Proton-coupled electron transfer in Fe-superoxide dismutase and Mn-superoxide dismutase

Catechol-Modified and MnO2-Nanozyme-Reinforced Hydrogel with Improved Antioxidant and Antibacterial Capacity for Periodontitis Treatment

Periodontitis is an inflammatory disease characterized by tooth loss and alveolar bone resorption. Bacteria are the original cause of periodontitis, and excess reactive oxygen species (ROS) encourage and intensify inflammation. In this study, a mussel-inspired and MnO2 NPs-reinforced adhesive hydrogel capable of alleviating periodontitis with improved antibacterial and antioxidant abilities was developed. The hydrogel was created by combining polyvinyl alcohol (PVA), 3,4-dihydroxy-d-phenylalanine (DOPA), and MnO2 nanoparticles (NPs) (named PDMO hydrogel). The hydrogel was demonstrated to be able to scavenge various free radicals (including total ROS─O2•- and OH•) and relieve the hypoxia in an inflammatory microenvironment by scavenging excess ROS and generating O2 due to its superoxide dismutase (SOD)/catalase (CAT)-like activity. Besides, under 808 nm near-infrared (NIR) light, the photothermal performance of the PDMO hydrogel displayed favorable antibacterial and antibiofilm effects toward Escherichia coli, Staphylococcus aureus, and Porphyromonas gingivalis (up to nearly 100% antibacterial rate). Furthermore, the PDMO hydrogel exhibited favorable therapeutic efficacy in alleviating gingivitis in Sprague-Dawley rats, even comparable to or better than the commercial PERIO. In addition, in the periodontitis models, the PDMO2 group showed the height of the residual alveolar bone and the smallest shadow area of low density among other groups, indicating the positive role of the PDMO2 hydrogel in bone regeneration. Finally, the biosafety of the PDMO hydrogel was comprehensively investigated, and the hydrogel was demonstrated to have good biocompatibility. Therefore, the developed PDMO hydrogel provided an effective solution to resolve biofilm recolonization and oxidative stress in periodontitis and could be a superior candidate for local drug delivery system in the clinical management of periodontitis with great potential for future clinical translation.

Interaction Network Construction and Functional Analysis of the Plasma Membrane H+-ATPase in Bangia fuscopurpurea (Rhodophyta)

Salinity is a serious threat to most land plants. Although seaweeds adapt to salty environments, intertidal species experience wide fluctuations in external salinities, including hyper- and hypo-saline stress. Bangia fuscopurpurea is an economic intertidal seaweed with a strong tolerance to hypo-salinity. Until now, the salt stress tolerance mechanism has remained elusive. Our previous study showed that the expression of B. fuscopurpurea plasma membrane H⁺-ATPase (BfPMHA) genes were the most upregulated under hypo-salinity. In this study, we obtained the complete sequence of BfPMHA, traced the relative expression of this BfPMHA gene in B. fuscopurpurea under hypo-salinity, and analyzed the protein structure and properties based on the gene's sequence. The result showed that the expression of BfPMHA in B. fuscopurpurea increased significantly with varying hypo-salinity treatments, and the higher the degree of low salinity stress, the higher the expression level. This BfPMHA had typical PMHA structures with a Cation-N domain, an E1-E2 ATPase domain, a Hydrolase domain, and seven transmembrane domains. In addition, through the membrane system yeast two-hybrid library, three candidate proteins interacting with BfPMHA during hypo-saline stress were screened, fructose-bisphosphate aldolase (BfFBA), glyceraldehyde 3-phosphate dehydrogenase (NADP⁺) (phosphorylating) (BfGAPDH), and manganese superoxide dismutase (BfMnSOD). The three candidates and BfPMHA genes were successfully transferred and overexpressed in a BY4741 yeast strain. All of them significantly enhanced the yeast tolerance to NaCl stress, verifying the function of BfPMHA in salt stress response. This is the first study to report the structure and topological features of PMHA in B. fuscopurpurea and its candidate interaction proteins in response to salt stress.

Development of bacterial resistance induced by low concentration of two-dimensional black phosphorus via mutagenesis

The wide use of nano-antibacterial materials has triggered concerns over the development of nanomaterials-associated bacterial resistance. Two-dimensional (2D) black phosphorus (BP) as a new class of emerging 2D nanomaterial has displayed excellent antibacterial performance. However, whether bacteria repeatedly exposed to 2D BP can develop resistance is not clear. We found that wild type E. coli K-12 MG 1655 strains can increase resistance to 2D-BP nanosheets after repeated exposure with subinhibitory concentration of 2D-BP nanosheets. Adaptive morphogenesis including the reinforced barrier function of cell membrane were observed in the resistant bacteria, which enhanced the resistance of bacteria to 2D-BP nanosheets. The whole-genome sequencing analysis showed that the three mutation genes including dmdA, mntP, and gyrA genes were observed in the 2D-BP resistant strains, which controlled catabolism, membrane structure, and DNA replication, respectively. Furthermore, transcriptional sequencing confirmed that these genes related to metabolization, membrane structure, and cell motility were upregulated in the 2D-BP resistant bacteria. The development of resistance to 2D-BP in bacteria mainly attributed to the changes in energy metabolism and membrane structure of bacteria caused by gene mutations. In addition, the up-regulated function of cell motility also helped the bacteria to develop resistance by escaping external stimuli. The results provided new evidence for understanding an important effect of nano-antibacterial materials on the development of bacterial resistance.

The wide use of nano-antibacterial materials has triggered concerns over the development of nanomaterials-associated bacterial resistance.

Antibacterial Zeolite Imidazole Frameworks with Manganese Doping for Immunomodulation to Accelerate Infected Wound Healing

Numerous nanomedicines currently emerge to reduce the dramatic threat in antibiotics resistance for antibacterial application against severe bacterial infections, while it is restricted by over-reacted immune response to pathogenic bacteria. Herein, enzymatic activity is introduced into the zeolitic imidazolate framework-8 (ZIF-8) to achieve sterilization by releasing Zn ions, as well as inflammation regulation through the variable valence of Mn ions that are uniformly doped into its framework. Within this simple metal organic framework (MOF) structure design, Mn-ZIF-8 possesses the co-existence of Mn²⁺ /Mn⁴⁺ to endow the nanocomposite with the anti-inflammatory capabilities, which can be adjusted through the redox environment. The enzymatic activity of Mn ions and superiority of pore structure of ZIF-8 are effectively combined to realize the substrate selection via reactant molecular size and high-efficiency internal catalytic performance. By such design, this nanocomposite would not only exhibit an excellent antibacterial performance against pathogenic bacteria, but also reshape the inflammatory immunity by regulating macrophage polarization to suppress over-reacted inflammation, leading to a favorably therapeutic efficiency on bacteria-infected wound healing in animal models. Taken together, this nanoplatform provides effective approach for accelerating infected wound healing via bacteria killing and inflammation modulation, and may be extended for the therapy of other severe bacteria-induced infections.

Metalloprotein catalysis: structural and mechanistic insights into oxidoreductases from neutron protein crystallography

Metalloproteins catalyze a range of reactions, with enhanced chemical functionality due to their metal cofactor. The reaction mechanisms of metalloproteins have been experimentally characterized by spectroscopy, macromolecular crystallography and cryo-electron microscopy. An important caveat in structural studies of metalloproteins remains the artefacts that can be introduced by radiation damage. Photoreduction, radiolysis and ionization deriving from the electromagnetic beam used to probe the structure complicate structural and mechanistic interpretation. Neutron protein diffraction remains the only structural probe that leaves protein samples devoid of radiation damage, even when data are collected at room temperature. Additionally, neutron protein crystallography provides information on the positions of light atoms such as hydrogen and deuterium, allowing the characterization of protonation states and hydrogen-bonding networks. Neutron protein crystallography has further been used in conjunction with experimental and computational techniques to gain insight into the structures and reaction mechanisms of several transition-state metal oxidoreductases with iron, copper and manganese cofactors. Here, the contribution of neutron protein crystallography towards elucidating the reaction mechanism of metalloproteins is reviewed.

Controlling the Reactivity of a Metal-Hydroxo Adduct with a Hydrogen Bond

The enzymes manganese lipoxygenase (MnLOX) and manganese superoxide dismutase (MnSOD) utilize mononuclear Mn centers to effect their catalytic reactions. In the oxidized Mn^III state, the active site of each enzyme contains a hydroxo ligand, and X-ray crystal structures imply a hydrogen bond between this hydroxo ligand and a cis carboxylate ligand. While hydrogen bonding is a common feature of enzyme active sites, the importance of this particular hydroxo-carboxylate interaction is relatively unexplored. In this present study, we examined a pair of Mn^III-hydroxo complexes that differ by a single functional group. One of these complexes, [Mn^III(OH)(PaPy₂N)]⁺, contains a naphthyridinyl moiety capable of forming an intramolecular hydrogen bond with the hydroxo ligand. The second complex, [Mn^III(OH)(PaPy₂Q)]⁺, contains a quinolinyl moiety that does not permit any intramolecular hydrogen bonding. Spectroscopic characterization of these complexes supports a common structure, but with perturbations to [Mn^III(OH)(PaPy₂N)]⁺, consistent with a hydrogen bond. Kinetic studies using a variety of substrates with activated O-H bonds, revealed that [Mn^III(OH)(PaPy₂N)]⁺ is far more reactive than [Mn^III(OH)(PaPy₂Q)]⁺, with rate enhancements of 15-100-fold. A detailed analysis of the thermodynamic contributions to these reactions using DFT computations reveals that the former complex is significantly more basic. This increased basicity counteracts the more negative reduction potential of this complex, leading to a stronger O-H BDFE in the [Mn^II(OH₂)(PaPy₂N)]⁺ product. Thus, the differences in reactivity between [Mn^III(OH)(PaPy₂Q)]⁺ and [Mn^III(OH)(PaPy₂N)]⁺ can be understood on the basis of thermodynamic considerations, which are strongly influenced by the ability of the latter complex to form an intramolecular hydrogen bond.

Direct detection of coupled proton and electron transfers in human manganese superoxide dismutase

Human manganese superoxide dismutase is a critical oxidoreductase found in the mitochondrial matrix. Concerted proton and electron transfers are used by the enzyme to rid the mitochondria of O₂^•−. The mechanisms of concerted transfer enzymes are typically unknown due to the difficulties in detecting the protonation states of specific residues and solvent molecules at particular redox states. Here, neutron diffraction of two redox-controlled manganese superoxide dismutase crystals reveal the all-atom structures of Mn³⁺ and Mn²⁺ enzyme forms. The structures deliver direct data on protonation changes between oxidation states of the metal. Observations include glutamine deprotonation, the involvement of tyrosine and histidine with altered pK_as, and four unusual strong-short hydrogen bonds, including a low barrier hydrogen bond. We report a concerted proton and electron transfer mechanism for human manganese superoxide dismutase from the direct visualization of active site protons in Mn³⁺ and Mn²⁺ redox states.

Human manganese superoxide dismutase (MnSOD) is an oxidoreductase that uses concerted proton and electron transfers to reduce the levels of superoxide radicals in mitochondria, but mechanistic insights into this process are limited. Here, the authors report neutron crystal structures of Mn³⁺SOD and Mn²⁺SOD, revealing changes in the protonation states of key residues in the enzyme active site during the redox cycle.

Redox manipulation of the manganese metal in human manganese superoxide dismutase for neutron diffraction

Human manganese superoxide dismutase (MnSOD) is one of the most significant enzymes in preventing mitochondrial dysfunction and related diseases by combating reactive oxygen species (ROS) in the mitochondrial matrix. Mitochondria are the source of up to 90% of cellular ROS generation, and MnSOD performs its necessary bioprotective role by converting superoxide into oxygen and hydrogen peroxide. This vital catalytic function is conducted via cyclic redox reactions between the substrate and the active-site manganese using proton-coupled electron transfers. Owing to protons being difficult to detect experimentally, the series of proton transfers that compose the catalytic mechanism of MnSOD are unknown. Here, methods are described to discern the proton-based mechanism using chemical treatments to control the redox state of large perdeuterated MnSOD crystals and subsequent neutron diffraction. These methods could be applicable to other crystal systems in which proton information on the molecule in question in specific chemical states is desired.

A Review of the Catalytic Mechanism of Human Manganese Superoxide Dismutase

Superoxide dismutases (SODs) are necessary antioxidant enzymes that protect cells from reactive oxygen species (ROS). Decreased levels of SODs or mutations that affect their catalytic activity have serious phenotypic consequences. SODs perform their bio-protective role by converting superoxide into oxygen and hydrogen peroxide by cyclic oxidation and reduction reactions with the active site metal. Mutations of SODs can cause cancer of the lung, colon, and lymphatic system, as well as neurodegenerative diseases such as Parkinson's disease and amyotrophic lateral sclerosis. While SODs have proven to be of significant biological importance since their discovery in 1968, the mechanistic nature of their catalytic function remains elusive. Extensive investigations with a multitude of approaches have tried to unveil the catalytic workings of SODs, but experimental limitations have impeded direct observations of the mechanism. Here, we focus on human MnSOD, the most significant enzyme in protecting against ROS in the human body. Human MnSOD resides in the mitochondrial matrix, the location of up to 90% of cellular ROS generation. We review the current knowledge of the MnSOD enzymatic mechanism and ongoing studies into solving the remaining mysteries.

Human Mn-superoxide dismutase inactivation by peroxynitrite: a paradigm of metal-catalyzed tyrosine nitration in vitro and in vivo

Nitration of human MnSOD at active site Tyr34 represents a biologically-relevant oxidative post-translational modification that causes enzyme inactivation.

Combined QM/MM and Monte Carlo study for redox leveling in Mn and Fe superoxide dismutase

Superoxide dismutases (SOD) are vital enzymes for disproportionation of superoxide molecules in mammals. Despite the high similarity between the Mn-SOD and Fe-SOD, they are inactive if the metals in the active sites are exchanged. Here, we use DFT, QM/MM and Monte Carlo sampling to optimize the crystal structure and to calculate the mid-point potential for the native and substituted Mn/Fe-SOD. The optimized DFT and QM/MM structures of the Mn-SOD show a major conformational change for the conserved TYR34 compared to the X-ray structure. These changes reduce the distance between TYR34 and Mn ion to 2.59 Å, which yields a lower reduction potential for the Mn. On contrary, there is no significant difference between optimized and crystal structures in the Fe-SOD. The calculated E _m values starting from the DFT structures of the active sites show similar pattern, in good agreement with those observed experimentally. However, the calculated E _m values starting with the QM/MM structures that include the whole protein are significantly higher due to the desolvation penalty. In addition, the pK _a values for the water ligand in the reduced state Mn(II) and Fe(II) were calculated. The water pK _a in Mn-SOD is higher than that in Fe-SOD by 3.5 pH units, which is similar to the shift measured experimentally. Finally, we investigated the role of HIS30 and the effect of its protonation state on the E _m values.

A Single Outer-Sphere Mutation Stabilizes apo-Mn Superoxide Dismutase by 35 °C and Disfavors Mn Binding

The catalytic active site of Mn-specific superoxide dismutase (MnSOD) is organized around a redox-active Mn ion. The most highly conserved difference between MnSODs and the homologous FeSODs is the origin of a Gln in the second coordination sphere. In MnSODs it derives from the C-terminal domain whereas in FeSODs it derives from the N-terminal domain, yet its side chain occupies almost superimposable positions in the active sites of these two types of SODs. Mutation of this Gln69 to Glu in Escherichia coli FeSOD increased the Fe3+/2+ reduction midpoint potential by >0.6 V without disrupting the structure or Fe binding [ Yikilmaz, E., Rodgers, D. W., and Miller, A.-F. ( 2006 ) Biochemistry 45 ( 4 ), 1151 - 1161 ]. We now describe the analogous Q146E mutant of MnSOD, explaining its low Mn content in terms increased stability of the apo-Mn protein. In 0.8 M guanidinium HCl, Q146E-apoMnSOD displays an apparent melting midpoint temperature (Tm) 35 °C higher that of wild-type (WT) apoMnSOD, whereas the Tm of WT-holoMnSOD is only 20 °C higher than that of WT-apoMnSOD. In contrast, the Tm attributed to Q146E-holoMnSOD is 40 °C lower than that of Q146E-apoMnSOD. Thus, our data refute the notion that the WT residues optimize the structural stability of the protein and instead are consistent with conservation on the basis of enzyme function and therefore ability to bind metal ion. We propose that the WT-MnSOD protein conserves a destabilizing amino acid at position 146 as part of a strategy to favor metal ion binding.

Substrate-analog binding and electrostatic surfaces of human manganese superoxide dismutase

Superoxide dismutases (SODs) are enzymes that play a key role in protecting cells from toxic oxygen metabolites by disproportionation of two molecules of superoxide into molecular oxygen and hydrogen peroxide via cyclic reduction and oxidation at the active site metal. The azide anion is a potent competitive inhibitor that binds directly to the metal and is used as a substrate analog to superoxide in studies of SOD. The crystal structure of human MnSOD-azide complex was solved and shows the putative binding position of superoxide, providing a model for binding to the active site. Azide is bound end-on at the sixth coordinate position of the manganese ion. Tetrameric electrostatic surfaces were calculated incorporating accurate partial charges for the active site in three states, including a state with superoxide coordinated to the metal using the position of azide as a model. These show facilitation of the anionic ligand to the active site pit via a 'valley' of positively-charged surface patches. Surrounding ridges of negative charge help guide the superoxide anion. Within the active site pit, Arg173 and Glu162 further guide and align superoxide for efficient catalysis. Superoxide coordination at the sixth position causes the electrostatic surface of the active site pit to become nearly neutral. A model for electrostatic-mediated diffusion, and efficient binding of superoxide for catalysis is presented.

Reinventing the Wheel: The NiSOD Story

The most recently discovered SOD requires nickel in its active site – NiSOD. Among the available metals, nickel seems an unlikely redox center. This chapter discusses the protein adaptations required in order to use nickel for SOD catalysis. Cysteine ligands are employed for the first time in an SOD, to suppress the potential of the Ni(ii/iii) couple. However, this adaptation alone is not sufficient to produce an SOD, since thiolate ligands are sensitive to oxidation by H2O2 and O2. Additional adaptations include the use of two unusual backbone N-donor ligands, an amidate and the N-terminal amine. Yet merely producing a stable Ni redox center is not sufficient for SOD catalysis. A source of protons is needed to produce H2O2 and the pH-independent catalysis that is characteristic of SODs. Thus, the cysteine thiolates were also employed to provide a site for protonation. In restricting active site access, NiSOD appears to have utilized the same strategy employed by MnSOD and FeSOD – a “gateway” formed by Tyr residues. Thus, NiSOD represents evolution that converged on the same criteria for catalysis as other SODs, where the adaptations to the metal site are uniquely suited to using nickel as a redox center.

Preliminary neutron diffraction analysis of challenging human manganese superoxide dismutase crystals

Superoxide dismutases (SODs) are enzymes that protect against oxidative stress by dismutation of superoxide into oxygen and hydrogen peroxide through cyclic reduction and oxidation of the active-site metal. The complete enzymatic mechanisms of SODs are unknown since data on the positions of hydrogen are limited. Here, methods are presented for large crystal growth and neutron data collection of human manganese SOD (MnSOD) using perdeuteration and the MaNDi beamline at Oak Ridge National Laboratory. The crystal from which the human MnSOD data set was obtained is the crystal with the largest unit-cell edge (240 Å) from which data have been collected via neutron diffraction to sufficient resolution (2.30 Å) where hydrogen positions can be observed.

Saturation kinetics in phenolic O-H bond oxidation by a mononuclear Mn(III)-OH complex derived from dioxygen

The mononuclear hydroxomanganese(III) complex, [Mn(III)(OH)(dpaq)](+), which is supported by the amide-containing N5 ligand dpaq (dpaq = 2-[bis(pyridin-2-ylmethyl)]amino-N-quinolin-8-yl-acetamidate) was generated by treatment of the manganese(II) species, [Mn(II)(dpaq)](OTf), with dioxygen in acetonitrile solution at 25 °C. This oxygenation reaction proceeds with essentially quantitative yield (greater than 98% isolated yield) and represents a rare example of an O2-mediated oxidation of a manganese(II) complex to generate a single product. The X-ray diffraction structure of [Mn(III)(OH)(dpaq)](+) reveals a short Mn-OH distance of 1.806(13) Å, with the hydroxo moiety trans to the amide function of the dpaq ligand. No shielding of the hydroxo group is observed in the solid-state structure. Nonetheless, [Mn(III)(OH)(dpaq)](+) is remarkably stable, decreasing in concentration by only 10% when stored in MeCN at 25 °C for 1 week. The [Mn(III)(OH)(dpaq)](+) complex participates in proton-coupled electron transfer reactions with substrates with relatively weak O-H and C-H bonds. For example, [Mn(III)(OH)(dpaq)](+) oxidizes TEMPOH (TEMPOH = 2,2'-6,6'-tetramethylpiperidine-1-ol), which has a bond dissociation free energy (BDFE) of 66.5 kcal/mol, in MeCN at 25 °C. The hydrogen/deuterium kinetic isotope effect of 1.8 observed for this reaction implies a concerted proton-electron transfer pathway. The [Mn(III)(OH)(dpaq)](+) complex also oxidizes xanthene (C-H BDFE of 73.3 kcal/mol in dimethylsulfoxide) and phenols, such as 2,4,6-tri-t-butylphenol, with BDFEs of less than 79 kcal/mol. Saturation kinetics were observed for phenol oxidation, implying an initial equilibrium prior to the rate-determining step. On the basis of a collective body of evidence, the equilibrium step is attributed to the formation of a hydrogen-bonding complex between [Mn(III)(OH)(dpaq)](+) and the phenol substrates.

Molecular mechanisms for generating transmembrane proton gradients

Membrane proteins use the energy of light or high energy substrates to build a transmembrane proton gradient through a series of reactions leading to proton release into the lower pH compartment (P-side) and proton uptake from the higher pH compartment (N-side). This review considers how the proton affinity of the substrates, cofactors and amino acids are modified in four proteins to drive proton transfers. Bacterial reaction centers (RCs) and photosystem II (PSII) carry out redox chemistry with the species to be oxidized on the P-side while reduction occurs on the N-side of the membrane. Terminal redox cofactors are used which have pKas that are strongly dependent on their redox state, so that protons are lost on oxidation and gained on reduction. Bacteriorhodopsin is a true proton pump. Light activation triggers trans to cis isomerization of a bound retinal. Strong electrostatic interactions within clusters of amino acids are modified by the conformational changes initiated by retinal motion leading to changes in proton affinity, driving transmembrane proton transfer. Cytochrome c oxidase (CcO) catalyzes the reduction of O2 to water. The protons needed for chemistry are bound from the N-side. The reduction chemistry also drives proton pumping from N- to P-side. Overall, in CcO the uptake of 4 electrons to reduce O2 transports 8 charges across the membrane, with each reduction fully coupled to removal of two protons from the N-side, the delivery of one for chemistry and transport of the other to the P-side.

Electrostatic effects on proton coupled electron transfer in oxomanganese complexes inspired by the oxygen-evolving complex of photosystem II

The influence of electrostatic interactions on the free energy of proton coupled electron transfer in biomimetic oxomanganese complexes inspired by the oxygen-evolving complex (OEC) of photosystem II (PSII) are investigated. The reported study introduces an enhanced multiconformer continuum electrostatics (MCCE) model, parametrized at the density functional theory (DFT) level with a classical valence model for the oxomanganese core. The calculated pKa's and oxidation midpoint potentials (E(m)'s) match experimental values for eight complexes, indicating that purely electrostatic contributions account for most of the observed couplings between deprotonation and oxidation state transitions. We focus on pKa's of terminal water ligands in [Mn(II/III)(H2O)6](2+/3+) (1), [Mn(III)(P)(H2O)2](3-) (2, P = 5,10,15,20-tetrakis(2,6-dichloro-3-sulfonatophenyl)porphyrinato), [Mn2(IV,IV)(μ-O)2(terpy)2(H2O)2](4+) (3, terpy = 2,2':6',2″-terpyridine), and [Mn3(IV,IV,IV)(μ-O)4(phen)4(H2O)2](4+) (4, phen = 1,10-phenanthroline) and the pKa's of μ-oxo bridges and Mn E(m)'s in [Mn2(μ-O)2(bpy)4] (5, bpy = 2,2'-bipyridyl), [Mn2(μ-O)2(salpn)2] (6, salpn = N,N'-bis(salicylidene)-1,3-propanediamine), [Mn2(μ-O)2(3,5-di(Cl)-salpn)2] (7), and [Mn2(μ-O)2(3,5-di(NO2)-salpn)2] (8). The analysis of complexes 6-8 highlights the strong coupling between electron and proton transfers, with any Mn oxidation lowering the pKa of an oxo bridge by 10.5 ± 0.9 pH units. The model also accounts for changes in the E(m)'s by ligand substituents, such as found in complexes 6-8, due to the electron withdrawing Cl (7) and NO2 (8). The reported study provides the foundation for analysis of electrostatic effects in other oxomanganese complexes and metalloenzymes, where proton coupled electron transfer plays a fundamental role in redox-leveling mechanisms.

Geometric and electronic structures of manganese-substituted iron superoxide dismutase

The active-site structures of the oxidized and reduced forms of manganese-substituted iron superoxide dismutase (Mn(Fe)SOD) are examined, for the first time, using a combination of spectroscopic and computational methods. On the basis of electronic absorption, circular dichroism (CD), magnetic CD (MCD), and variable-temperature variable-field MCD data obtained for oxidized Mn(Fe)SOD, we propose that the active site of this species is virtually identical to that of wild-type manganese SOD (MnSOD), with both containing a metal ion that resides in a trigonal bipyramidal ligand environment. This proposal is corroborated by quantum mechanical/molecular mechanical (QM/MM) computations performed on complete protein models of Mn(Fe)SOD in both its oxidized and reduced states and, for comparison, wild-type (WT) MnSOD. The major differences between the QM/MM optimized active sites of WT MnSOD and Mn(Fe)SOD are a smaller (His)N-Mn-N(His) equatorial angle and a longer (Gln146(69))NH···O(sol) H-bond distance in the metal-substituted protein. Importantly, these modest geometric differences are consistent with our spectroscopic data obtained for the oxidized proteins and high-field electron paramagnetic resonance spectra reported previously for reduced Mn(Fe)SOD and MnSOD. As Mn(Fe)SOD exhibits a reduction midpoint potential (E(m)) almost 700 mV higher than that of MnSOD, which has been shown to be sufficient for explaining the lack of SOD activity displayed by the metal-subtituted species (Vance, C. K.; Miller, A. F. Biochemistry 2001, 40, 13079-13087), E(m)'s were computed for our experimentally validated QM/MM optimized models of Mn(Fe)SOD and MnSOD. These computations properly reproduce the experimental trend and reveal that the drastically elevated E(m) of the metal substituted protein stems from a larger separation between the second-sphere Gln residue and the coordinated solvent in Mn(Fe)SOD relative to MnSOD, which causes a weakening of the corresponding H-bond interaction in the oxidized state and alleviates steric crowding in the reduced state.

Superoxide dismutases: ancient enzymes and new insights

Superoxide dismutases (SODs) catalyze the de toxification of superoxide. SODs therefore acquired great importance as O(2) became prevalent following the evolution of oxygenic photosynthesis. Thus the three forms of SOD provide intriguing insights into the evolution of the organisms and organelles that carry them today. Although ancient organisms employed Fe-dependent SODs, oxidation of the environment made Fe less bio-available, and more dangerous. Indeed, modern lineages make greater use of homologous Mn-dependent SODs. Our studies on the Fe-substituted MnSOD of Escherichia coli, as well as redox tuning in the FeSOD of E. coli shed light on how evolution accommodated differences between Fe and Mn that would affect SOD performance, in SOD proteins whose activity is specific to one or other metal ion.

Reduction potentials and acidity constants of Mn superoxide dismutase calculated by QM/MM free-energy methods

We used two theoretical methods to estimate reduction potentials and acidity constants in Mn superoxide dismutase (MnSOD), namely combined quantum mechanical and molecular mechanics (QM/MM) thermodynamic cycle perturbation (QTCP) and the QM/MM-PBSA approach. In the latter, QM/MM energies are combined with continuum solvation energies calculated by solving the Poisson-Boltzmann equation (PB) or by the generalised Born approach (GB) and non-polar solvation energies calculated from the solvent-exposed surface area. We show that using the QTCP method, we can obtain accurate and precise estimates of the proton-coupled reduction potential for MnSOD, 0.30±0.01 V, which compares favourably with experimental estimates of 0.26-0.40 V. However, the calculated potentials depend strongly on the DFT functional used: The B3LYP functional gives 0.6 V more positive potentials than the PBE functional. The QM/MM-PBSA approach leads to somewhat too high reduction potentials for the coupled reaction and the results depend on the solvation model used. For reactions involving a change in the net charge of the metal site, the corresponding results differ by up to 1.3 V or 24 pK(a) units, rendering the QM/MM-PBSA method useless to determine absolute potentials. However, it may still be useful to estimate relative shifts, although the QTCP method is expected to be more accurate.

Concerted proton-electron transfers: electrochemical and related approaches

Proton-coupled electron transfers (PCETs) are omnipresent in natural and artificial chemical processes. Given the contemporary challenges associated with energy conversion, pollution abatement, and the development of high-performance sensors, a greater understanding of the mechanisms that underlie the practical efficiency of PCETs is a timely research topic. In contrast to hydrogen-atom transfers, proton and electron transfers involve different centers in PCET reactions. The reaction may go through an electron- or proton-transfer intermediate, giving rise to the electron-proton transfer (EPT) and the proton-electron transfer (PET) pathways. When the proton and electron transfers are concerted (the CPET pathway), the high-energy intermediates of the stepwise pathways are bypassed, although this thermodynamic benefit may have a kinetic cost. The primary task of kinetics-based mechanism analysis is therefore to distinguish the three pathways, quantifying the factors that govern the competition between them, which requires modeling of CPET reactivity. CPET models of varying sophistication have appeared, but the large number of parameters involved and the uncertainty of the quantum chemical calculations they may have to resort to make experimental confrontation and inspiration a necessary component of model testing and refinement. Electrochemical PCETs are worthy of particular attention, if only because most applications in which PCET mechanisms are operative involve collection or injection of electricity through electrodes. More fundamentally, changing the electrode potential is an easy and continuous means of varying the driving force of the reaction, whereas the current flowing through the electrode is a straightforward measure of its rate. Consequently, the current-potential response in nondestructive techniques (such as cyclic voltammetry) can be read as an activation-driving force relationship, provided the contribution of diffusion has been taken into account. Intrinsic properties (properties at zero driving force) are consequently a natural outcome of the electrochemical approach. In this Account, we begin by examining the modeling of CPET reactions and then describe illustrating experimental examples inspired by two biological systems, photosystem II and superoxide dismutase. One series of studies examined the oxidation of phenols with, as proton acceptor, either an attached nitrogen base or water (in water as solvent). Another addressed interconversion of aquo-hydroxo-oxo couples of transition metal complexes, using osmium complexes as prototypes. Finally, the reduction of superoxide ion, which is closely related to its dismutation, allowed the observation and rationalization of the remarkable properties of water as a proton donor. Water is also an exceptional proton acceptor in the oxidation of phenols, requiring very small reorganization energies, both in the electrochemical and homogeneous cases. These varied examples reveal general features of PCET reactions that may serve as guidelines for future studies, suggesting that research emphasis might be profitably directed toward new biological systems on the one hand and on the role of hydrogen bonding and hydrogen-bonded environments (such as water or proteins) on the other.

Nickel superoxide dismutase: structural and functional roles of Cys2 and Cys6

Nickel superoxide dismutase (NiSOD) is unique among the family of superoxide dismutase enzymes in that it coordinates Cys residues (Cys2 and Cys6) to the redox-active metal center and exhibits a hexameric quaternary structure. To assess the role of the Cys residues with respect to the activity of NiSOD, mutations of Cys2 and Cys6 to Ser (C2S-NiSOD, C6S-NiSOD, and C2S/C6S-NiSOD) were carried out. The resulting mutants do not catalyze the disproportionation of superoxide, but retain the hexameric structure found for wild-type NiSOD and bind Ni(II) ions in a 1:1 stoichiometry. X-ray absorption spectroscopic studies of the Cys mutants revealed that the nickel active-site structure for each mutant resembles that of C2S/C6S-NiSOD and demonstrate that mutation of either Cys2 or Cys6 inhibits coordination of the remaining Cys residue. Mutation of one or both Cys residue(s) in NiSOD induces the conversion of the low-spin Ni(II) site in the native enzyme to a high-spin Ni(II) center in the mutants. This result indicates that coordination of both Cys residues is required to generate the native low-spin configurations and maintain catalytic activity. Analysis of the quaternary structure of the Cys mutants by differential scanning calorimetry, mass spectrometry, and size-exclusion chromatography revealed that the Cys ligands, particularly Cys2, are also important for stabilizing the hexameric quaternary structure of the native enzyme.

The structural biochemistry of the superoxide dismutases

The discovery of superoxide dismutases (SODs), which convert superoxide radicals to molecular oxygen and hydrogen peroxide, has been termed the most important discovery of modern biology never to win a Nobel Prize. Here, we review the reasons this discovery has been underappreciated, as well as discuss the robust results supporting its premier biological importance and utility for current research. We highlight our understanding of SOD function gained through structural biology analyses, which reveal important hydrogen-bonding schemes and metal-binding motifs. These structural features create remarkable enzymes that promote catalysis at faster than diffusion-limited rates by using electrostatic guidance. These architectures additionally alter the redox potential of the active site metal center to a range suitable for the superoxide disproportionation reaction and protect against inhibition of catalysis by molecules such as phosphate. SOD structures may also control their enzymatic activity through product inhibition; manipulation of these product inhibition levels has the potential to generate therapeutic forms of SOD. Markedly, structural destabilization of the SOD architecture can lead to disease, as mutations in Cu,ZnSOD may result in familial amyotrophic lateral sclerosis, a relatively common, rapidly progressing and fatal neurodegenerative disorder. We describe our current understanding of how these Cu,ZnSOD mutations may lead to aggregation/fibril formation, as a detailed understanding of these mechanisms provides new avenues for the development of therapeutics against this so far untreatable neurodegenerative pathology.

15N-NMR characterization of His residues in and around the active site of FeSOD

We have exploited (15)N-NMR to observe histidine (His) side chains in and around the active site of Fe-containing superoxide dismutase (FeSOD). In the oxidized state, we observe all the non-ligand His side chains and in the reduced state we can account for all the signals in the imidazole spectral region in terms of the non-ligand His', paramagnetically displaced signals from two backbone amides, and the side chain of glutamine 69 (Gln69). We also observe signals from the His' that ligate Fe(II). These confirm that neither the Q69H nor the Q69E mutation strongly affects the Fe(II) electronic structure, despite the 250 mV and >660 mV increases in E(m) they produce, respectively. In the Q69H mutant, we observe two new signals attributable to the His introduced into the active site in place of Gln69. One corresponds to a protonated N and the other is strongly paramagnetically shifted, to 500 ppm. The strong paramagnetic effects support the existence of an H-bond between His69 and the solvent molecule coordinated to Fe(II), as proposed based on crystallography. Based on previous information that His69 is neutral, we infer that the shifted N is not protonated. Therefore, we propose that this N represents a site of H-bond acceptance from coordinated solvent, representing a reversal of the polarity of this H-bond from that in WT (wild-type) FeSOD protein. We also present evidence that substrate analogs bind to Fe(II)SOD outside the Fe(II) coordination sphere, affecting Gln69 but without direct involvement of His30.

Superoxide dismutases-a review of the metal-associated mechanistic variations

Superoxide dismutases are enzymes that function to catalytically convert superoxide radical to oxygen and hydrogen peroxide. These enzymes carry out catalysis at near diffusion controlled rate constants via a general mechanism that involves the sequential reduction and oxidation of the metal center, with the concomitant oxidation and reduction of superoxide radicals. That the catalytically active metal can be copper, iron, manganese or, recently, nickel is one of the fascinating features of this class of enzymes. In this review, we describe these enzymes in terms of the details of their catalytic properties, with an emphasis on the mechanistic differences between the enzymes. The focus here will be concentrated mainly on two of these enzymes, copper, zinc superoxide dismutase and manganese superoxide dismutase, and some relatively subtle variations in the mechanisms by which they function.

Contribution of human manganese superoxide dismutase tyrosine 34 to structure and catalysis

Superoxide dismutase (SOD) enzymes are critical in controlling levels of reactive oxygen species (ROS) that are linked to aging, cancer, and neurodegenerative disease. Superoxide (O(2)(*-)) produced during respiration is removed by the product of the SOD2 gene, the homotetrameric manganese superoxide dismutase (MnSOD). Here, we examine the structural and catalytic roles of the highly conserved active-site residue Tyr34, based upon structure-function studies of MnSOD enzymes with mutations at this site. Substitution of Tyr34 with five different amino acids retained the active-site protein structure and assembly but caused a substantial decrease in the catalytic rate constant for the reduction of superoxide. The rate constant for formation of the product inhibition complex also decreases but to a much lesser extent, resulting in a net increase in the level of product inhibited form of the mutant enzymes. Comparisons of crystal structures and catalytic rates also suggest that one mutation, Y34V, interrupts the hydrogen-bonded network, which is associated with a rapid dissociation of the product-inhibited complex. Notably, with three of the Tyr34 mutants, we also observe an intermediate in catalysis, which has not been reported previously. Thus, these mutants establish a means of trapping a catalytic intermediate that promises to help elucidate the mechanism of catalysis.

Role of conserved tyrosine residues in NiSOD catalysis: a case of convergent evolution

Superoxide dismutases rely on protein structural elements to adjust the redox potential of the metallocenter to an optimum value near 300 mV (vs NHE), to provide a source of protons for catalysis, and to control the access of anions to the active site. These aspects of the catalytic mechanism are examined herein for recombinant preparations of the nickel-dependent SOD (NiSOD) from Streptomyces coelicolor and for a series of mutants that affect a key tyrosine residue, Tyr9 (Y9F-, Y62F-, Y9F/Y62F-, and D3A-NiSOD). Structural aspects of the nickel sites are examined by a combination of EPR and X-ray absorption spectroscopies, and by single-crystal X-ray diffraction at approximately 1.9 A resolution in the case of Y9F- and D3A-NiSODs. The functional effects of the mutations are examined by kinetic studies employing pulse radiolytic generation of O2- and by redox titrations. These studies reveal that although the structure of the nickel center in NiSOD is unique, the ligand environment is designed to optimize the redox potential at 290 mV and results in the oxidation of 50% of the nickel centers in the oxidized hexamer. Kinetic investigations show that all of the mutant proteins have considerable activity. In the case of Y9F-NiSOD, the enzyme exhibits saturation behavior that is not observed in wild-type (WT) NiSOD and suggests that release of peroxide is inhibited. The crystal structure of Y9F-NiSOD reveals an anion binding site that is occupied by either Cl- or Br- and is located close to but not within bonding distance of the nickel center. The structure of D3A-NiSOD reveals that in addition to affecting the interaction between subunits, this mutation repositions Tyr9 and leads to altered chemistry with peroxide. Comparisons with Mn(SOD) and Fe(SOD) reveal that although different strategies for adjusting the redox potential and supply of protons are employed, NiSOD has evolved a similar strategy for controlling the access of anions to the active site.

Combined NMR and computational study for azide binding to human manganese superoxide dismutase

Human manganese superoxide dismutase (MnSOD) labeled with 3-fluorotyrosine (Tyf) was complexed with the (15)N-labeled inhibitor azide ([(15)N(3)(-)]). The sample was characterized by solid-state NMR (SSNMR) spectroscopy ((19)F-MAS and (15)N-CPMAS). Employing (19)F-(15)N-REDOR spectroscopy, we determined the distances between the fluorine label in Tyrosine-34 and the three (15)N-nuclei of the azide and the relative orientation of the azide in the binding pocket of the MnSOD. A distance of R(1)=4.85A between the (19)F-label of Tyf34 and the nearest (15)N of the azide and an azide-fluorotyrosine Tyf34 angle of 90 degrees were determined. These geometry data are employed as input for molecular modeling of the location of the inhibitor in the active site of the enzyme. In the computations, several possible binding geometries of the azide near the Mn-complex were assumed. Only when the azide replaces the water ligand at the Mn-complex we obtained a geometry of the azide-Mn-complex, which is consistent with the present NMR data. This indicates that the water molecule ligating to the Mn-complex is removed and the azide is placed at this position. As a consequence the azide forms an H bond with Gln143 instead with Tyf34, in contrast to non-(19)F-labeled MnSOD, where the azide is hydrogen bonded to the hydroxy group of Tyr34.

Redox tuning over almost 1 V in a structurally conserved active site: lessons from Fe-containing superoxide dismutase

Metalloenzymes catalyze some of the most demanding reactions in biochemistry, thereby enabling organisms to extract energy from redox reactions and utilize inorganic starting materials such as N 2 and CH 4. Bound metal ions bring to enzymes greater chemical versatility and reactivity than would be possible from amino acids alone. However the host proteins must control this broad reactivity, activating the metal for the intended reaction while excluding the rest of its chemical repertoire. To this end, metalloproteins must control the metal ion reduction midpoint potential ( E m), because the E m determines what redox reactions are possible. We have documented potent redox tuning in Fe- and Mn-containing superoxide dismutases (FeSODs and MnSODs), and manipulated it to generate FeSOD variants with E ms spanning 900 mV (21 kcal/mol or 87 kJ/mol) with retention of overall structure. This achievement demonstrates possibilities and strategies with great promise for efforts to design or modify catalytic metal sites. FeSODs and MnSODs oxidize and reduce superoxide in alternating reactions that are coupled to proton transfer, wherein the metal site is believed to cycle between M3+ x OH- and M2+ x OH2 (M = Fe or Mn). Thus the E m reflects the ease both of reducing the metal ion and of protonating the coordinated solvent molecule. Moreover similar E ms are achieved by Fe-specific and Mn-specific SODs despite the very different intrinsic E(m)s of high-spin Fe3+/2+ and Mn3+/2+. We provide evidence that E(m) depression by some 300 mV can be achieved via a key enforced H-bond that appears able to disfavor proton acquisition by coordinated solvent. Based on 15N-nuclear magnetic resonance (NMR), stronger H-bond donation to coordinated solvent can explain the greater redox depression achieved by the Mn-specific SOD protein compared with the Fe-specific protein. Furthermore, by manipulating the strength and polarity of this one H-bond, with comparatively minor perturbation to active site atomic and electronic structure, we succeeded in raising the E m of FeSOD by more than 660 mV, apparently by a combination of promoting protonation of coordinated solvent and providing an energetically favorable source of a redox-coupled proton. These studies have combined the use of electron paramagnetic resonance (EPR), NMR, magnetic circular dichroism (MCD), and optical spectrophotometry to characterize the electronic structures of the various metal sites, with complementary density functional theoretical (DFT) calculations, NMR spectroscopy, and X-ray crystallography to define the protein structures and protonation states. Overall, we have generated structurally homologous Fe sites that span some 900 mV, and have demonstrated the enormous redox tuning accessible via the energies associated with proton transfer coupled to electron transfer. In this regard, we note the possible significance of coordinated solvent molecules in numerous biological redox-active metal sites besides that of SOD.

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.

Accurate metal-site structures in proteins obtained by combining experimental data and quantum chemistry

The use of molecular mechanics calculations to supplement experimental data in standard X-ray crystallography and NMR refinements is discussed and it is shown that structures can be locally improved by the use of quantum chemical calculations. Such calculations can also be used to interpret the structures, e.g. to decide the protonation state of metal-bound ligands. They have shown that metal sites in crystal structures are frequently photoreduced or disordered, which makes the interpretation of the structures hard. Similar methods can be used for EXAFS refinements to obtain a full atomic structure, rather than a set of metal-ligand distances.

The reaction mechanism of iron and manganese superoxide dismutases studied by theoretical calculations

We have studied the detailed reaction mechanism of iron and manganese superoxide dismutase with density functional calculations on realistic active-site models, with large basis sets and including solvation, zero-point, and thermal effects. The results indicate that the conversion of O2- to O2 follows an associative mechanism, with O2- directly binding to the metal, followed by the protonation of the metal-bound hydroxide ion, and the dissociation of 3O2. All these reaction steps are exergonic. Likewise, we suggest that the conversion of O2- to H2O2 follows an at least a partly second-sphere pathway. There are small differences in the preferred oxidation and spin states, as well as in the geometries, of Fe and Mn, but these differences have little influence on the energetics, and therefore on the reaction mechanism of the two types of superoxide dismutases. For example, the two metals have very similar reduction potentials in the active-site models, although they differ by 0.7 V in water solution. The reaction mechanisms and spin states seem to have been designed to avoid spin conversions or to facilitate them by employing nearly degenerate spin states.

Structure, redox, pK a, spin. A golden tetrad for understanding metalloenzyme energetics and reaction pathways

After a review of the current status of density functional theory (DFT) for spin-polarized and spin-coupled systems, we focus on the resting states and intermediates of redox-active metalloenzymes and electron transfer proteins, showing how comparisons of DFT-calculated spectroscopic parameters with experiment and evaluation of related energies and geometries provide important information. The topics we examine include (1) models for the active-site structure of methane monooxygenase intermediate Q and ribonucleotide reductase intermediate X; (2) the coupling of electron transfer to proton transfer in manganese superoxide dismutase, with implications for reaction kinetics; (3) redox, pK(a), and electronic structure issues in the Rieske iron-sulfur protein, including their connection to coupled electron/proton transfer, and an analysis of how partial electron delocalization strongly alters the electron paramagnetic resonance spectrum; (4) the connection between protein-induced structural distortion and the electronic structure of oxidized high-potential 4Fe4S proteins with implications for cluster reactivity; (5) an analysis of cluster assembly and central-atom insertion into the FeMo cofactor center of nitrogenase based on DFT structural and redox potential calculations.

Assignment strategy for fast relaxing signals: complete aminoacid identification in thulium substituted calbindin D 9K

Paramagnetic proteins generally contain regions with diverse relaxation properties. Nuclei in regions far from the metal center may behave like those in diamagnetic proteins, but those closer to the metal experience rapid relaxation with accompanying line broadening. We have used a set of NMR experiments optimized to capture data from these various concentric regions in assigning the signals from a paramagnetic Calbindin D 9K derivative in which one of the two calcium ions has been replaced by thulium(III). Normal double- and triple-resonance experiments with 1H detection were used in collecting data from nuclei in the diamagnetic-like region; these approaches identified signals from fewer than 50% of the amino acid residues (those with d > 17.5 A from thulium(III)). Paramagnetism-optimized two-dimensional NMR experiments with 1H detection were used in collecting data from nuclei in the next nearer region (d > 15 A). Standard (d > 14 A) and optimized (d > 9 A) 13C direct-detection experiments were used to capture data from nuclei in the next layer. Finally nuclei closest to the metal were detected by one-dimensional 13C (d > 5 A) and one-dimensional 15N data collection (d > 4.2 A). NMR signals were assigned on the basis of through-bond correlations and, for signals closest to the metal, pseudocontact shifts. The latter were determined from chemical shift differences between assigned signals in thulium(III) and lanthanum(III) derivatives of Calbindin D 9K and they were interpreted on the basis of a structural model for the lanthanide-substituted protein. This approach yielded assignments of at least one resonance per amino acid residue, including those in the thulium(III) coordination sphere.

Crystal structure of nitrated human manganese superoxide dismutase: mechanism of inactivation

A cellular consequence of the reaction of superoxide and nitric oxide is enhanced peroxynitrite levels. Reaction of peroxynitrite with manganese superoxide dismutase (MnSOD) causes nitration of the active-site residue Tyr34 and nearly complete inhibition of catalysis. We report the crystal structures at 2.4 A resolution of human MnSOD nitrated by peroxynitrite and the unmodified MnSOD. A comparison of these structures showed no significant conformational changes of active-site residues or solvent displacement. The side chain of 3-nitrotyrosine 34 had a single conformation that extended toward the manganese with O1 of the nitro group within hydrogen-bonding distance (3.1 A) of Nepsilon2 of the second-shell ligand Gln143. Also, nitration of Tyr34 caused a weakening, as evidenced by the lengthening, of a hydrogen bond between its phenolic OH and Gln143, part of an extensive hydrogen-bond network in the active site. Inhibition of catalysis can be attributed to a steric effect of 3-nitrotyrosine 34 that impedes substrate access and binding, and alteration of the hydrogen-bond network that supports proton transfer in catalysis. It is also possible that an electrostatic effect of the nitro group has altered the finely tuned redox potential necessary for efficient catalysis, although the redox potential of nitrated MnSOD has not been measured.