase. Dynamical transformations play a crucial role in catalysis. We have provided direct insight into the motions of the first-shell ligands with emphasis on the chelating and bridging carboxylates. In the case of the terminal Asp234 residue we observe changes in the binding mode (carboxylate shifts). AIMD dynamics of sufficient duration has allowed us to observe proton transfer from the bridging nucleophile to the catalytically essential Asp 128 residue and to map the underlying free energy surface in terms of simple reaction coordinates, such as the oxygen-oxygen distance Ro-o and the asymmetric stretch delta. This has provided valuable insight into the nature of the last step of the catalytic cycle. In addition, constrained molecular dynamics permitted us to compare the deprotonation free energy of the bridging nucleophile in the case of native versus metal-depleted arginase.

Synthesis, characterisation and catalase-like activity of dimanganese(III) complexes of 1,5-bis(5-X-salicylidenamino)pentan-3-ol (X=nitro and chloro)

The dimanganese(III,III) complexes [Mn(2)(III)(5-NO(2)-salpentO)(mu-AcO)(mu-MeO)(methanol)(2)]Y (1: Y=Br, 2a: Y=I, 2b: Y=I(3)), [Mn(2)(III)(5-NO(2)-salpentO)(mu-AcO)(mu-MeO)(methanol)(ClO(4))] (3) and [Mn(2)(III)(5-Cl-salpentO)(mu-AcO)(mu-MeO)(methanol)(2)]Br (4), where salpentOH is the symmetrical Schiff base ligand 1,5-bis(salicylidenamino)pentan-3-ol, were synthesised and structurally characterized. Complex 2b crystallises in the monoclinic system, space group P2(1)/c, and exhibits Mn. . .Mn separation of 2.911 A. This Mn. . .Mn separation is very close to the other characterized (mu-alkoxo)(2)(mu-acetato)Mn(2)(III) complexes of X-salpentOH (X=MeO, Br and H) and reveals that the aromatic substituent has little influence on the geometric parameters of the bimetallic core. A correlation between the electronic character of the different ring substituents, the redox potentials of the dinuclear complexes and their catalase activity was evidenced. Complexes 1-4 show saturation kinetics with [H(2)O(2)] and the H(2)O(2) disproportionation involves redox cycling between the Mn(2)(III)/Mn(2)(IV) levels. The catalytic activity studies show that bound acetate is required for catalase activity and that the acetato and alkoxo bridges serve as internal bases facilitating the proton transfer coupled to oxidation of the metal centre.

A Comparative XAS and X-ray Diffraction Study of New Binuclear Mn(III) Complexes with Catalase Activity. Indirect Effect of the Counteranion on Magnetic Properties

Four new binuclear Mn(III) complexes with carboxylate bridges have been synthesized: [[Mn(nn)(H(2)O)](2)(mu-ClCH(2)COO)(2)(mu-O)](ClO(4))(2) with nn = bpy (1) or phen (2) and [[Mn(bpy)(H(2)O)](2)(mu-RCOO)(2)(mu-O)](NO(3))(2) with RCOO = ClCH(2)COO (3) or CH(3)COO (4). The characterization by X-ray diffraction (1 and 3) and X-ray absorption spectroscopy (XAS) (1-4) displays the relevance of this spectroscopy to the elucidation of the structural environment of the manganese ions in this kind of compound. Magnetic susceptibility data show an antiferromagnetic coupling for all the compounds: J = -2.89 cm(-1) (for 1), -8.16 cm(-1) (for 2), -0.68 cm(-1) (for 3), and -2.34 cm(-1) (for 4). Compounds 1 and 3 have the same cation complex [[Mn(bpy)(H(2)O)](2)(mu-ClCH(2)COO)(2)(mu-O)](2+), but, while 1 shows an antiferromagnetic coupling, for 3 the magnetic interaction between Mn(III) ions is very weak. The four compounds show catalase activity, and when the reaction stopped, Mn(II) compounds with different nuclearity could be obtained: binuclear [[Mn(phen)(2)](mu-ClCH(2)COO)(2)](ClO(4))(2), trinuclear [Mn(3)(bpy)(2)(mu-ClCH(2)COO)(6)], or mononuclear complexes without carboxylate. Two Mn(II) compounds without carboxylate have been characterized by X-ray diffraction: [Mn(NO(3))(2)(bpy)(2)][Mn(NO(3))(bpy)(2)(H(2)O)]NO(3) (5) and [Mn(bpy)(3)](ClO(4))(2).0.5 C(6)H(4)-1,2-(COOEt)(2).0.5H(2)O (8).

Thermus thermophilus as a cell factory for the production of a thermophilic Mn-dependent catalase which fails to be synthesized in an active form in Escherichia coli

Thermostable Mn-dependent catalases are promising enzymes in biotechnological applications as H(2)O(2)-detoxifying systems. We cloned the genes encoding Mn-dependent catalases from Thermus thermophilus HB27 and HB8 and a less thermostable mutant carrying two amino acid replacements (M129V and E293G). When the wild-type and mutant genes were overexpressed in Escherichia coli, unmodified or six-His-tagged proteins of the expected size were overproduced as inactive proteins. Several attempts to obtain active forms or to activate the overproduced proteins were unsuccessful, even when soluble and thermostable proteins were used. Therefore, a requirement for a Thermus-specific activation factor was suggested. To overcome this problem, the Mn-dependent catalase genes were overexpressed directly in T. thermophilus under the control of the Pnar promoter. This promoter belongs to a respiratory nitrate reductase from of T. thermophilus HB8, whose transcription is activated by the combined action of nitrate and anoxia. Upon induction in T. thermophilus HB8, a 20- to 30-fold increase in catalase specific activity was observed, whereas a 90- to 110-fold increase was detected when the laboratory strain T. thermophilus HB27::nar was used as the host. The thermostability of the overproduced wild-type catalase was identical to that previously reported for the native enzyme, whereas decreased stability was detected for the mutant derivative. Therefore, our results validate the use of T. thermophilus as an alternative cell factory for the overproduction of thermophilic proteins that fail to be expressed in well-known mesophilic hosts.

Controlled Redox Conversion of New X-ray-Characterized Mono- and Dinuclear Heptacoordinated Mn(II) Complexes into Di-μ-oxo-dimanganese Core Complexes

Two heptacoordinated Mn(II) complexes are isolated and X-ray characterized using the well-known tpen ligand (tpen = N,N,N',N'-tetrakis(2-pyridylmethyl)-1,2-ethanediamine): [(tpen)Mn(OH(2))](ClO(4))(2) (1(ClO(4))(2)) and [(tpen)Mn(micro-OAc)Mn(tpen)](ClO(4))(3).2H(2)O (2(ClO(4))(3).2H(2)O). Crystallographic data for 1(ClO(4))(2) at 110(2) K (respectively at 293(2) K): monoclinic, space group C2/c, a = 15.049(3) A (15.096(3) A), b = 9.932(2) A (10.105(2) A), c = 19.246(4) A (19.443(4) A), beta = 94.21(3) degrees (94.50(3) degrees ), Z = 4. Crystallographic data for 2(ClO(4))(3).0.5(C(2)H(5))(2)O at 123(2) K: triclinic, space group P, a = 12.707(3) A, b = 12.824(3) A, c = 19.052(4) A, alpha = 102.71(3) degrees, beta = 97.83(3) degrees, gamma = 98.15(3) degrees, Z = 2. Investigation of the variation upon temperature of the molar magnetic susceptibility of compound 2(ClO(4))(3).2H(2)O reveals a weak antiferromagnetic exchange interaction between the two high-spin Mn(II) ions (J = -0.65 +/- 0.05 cm(-)(1), H = -JS(1).S(2)). EPR spectra are recorded on powder samples and on frozen acetonitrile solutions, demonstrating the maintenance upon dissolution of the heptacoordination of Mn in complex 1 while complex 2 partially dissociates. Electrochemical responses of complexes 1 and 2 are investigated in acetonitrile, and bulk electrolyses are performed at oxidative potential in the presence of various amounts of 2,6-lutidine (0-2.65 equiv per Mn ion). The formation from either 1 or 2 of the mixed-valent complex [(tpen)Mn(III)(micro-O)(2)Mn(IV)(tpen)](3+) (3) is established from mass spectrometry and EPR and IR spectroscopy measurements. When reaction is started from 2, formation of [(tpen)Mn(IV)(micro-O)(2)(micro-OAc)Mn(IV)](3+) (4) is evidenced from cyclic voltammetry, EPR, and UV-vis data. The Mn vs tpen ratio in the electrogenerated complexes is accurately controlled by the quantity of additional 2,6-lutidine. The role of tpen as a base is discussed.

A new class of oxo-bridged high-valent hexamanganese clusters supported by sterically hindered carboxylate ligands

A new class of oxo-bridged high-valent hexamanganese (Mn6) clusters containing a novel (Mn6O8)6+ core, [MnIV(4)MnIII2(mu-O)4(mu3-O)4(dmb)6(O2CR)2]4+ (where dmb=4,4'-dimethyl-2,2'-bipyridine, and RCO2=2,6-di(p-tolyl)benzoate (Ar(Tol)CO2-) (3) or 2,6-di(4-tert-butylphenyl)benzoate (Ar(4-tBuPh)CO2-) (4)), was synthesized using sterically hindered m-terphenyl-derived carboxylate ligands. These complexes can be synthesized by oxidizing the MnII mononuclear complexes, [Mn(dmb)2(OH2)(O2CR)]+ (where RCO2=Ar(Tol)CO2- (1) or Ar(4-tBuPh)CO2- (2)) with (n-Bu4N)MnO4, by direct Mn(II) + Mn(VII) in situ comproportionation reactions, or by ligand substitution on the dinuclear manganese (III,IV) or (IV,IV) complexes, [(Mn2(mu-O)2(dmb)4)](3+/4+). The compound [MnIV4MnIII2(mu-O)4(mu3-O)4(dmb)6(Ar(Tol)CO2)2](OTf)4 [3(OTf)4] crystallizes in the monoclinic space group P2(1)/n, with the cell parameters a=15.447(1) A, b=15.077(2) A, c=27.703(2) A, beta=91.68(2) degrees, V=6449.3(6) A3, and Z=2. The X-ray structure reveals that there are three different bridging modes for the oxo groups: mu, "pyramidal" mu3, and "T-shaped" mu3. Solid-state variable temperature magnetic susceptibility studies suggest that the Mn centers are net antiferromagnetically coupled to yield a diamagnetic ST=0 ground spin state with a large number of low-lying, thermally accessible states with ST>0. 1H NMR spectra were recorded for both Mn6 clusters and selected resonances assigned. The electronic and redox properties of these complexes along with the effect of the presence of the bulky carboxylate ligands are also described here.

Water oxidation in PSII—H atom abstraction revisited

A model for the water oxidation reaction in Photosystem II (PSII) is presented, based on an H atom abstraction mechanism. The model rationalises the S-state dependence of observed substrate water exchange kinetics [Biochim. Biophys. Acta 1503 (2001) 197] and assumes that H transfer occurs to an oxidised micro-oxo bridge oxygen on the S(3)-->S(4)-->S(0) transition. The model requires that only one Mn-pair and a Ca ion be directly involved in the substrate binding and catalytic function. The multiline signal observed in the S(0) state is shown to plausibly arise from such a system. A detailed molecular model of the three-metal site, assuming ligation by those residues identified by mutagenesis as Ca/Mn ligands is presented. This bears a resemblance to the dinuclear Mn site in Mn catalase and is generally consistent with the electron density map of cyanobacterial PSII recently presented [Proc. Natl. Acad. Sci. U. S. A. 100 (2003) 98].

Calculating the Electron Paramagnetic Resonance Parameters of Exchange Coupled Transition Metal Complexes Using Broken Symmetry Density Functional Theory: Application to a MnIII/MnIV Model Compound

The capability of the density functional broken symmetry approach for the calculation of various EPR parameters of exchange coupled metal clusters is demonstrated by studying the experimentally well-investigated [Mn(III)Mn(IV)(mu-O)(2)(mu-OAc)DTNE](2+) complex. Geometry optimizations of the complex in its broken symmetry and high spin states yielded structures with two distinct manganese sites and geometrical parameters in good agreement with the X-ray structure. Exchange coupling constants were calculated from the energy differences between the high spin and broken symmetry states using the Heisenberg spin Hamiltonian. Very good agreement between theory and experiment was achieved with the B3LYP hybrid functional. The g-tensor calculations were performed employing the coupled perturbed Kohn-Sham equations. A strategy for the computation of g-tensor site values is presented and provides single-site g-tensors that are in good agreement with the expectations for Mn(III) and Mn(IV), respectively. Spin projection gave the g-tensor of the coupled manganese complex in very good agreement with the experimental results. Complete (55)Mn hyperfine tensors, including spin-orbit contributions, were calculated and spin-projected. The source of anisotropy in this system could be traced back to the Mn(III) ion in line with the experimental results. The isotropic manganese hyperfine coupling constants were underestimated by factors between 1.4 and 2.5. It is shown that this deficiency is systematic in character and not anchored in the broken symmetry approach. Nuclear quadrupole splitting of the (55)Mn nuclei is shown to be small in this system. In addition, (14)N and (1)H ligand hyperfine data were calculated and compared well with the experimental results. The quality of the extended point-dipole model was demonstrated in application to (1)H anisotropic hyperfine coupling constants.

First principles computational study of the active site of arginase

Ab initio density functional theory (DFT) methods were used to investigate the structural features of the active site of the binuclear enzyme rat liver arginase. Special emphasis was placed on the crucial role of the second shell ligand interactions. These interactions were systematically studied by performing calculations on models of varying size. It was determined that a water molecule, and not hydroxide, is the bridging exogenous ligand. The carboxylate ligands facilitate the close approach of the Mn (II) ions by attenuating the metal-metal electrostatic repulsion. Of the two metals, Mn(A) was shown to carry a larger positive charge. Analysis of the electronic properties of the active site revealed that orbitals involving the terminal Asp234 residue, as well as the flexible micro-1,1 bridging Asp232, lie at high energies, suggesting weaker coordination. This is reflected in certain structural variability present in our models and is also consistent with recent experimental findings. Finally, implications of our findings for the biological function of the enzyme are delineated.

Biochemical properties and regulated gene expression of the superoxide dismutase from the facultatively aerobic hyperthermophile Pyrobaculum calidifontis

Superoxide dismutase (SOD) was purified from a facultatively aerobic hyperthermophilic archaeon, Pyrobaculum calidifontis VA1. The purified native protein from aerobically grown cells exhibited 1,960 U of SOD activity/mg and contained 0.86 +/- 0.04 manganese and <0.01 iron atoms per subunit. The gene encoding SOD was cloned and expressed in Escherichia coli. Although the recombinant protein was soluble, little activity was observed due to the lack of metal incorporation. Reconstitution of the enzyme by heat treatment with either Mn or Fe yielded a highly active protein with specific activities of 1,970 and 434 U/mg, respectively. This indicated that the SOD from P. calidifontis was a cambialistic SOD with a preference toward Mn in terms of activity. Interestingly, reconstitution experiments in vitro indicated a higher tendency of the enzyme to incorporate Fe than Mn. When P. calidifontis was grown under anaerobic conditions, a majority of the native SOD was incorporated with Fe, indicating the cambialistic property of this enzyme in vivo. We further examined the expression levels of SOD and a previously characterized Mn catalase from this strain in the presence or absence of oxygen. Northern blot, Western blot, and activity measurement analyses revealed that both genes are expressed at much higher levels under aerobic conditions. We also detected a rapid response in the biosynthesis of these enzymes once the cells were exposed to oxygen.

Quantitative analysis of dinuclear manganese(II) EPR spectra

A quantitative method for the analysis of EPR spectra from dinuclear Mn(II) complexes is presented. The complex [(Me(3)TACN)(2)Mn(II)(2)(mu-OAc)(3)]BPh(4) (1) (Me(3)TACN=N, N('),N(")-trimethyl-1,4,7-triazacyclononane; OAc=acetate(1-); BPh(4)=tetraphenylborate(1-)) was studied with EPR spectroscopy at X- and Q-band frequencies, for both perpendicular and parallel polarizations of the microwave field, and with variable temperature (2-50K). Complex 1 is an antiferromagnetically coupled dimer which shows signals from all excited spin manifolds, S=1 to 5. The spectra were simulated with diagonalization of the full spin Hamiltonian which includes the Zeeman and zero-field splittings of the individual manganese sites within the dimer, the exchange and dipolar coupling between the two manganese sites of the dimer, and the nuclear hyperfine coupling for each manganese ion. All possible transitions for all spin manifolds were simulated, with the intensities determined from the calculated probability of each transition. In addition, the non-uniform broadening of all resonances was quantitatively predicted using a lineshape model based on D- and r-strain. As the temperature is increased from 2K, an 11-line hyperfine pattern characteristic of dinuclear Mn(II) is first observed from the S=3 manifold. D- and r-strain are the dominate broadening effects that determine where the hyperfine pattern will be resolved. A single unique parameter set was found to simulate all spectra arising for all temperatures, microwave frequencies, and microwave modes. The simulations are quantitative, allowing for the first time the determination of species concentrations directly from EPR spectra. Thus, this work describes the first method for the quantitative characterization of EPR spectra of dinuclear manganese centers in model complexes and proteins. The exchange coupling parameter J for complex 1 was determined (J=-1.5+/-0.3 cm(-1); H(ex)=-2JS(1).S(2)) and found to be in agreement with a previous determination from magnetization. The phenomenon of exchange striction was found to be insignificant for 1.

Direct measurement of the hyperfine and g-tensors of a Mn(III)-Mn(IV) complex in polycrystalline and frozen solution samples by high-field EPR

The g-tensors and hyperfine tensors of the S = (1)/(2) ground state of the mixed valence [LMn(IIImu-O)(2)Mn(IV)L](3+) complex (L = N,N'-dimethyl-N,N'-bis(2-pyridylmethyl)ethane-1,2-diamine) was deter-mined in the solid-state and frozen acetonitrile solution by high-field EPR. Both samples exhibited complex anisotropic temperature behaviors that precluded the use of routine spectrum simulation procedures to extract the spin parameters. To circumvent this problem, the parameters were measured directly by using multifrequency techniques. In the case of the frozen solution, this approach yielded seven of the nine spin parameters with varying uncertainty, the two extreme principal g-values, the four hyperfine couplings associated with each of these two g-values, and the middle g-value. This latter parameter was obtained from a first moment analysis. Unlike simulations, the statistical errors associated with each value could be assigned in a straightforward and rigorous manner. The directly measured g-values were different in frozen solution and polycrystalline powder. The temperature dependence of the high-field EPR spectra of the polycrystalline powder revealed a spin-spin interaction between the neighboring binuclear complexes.

Binuclear manganese compounds of potential biological significance. Part 2. Mechanistic study of hydrogen peroxide disproportionation by dimanganese complexes: the two oxygen atoms of the peroxide end up in a dioxo intermediate

The dimanganese(II,II) complexes 1a [Mn(2)(L)(OAc)(2)(CH(3)OH)](ClO(4)) and 1b [Mn(2)(L)(OBz)(2)(H(2)O)](ClO(4)), where HL is the unsymmetrical phenol ligand 2-(bis-(2-pyridylmethyl)aminomethyl)-6-((2-pyridylmethyl)(benzyl)aminomethyl)-4-methylphenol, react with hydrogen peroxide in acetonitrile solution. The disproportionation reaction was monitored by electrospray ionization mass spectrometry (ESI-MS) and EPR and UV-visible spectroscopies. Extensive EPR studies have shown that a species (2) exhibiting a 16-line spectrum at g approximately 2 persists during catalysis. ESI-MS experiments conducted similarly during catalysis associate 2a with a peak at 729 (791 for 2b) corresponding to the formula [Mn(III)Mn(IV)(L)(O)(2)(OAc)](+) ([Mn(III)Mn(IV)(L)(O)(2)(OBz)](+) for 2b). At the end of the reaction, it is partly replaced by a species (3) possessing a broad unfeatured signal at g approximately 2. ESI-MS associates 3a with a peak at 713 (775 for 3b) corresponding to the formula [Mn(II)Mn(III)(L)(O)(OAc)](+) ([Mn(II)Mn(III)(L)(O)(OBz)](+) for 3b). In the presence of H(2)(18)O, these two peaks move to 733 and to 715 indicating the presence of two and one oxo ligands, respectively. When H(2)(18)O(2) is used, 2a and 3a are labeled showing that the oxo ligands come from H(2)O(2). Interestingly, when an equimolar mixture of H(2)O(2) and H(2)(18)O(2) is used, only unlabeled and doubly labeled 2a/b are formed, showing that its two oxo ligands come from the same H(2)O(2) molecule. All these experiments lead to attribute the formula [Mn(III)Mn(IV)(L)(O)(2)(OAc)](+) to 2a and to 3a the formula [Mn(II)Mn(III)(L)(O)(OAc)](+). Freeze-quench/EPR experiments revealed that 2a appears at 500 ms and that another species with a 6-line spectrum is formed transiently at ca. 100 ms. 2a was prepared by reaction of 1a with tert-butyl hydroperoxide as shown by EPR and UV-visible spectroscopies and ESI-MS experiments. Its structure was studied by X-ray absorption experiments which revealed the presence of two or three O atoms at 1.87 A and three or two N/O atoms at 2.14 A. In addition one N atom was found at a longer distance (2.3 A) and one Mn at 2.63 A. 2a can be one-electron oxidized at E(1/2) = 0.91 V(NHE) (DeltaE(1/2) = 0.08 V) leading to its Mn(IV)Mn(IV) analogue. The formation of 2a from 1a was monitored by UV-visible and X-ray absorption spectroscopies. Both concur to show that an intermediate Mn(II)Mn(III) species, resembling 4a [Mn(2)(L)(OAc)(2)(H(2)O)](ClO(4))(2), the one-electron-oxidized form of 1a, is formed initially and transforms into 2a. The structures of the active intermediates 2 and 3 are discussed in light of their spectroscopic properties, and potential mechanisms are considered and discussed in the context of the biological reaction.

Structure of the manganese-bound manganese transport regulator of Bacillus subtilis

The Bacillus subtilis manganese transport regulator, MntR, binds Mn2+ as an effector and is a repressor of transporters that import manganese. A member of the diphtheria toxin repressor (DtxR) family of metalloregulatory proteins, MntR exhibits selectivity for Mn2+ over Fe2+. Replacement of a metal-binding residue, Asp8, with methionine (D8M) relaxes this specificity. We report here the X-ray crystal structures of wild-type MntR and the D8M mutant bound to manganese with 1.75 A and 1.61 A resolution, respectively. The 142-residue MntR homodimer has substantial structural similarity to the 226-residue DtxR but lacks the C-terminal SH3-like domain of DtxR. The metal-binding pockets of MntR and DtxR are substantially different. The cation-to-cation distance between the two manganese ions bound by MntR is 3.3 A, whereas that between the metal ions bound by DtxR is 9 A. D8M binds only a single Mn2+ per monomer, owing to alteration of the metal-binding site. The sole retained metal site adopts pseudo-hexacoordinate geometry rather than the pseudo-heptacoordinate geometry of the MntR metal sites.

X- and Q-band EPR studies of the dinuclear Mn(II) complex [(Bpmp)Mn2(mu-OAc)2]+. Determination of the spin parameters for the S = 1 and S = 2 spin states

A new mu-phenoxo-bis-mu-acetato di-Mn(II) complex using the BpmpH ligand was isolated as a perchlorate salt (BpmpH = 2,6-bis[bis(2-pyridylmethyl)aminomethyl]-4-methyl-phenol). The X-ray structure has been solved showing that the two Mn(II) ions are in a distorted octahedral environment. Investigation of the variation of the molar magnetic susceptibility upon temperature reveals an antiferromagnetic exchange interaction between the two high-spin Mn(II) ions. Fitting of the experimental data led to g = 1.99 and J = 9.6 cm(-1) (H(HDvV) = JS(A).S(B)). EPR spectra recorded on a powder sample of [(Bpmp)Mn(2)(mu-OAc)(2)](ClO(4)).0.5H(2)O at X-band between 4.3 K and room temperature and at Q-band between 5 and 298 K are presented. A new method based on a scrupulous examination of the variation upon temperature of these experimental spectra is developed here to first assign the transitions to the relevant spin states and second to determine the associated spin parameters. This approach is compared to the deconvolution process that has been previously applied to dinuclear Mn(II) complexes or metalloenzyme active sites. Crystallographic data is as follows: triclinic, space group P one macro, a = 10.154(2) A, b = 12.0454(2) A, c = 17.743(4) A, alpha = 101.69(3) degrees, beta = 93.62(3) degrees, gamma = 94.67(3) degrees, Z = 2.

Insights into the effects on metal binding of the systematic substitution of five key glutamate ligands in the ferritin of Escherichia coli

Ferritins are nearly ubiquitous iron storage proteins playing a fundamental role in iron metabolism. They are composed of 24 subunits forming a spherical protein shell encompassing a central iron storage cavity. The iron storage mechanism involves the initial binding and subsequent O2-dependent oxidation of two Fe2+ ions located at sites A and B within the highly conserved dinuclear "ferroxidase center" in individual subunits. Unlike animal ferritins and the heme-containing bacterioferritins, the Escherichia coli ferritin possesses an additional iron-binding site (site C) located on the inner surface of the protein shell close to the ferroxidase center. We report the structures of five E. coli ferritin variants and their Fe3+ and Zn2+ (a redox-stable alternative for Fe2+) derivatives. Single carboxyl ligand replacements in sites A, B, and C gave unique effects on metal binding, which explain the observed changes in Fe2+ oxidation rates. Binding of Fe2+ at both A and B sites is clearly essential for rapid Fe2+ oxidation, and the linking of FeB2+ to FeC2+ enables the oxidation of three Fe2+ ions. The transient binding of Fe2+ at one of three newly observed Zn2+ sites may allow the oxidation of four Fe2+ by one dioxygen molecule.

Emerging themes in manganese transport, biochemistry and pathogenesis in bacteria

Though an essential trace element, manganese is generally accorded little importance in biology other than as a cofactor for some free radical detoxifying enzymes and in the photosynthetic photosystem II. Only a handful of other Mn2+-dependent enzymes are known. Recent data, primarily in bacteria, suggest that Mn2+-dependent processes may have significantly greater physiological importance. Two major classes of prokaryotic Mn2+ uptake systems have now been described, one homologous to eukaryotic Nramp transporters and one a member of the ABC-type ATPase superfamily. Each is highly selective for Mn2+ over Fe2+ or other transition metal divalent cations, and each can accumulate millimolar amounts of intracellular Mn2+ even when environmental Mn2+ is scarce. In Salmonella enterica serovar Typhimurium, simultaneous mutation of both types of transporter results in avirulence, implying that one or more Mn2+-dependent enzymes is essential for pathogenesis. This review summarizes current literature on Mn2+ transport, primarily in the Bacteria but with relevant comparisons to the Archaea and Eukaryota. Mn2+-dependent enzymes are then discussed along with some speculations as to their role(s) in cellular physiology, again primarily in Bacteria. It is of particular interest that most of the enzymes which interconvert phosphoglycerate, pyruvate, and oxaloacetate intermediates are either strictly Mn2+-dependent or highly stimulated by Mn2+. This suggests that Mn2+ may play an important role in central carbon metabolism. Further studies will be required, however, to determine whether these or other actions of Mn2+ within the cell are the relevant factors in pathogenesis.

Raman spectra and normal coordinate analyses of low-frequency vibrations of oxo-bridged manganese complexes

The active sites of certain metalloenzymes involved in oxygen metabolism, such as manganese catalase and the oxygen-evolving complex of photosystem II, contain micro -oxo-bridged Mn clusters with ligands that include H(2)O and micro (1,3)-carboxylato bridges provided by protein side chains. In order to understand better the vibrational spectra of such clusters, the low-frequency resonance Raman spectra of a series of structurally characterized Mn-oxo model complexes were examined. The series includes complexes of the type [Mn(2)(O)(OAc)(2)(bpy)(2)(L)(2)] and [Mn(2)(O)(2)(OAc)(bpy)(2)(L)(2)], where bpy=2,2'-bipyridine, OAc=acetate and L=H(2)O or Cl(-). Complexes containing the isotopomers OAc- d(3) and D(2)O, as well as those containing both isotopomers, were also examined. Normal coordinate analyses (NCA) were performed on the various complexes using theGF matrix method. Selected vibrational modes in the 200-600 cm(-1) region were assigned based on the spectra and NCA, which identify vibrational modes arising from the metal-ligand bonds. These results will be useful in interpreting the vibrational spectra obtained from metalloproteins containing Mn-oxo complexes in their active sites.

Outer sphere mutagenesis of Lactobacillus plantarum manganese catalase disrupts the cluster core. Mechanistic implications

X-ray crystallography of the nonheme manganese catalase from Lactobacillus plantarum (LPC) [Barynin, V.V., Whittaker, M.M., Antonyuk, S.V., Lamzin, V.S., Harrison, P.M., Artymiuk, P.J. & Whittaker, J.W. (2001) Structure9, 725-738] has revealed the structure of the dimanganese redox cluster together with its protein environment. The oxidized [Mn(III)Mn(III)] cluster is bridged by two solvent molecules (oxo and hydroxo, respectively) together with a micro 1,3 bridging glutamate carboxylate and is embedded in a web of hydrogen bonds involving an outer sphere tyrosine residue (Tyr42). A novel homologous expression system has been developed for production of active recombinant LPC and Tyr42 has been replaced by phenylalanine using site-directed mutagenesis. Spectroscopic and structural studies indicate that disruption of the hydrogen-bonded web significantly perturbs the active site in Y42F LPC, breaking one of the solvent bridges and generating an 'open' form of the dimanganese cluster. Two of the metal ligands adopt alternate conformations in the crystal structure, both conformers having a broken solvent bridge in the dimanganese core. The oxidized Y42F LPC exhibits strong optical absorption characteristic of high spin Mn(III) in low symmetry and lower coordination number. MCD and EPR measurements provide complementary information defining a ferromagnetically coupled electronic ground state for a cluster containing a single solvent bridge, in contrast to the diamagnetic ground state found for the native cluster containing a pair of solvent bridges. Y42F LPC has less than 5% of the catalase activity and much higher Km for H2O2 ( approximately 1.4 m) at neutral pH than WT LPC, although the activity is slightly restored at high pH where the cluster is converted to a diamagnetic form. These studies provide new insight into the contribution of the outer sphere tyrosine to the stability of the dimanganese cluster and the role of the solvent bridges in catalysis by dimanganese catalases.