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At least 19 recordsLinked to original sources
PubMed2006

Infrared spectra of M(OH)(1,2,3) (M = Mn, Fe, Co, Ni) molecules in solid argon and the character of first row transition metal hydroxide bonding.

Reactions of laser-ablated Mn, Fe, Co, and Ni atoms with H(2)O(2) and with H(2) + O(2) mixtures in excess argon give new absorptions in the O-H and M-O stretching regions, which are assigned to metal dihydroxide and trihydroxide molecules, M(OH)(2) and M(OH)(3). Isotopic substitutions (D(2)O(2), (18)O(2), (16,18)O(2), D(2)) confirmed the assignments and DFT calculations reproduced the experimental results. The O-H stretching frequencies decreased in the dihydroxides from Sc to Zn. Mulliken and natural charge distributions indicate significant electron transfer from metal d orbitals to OH ligands that decreases from Sc to Zn, suggesting that the early transition metal hydroxides are more ionic and that the later transition metal hydroxides are more covalent.

Journal Article↗
PubMed2026

CeOx-Induced Spatial and Electronic Modulation for General Direct Oxo Coupling in Transition Metal Hydroxides.

Electrochemical water splitting has emerged as a sustainable paradigm for hydrogen generation, where sluggish kinetics of the oxygen evolution reaction (OER) catalyzed by transition metal-based materials remain the critical bottleneck. Herein, we present a strategy that anchors CeOx nanoparticles (∼2 nm) onto two-dimensional Ni(OH)2 nanosheets, enabling dual modulation of spatial configuration and electronic states to accelerate O-O coupling. Spatially, interfacial lattice distortion between CeOx and Ni(OH)2 optimizes Ni-Ni dual-metal sites with reduced interatomic spacing. Electronically, dynamic modulation through reversible Ce3+/Ce4+ redox cycling positions Ce as an electronic regulation hub, stabilizing Ni species at the catalytically favorable +3 oxidation state through Ce─O─Ni interactions. This synergistic effect shifts the pathway from adsorbate evolution mechanism (AEM) to oxide pathway mechanism (OPM). The prepared CeOx@Ni(OH)2 achieves an overpotential of 152 mV at 10 mA cm-2 and operates continuously over 2000 h with limited performance decay. When integrated into an alkaline anion exchange membrane water electrolyzer (AEMWE), it requires 1.91 V to attain 1 A cm-2 and maintains stable operation for 450 h. This OPM activation strategy shows potential applicability across CeOx-loaded transition metal hydroxides, including Ni(OH)2, Co(OH)2, NiCo, and NiFe layered double hydroxides, offering a promising approach for alkaline OER enhancement.

alkaline water oxidation↗
PubMed2007

General synthesis and delamination of highly crystalline transition-metal-bearing layered double hydroxides.

In this paper, we describe a general process for the synthesis and delamination of a family of highly crystalline Al-based and transition-metal-bearing layered double hydroxides (LDHs). Large-sized and monodispersed hexagonal platelike particles of binary-component M(II)-Al-CO3 and ternary-component M(II)-M'(II)-Al-CO3 LDHs (M(II) and M'(II) = Fe, Co, Ni, or Zn) were prepared at first by the urea method under optimized conditions. These CO32--LDHs were converted into other monovalent-anion-containing LDHs by means of a salt-acid treatment and subsequent anion-exchange processes. Finally, well-defined LDH nanosheets were obtained by delamination of NO3--LDHs in formamide.

Journal Article↗
PubMed2001

Interactions of metal ions with water: ab initio molecular orbital studies of structure, vibrational frequencies, charge distributions, bonding enthalpies, and deprotonation enthalpies. 2. Monohydroxides.

The formation and properties of a wide range of metal ion monohydroxides, M(n)(+)[OH(-)], where n = 1 and 2, have been studied by ab initio molecular orbital calculations at the MP2(FULL)/6-311++G**//MP2(FULL)/6-311++G** and CCSD(T)(FULL)/6-311++G**//MP2(FULL)/6-311++G** computational levels. The ions M(n)()(+) are from groups 1A, 2A, 3A, and 4A in the second, third, and fourth periods of the Periodic Table and from the first transition series. Geometrical parameters, vibrational frequencies, atomic charge distributions, orbital occupancies, and bonding enthalpies are reported. The M(n)(+)-O distances are shorter in the hydroxides than in the corresponding hydrates (published previously as Part 1, Inorg. Chem. 1998, 37, 4421-4431) due to a greater electrostatic interaction in the hydroxides. The natural bond orbitals for most of the first-row transition metal ion hydroxides do not contain a formal metal-oxygen bonding orbital; nevertheless the atomic charge distributions show that for both n = 1 and 2 a significant amount of electron density is consistently transferred from the hydroxide ion to the bound metal ion. Deprotonation enthalpies for the hydrates have been evaluated according to the simple dissociation process, M(n)(+)[OH(2)] --> M(n)(+)[OH(-)] + H(+), and also via proton transfer to another water molecule, M(n)(+)[OH(2)] + H(2)O --> M(n)(+)[OH(-)] + H(3)O(+). The drastic reduction in these deprotonation enthalpies as H(2)O molecules are sequentially bonded in the first coordination shell of the metal ion (amounting to 71, 64, 85, and 91 kcal/mol for the bonding of six water molecules to Mg(2+), Ca(2+), Mn(2+), and Zn(2+), respectively) is found to be due to the greater decrease in the bonding enthalpies for the hydroxides relative to the hydrates. Proton transfer to bases other than water, for example side chain groups of certain amino acids, could more than offset the decrease in deprotonation energy due to the filling of the first coordination shell. Linear relationships have been found between the pK(a) values for ionization of the Mg(2+), Ca(2+), Mn(2+), Fe(2+), Co(2+), Ni(2+), Cu(2+), and Zn(2+) aquo ions, and Delta for the bonding of the first water molecule, for the bonding of the hydroxide ion, and for proton dissociation from the monohydrate. Similar relationships have also been found between the pK(a) values and the reciprocal of the M-O bond lengths in both the monohydrates and hydroxides. Thus the ionization of metal hydrates in water echoes the properties of the monomeric species M(n)(+)[OH(2)].

Metals↗
PubMed1999

Arginase-boronic acid complex highlights a physiological role in erectile function.

The crystal structure of the complex between the binuclear manganese metalloenzyme arginase and the boronic acid analog of L-arginine, 2(S)-amino-6-boronohexanoic acid (ABH), has been determined at 1.7 A resolution from a crystal perfectly twinned by hemihedry. ABH binds as the tetrahedral boronate anion, with one hydroxyl oxygen symmetrically bridging the binuclear manganese cluster and a second hydroxyl oxygen coordinating to Mn2+A. This binding mode mimics the transition state of a metal-activated hydroxide mechanism. This transition state structure differs from that occurring in NO biosynthesis, thereby explaining why ABH does not inhibit NO synthase. We also show that arginase activity is present in the penis. Accordingly, the tight binding and specificity of ABH allows us to probe the physiological role of arginase in modulating the NO-dependent smooth muscle relaxation required for erection. Strikingly, ABH causes significant enhancement of nonadrenergic, noncholinergic nerve-mediated relaxation of penile corpus cavernosum smooth muscle, suggesting that arginase inhibition sustains L-arginine concentrations for NO synthase activity. Therefore, human penile arginase is a potential target for therapeutic intervention in the treatment of erectile dysfunction.

Aminocaproates↗
PubMed2003

Solid-state chelation of metal ions by ethylenediaminetetraacetate intercalated in a layered double hydroxide.

The solid-state chelation of transition metal ions (Co(2+), Ni(2+), and Cu(2+)) from aqueous solutions into the lithium aluminum layered double hydroxide ([LiAl(2)(OH)(6)]Cl x 0.5H(2)O or LDH) which has been pre-intercalated with EDTA (ethylenediaminetetraacetate) ligand has been investigated. The intercalated metal cations form [M(edta)](2)(-) complexes between the LDH layers as indicated by elemental analysis, powder X-ray diffraction, and IR and UV-vis spectroscopies. If metal chloride or nitrate salts are used in the reaction with the LDH then co-intercalation of either the Cl(-) or NO(3)(-) anions is observed. In the case of metal acetate salts the cations intercalate without the accompanying anion. This can be explained by the different intercalation selectivity of the anions in relation to the LDH. In the latter case the introduction of the positive charge into LDH structure was compensated for by the release from the solid of the equivalent quantity of lithium and hydrogen cations. Time-resolved in-situ X-ray diffraction measurements have revealed that the chelation/intercalation reactions proceed very quickly. The rate of the reaction found for nickel acetate depends on concentration as approximately k[Ni(Ac)(2)](3).

Journal Article↗
PubMed2001

Retention behavior of transition metals on a bifunctional ion-exchange column with oxalic acid as eluent.

The common eluents used with a bifunctional ion-exchange column (IonPac CS5A) for separating transition metals are pyridine-2,6-dicarboxylic acid and oxalic acid (Ox). When Ox is used, cadmium and manganese co-elute. Although much research has been done to overcome the Cd2+-Mn2+ co-elution problem, the role of lithium hydroxide in separating the transition metals has received little attention. In this study, it is found that when the Ox concentration is higher than 35 mM, Cu2+ elutes after Pb2+ and Ox plays a predominant role in the retention behavior of the seven metals. When Ox concentration is lower than 35 mM especially when its concentration (25 mM) is half of the usually used standard concentration (50 mM), Cu2+ elutes before Pb2+, and at the same time, Mn2+ and Cd2+ can also be baseline separated. Lithium hydroxide plays a predominant role in the separation of the metals separated by cation exchange. So, lithium hydroxide is used to adjust the pH of the eluent. The use of an isocratic elution (25 mM Ox/LiOH/2 mM Na2SO4, pH 3.88) allows the separation of seven metals (Cu2+, Pb2+, Co2+, Mn2+, Cd2+, Zn2+ and Ni2+) in a single run. The effects of inorganic modifiers such as NaNO3, Na2SO4 and Na4P3O7 on retention behavior of the metals are also investigated.

Chromatography, Ion Exchange↗
PubMed1987

A new redox loop formality involving metal-catalysed hydroxide-ion translocation. A hypothetical Cu loop mechanism for cytochrome oxidase.

A new hypothetical type of redox loop is described, which translocates hydroxide instead of protons. Conventional protonmotive redox loops use carriers of protons with electrons (e.g. QH2/Q systems) to couple electron transfer to the translocation of protons. The putative hydroxidemotive redox loop uses carriers of hydroxide ions against electrons (e.g. transition-metal centres) to couple electron transfer to the translocation of hydroxide ions. This simple idea leads to the proposal of a hydroxidemotive Cu loop mechanism that may possibly be applicable to the CuA or CuB centre of cytochrome oxidase, and might thus account for the coupling of electron transfer to net proton translocation in that osmoenzyme.

Copper↗
PubMed2006

Reactivity of lanthanocene hydroxides toward ketene, isocyanate, lanthanocene alkyl, and triscyclopentadienyllanthanide complexes.

The reactivity of [Cp(2)Ln(mu-OH)(THF)]2 (Ln = Y (1), Er (2), Yb (3)) toward PhEtCCO, PhNCO, Cp3Ln, [Cp2Ln(mu-CH3)]2, and the LiCl adduct of Cp2Ln(n)Bu(THF)x was examined. In all cases, OH-centered reactivity is observed: complexes 1-3 react with PhEtCCO to form the O-H addition products [Cp2Ln(mu-eta1:eta2-O2CCHEtPh)]2 (Ln = Yb (5), Er (6), Y (7), respectively, for 1-3), whereas treatment of 1 with PhNCO affords the addition/CpH-elimination/rearrangement product [{Cp2Y(THF)}2(mu-eta2:eta2-O2CNPh)] (8), which contains an unusual PhNCO(2) dianionic ligand. Analogous compound [Cp2Ln(THF)]2(mu-eta2:eta2-O2CNPh) (Ln = Yb (9), Er (10)) and 8 can be obtained in a higher yield by treatment of [Cp2Ln(mu-OH)(THF)]2 with PhNCO followed by reaction with the corresponding Cp3Ln. However, attempts to prepare the corresponding heterobimetallic complex by reacting stoichiometric amounts of [Cp2Y(mu-OH)(THF)]2 with PhNCO followed by treating it with Cp3Yb are unsuccessful. Instead, only rearrangement products 8 and 9 are obtained. Furthermore, the reaction of 3 with [Cp2Yb(mu-CH3)]2 or Cp3Yb forms oxo-bridged compound [Cp2Yb(THF)]2(mu-O) (11), whereas the reaction of [Cp2ErCl]2 with Li(n)Bu followed by treatment with 2 affords unexpected mu-oxo lanthanocene cluster (Cp2Er)3(mu-OH)(mu3-O)(mu-Cl)Li(THF)4 (12). In contrast to 1 and 2, 3 shows a strong tendency to undergo the intermolecular elimination of CpH at room temperature, giving trinuclear species [Cp2Yb(mu-OH)]2[CpYb(THF)](mu3-O) (4). The single-crystal X-ray diffraction structures of 1, 2, and 4-12 are described. All the results offer an interesting contrast to transition- and main-metal hydroxide complexes.

Journal Article↗
PubMed1996

Binding Patterns in Single-Ligand Complexes of NH(3), H(2)O, OH(-), and F(-) with First Series Transition Metals.

Single-ligand complexes of first series transition metals with ammonia, water, hydroxide, and fluoride, many known in the gas phase, have been studied in calculations covering the 20 mono- and divalent cations and some very unusual binding patterns have been found. Binding energies and binding geometries were calculated at MP2 level, using a basis with a (6d/4d) contraction in the metal d space and 6-311+G sets for the ligands. The results were used to distinguish the effect of steadily increasing nuclear charge across the series from the varying effects of d shell occupation. Even with only one ligand, the M(2+) adducts displayed the familiar ligand field effects, d shell repulsion in the expected d(delta) < d(pi) < d(sigma) order being superimposed on a regular progression to stronger binding and shorter bonds; that progression was disturbed only at the d(5) and d(10) positions, when the d(sigma) orbital was occupied. Monovalent metal adducts behaved in strikingly different fashion, with irregular changes across early and late series metals in both bond length and bond strength. The irregularities are only partly attributable to the presence of both d(n)()(-)(1)s and d(n)() ground states in the series. The other part of the explanation is the binding of anionic ligands inside the radial maximum of the 4s orbital. At these distances the normal binding preference shown by H(2)O and NH(3) for d(n)() over sd(n)()(-)(1) cations is reversed. In contrast to steeply rising binding energies across the divalent metal ion adducts, the trend lines for the monovalent series are flat, the increments in nuclear charge being insufficent to offset the extra repulsion of electrons added to the d shell.

Journal Article↗
PubMed2006

Protein-bound 3,4-dihydroxy-phenylanine (DOPA), a redox-active product of protein oxidation, as a trigger for antioxidant defences.

Protein hydroperoxides and protein-bound 3,4-dihydroxy-phenylanine are amongst the major long-lived redox-active products during free radical attack on proteins. Protein-bound 3,4-dihydroxy-phenylanine can redox cycle between catechol and quinone form, and bind transition metals, whereas hydroperoxides are converted to stable hydroxides. The free amino acid 3,4-dihydroxy-phenylanine is a normal metabolite, an oxidation product of tyrosine, involved in pathways of dopamine and melanin production, and we have shown that it may be incorporated into protein-by-protein synthesis. However, physiological levels of protein-bound 3,4-dihydroxy-phenylanine are very low; yet remarkably elevated levels occur in some pathologies. We propose that, unlike free 3,4-dihydroxy-phenylanine, protein-bound 3,4-dihydroxy-phenylanine is a signal for the activation of cellular defences both against the oxidative fluxes during oxidative stress and against the oxidative damage which sometimes ensues. Unlike free 3,4-dihydroxy-phenylanine, the levels of protein-bound 3,4-dihydroxy-phenylanine can change 5-10-fold during oxidative damage in vivo, an appropriate property for a signalling molecule. We suggest mechanisms by which protein-bound 3,4-dihydroxy-phenylanine might trigger oxidative defences, via NF-kappaB and other transcription factors. Little evidence yet bears directly on this, but we discuss some implications of observations on free 3,4-dihydroxy-phenylanine supply to cells in vitro, to Parkinson's patients, and to animal models of the disease. Several of the effects of 3,4-dihydroxy-phenylanine in these situations may be mediated by the production and actions of protein-bound 3,4-dihydroxy-phenylanine. Some experimental tests of the hypothesis are outlined and some possible therapeutic implications.

Animals↗
PubMed2002

Activation of acyl phosphate monoesters by lanthanide ions: enhanced reactivity of benzoyl methyl phosphate.

Acyl phosphate monoesters are intermediates in many biochemical acylation reactions, such as those involving aminoacyl adenylates. Benzoyl methyl phosphate, a typical acyl phosphate monoester, is slowly hydrolyzed in neutral solutions but reacts rapidly with amines. Since biochemical processes of acyl phosphate monoesters involve accelerated reactions with oxygen-centered nucleophiles, we sought catalysts for hydrolysis and methanolysis of benzoyl methyl phosphate to mimic the biochemical outcome. Lanthanide ions are particularly effective catalysts, accelerating reactions much more than comparable levels of magnesium ion. Detailed kinetic analysis of the hydrolysis reactions reveals formation of a 1:1 complex, followed by rapid reaction with a nucleophile. The hydroxide-dependent hydrolysis rate in the europium complex is about 10(5) times that of free substrate with hydroxide. A mechanism that accounts for the data and observed behavior involves bidentate coordination of the metal ion by the acyl phosphate through phosphate and carbonyl oxygens, lowering the energy of the tetrahedral addition intermediate and the associated transition states. The dependence of the metal ion catalyzed process on the concentration of hydroxide ion is consistent with coordinated hydroxide acting as a nucleophile. The reaction of benzoyl methyl phosphate with methanol to form methyl benzoate and methyl phosphate is 30 000 times more rapid in the presence of 0.0001 M lanthanum triflate (in the absence of the metal ion k(obs) = 2.1 x 10(-7) s(-1), at 25 degrees C). Thus, the combination of acyl phosphate esters and lanthanide salts appears to be a promising method for biomimetic acylation of hydroxyl groups.

Catalysis↗
PubMed2006

Vanadium recovery from oil fly ash by leaching, precipitation and solvent extraction processes.

In order to reduce the environmental impact due to land disposal of oil fly ash from power plants and to valorize this waste material, the removal of vanadium was investigated using leaching processes (acidic and alkaline treatments), followed by a second step of metal recovery from leachates involving either solvent extraction or selective precipitation. Despite a lower leaching efficiency (compared to sulfuric acid), sodium hydroxide was selected for vanadium leaching since it is more selective for vanadium (versus other transition metals). Precipitation was preferred to solvent extraction for the second step in the treatment since: (a) it is more selective; enabling complete recovery of vanadate from the leachate in the form of pure ammonium vanadate; and (b) stripping of the loaded organic phase (in the solvent extraction process) was not efficient. Precipitation was performed in a two-step procedure: (a) aluminum was first precipitated at pH 8; (b) then ammonium chloride was added at pH 5 to bring about vanadium precipitation.

Aluminum↗
PubMed2002

Reactivity of 3d transition metal cations in diethylene glycol solutions. Synthesis of transition metal ferrites with the structure of discrete nanoparticles complexed with long-chain carboxylate anions.

Study of the reactivity of 3d transition metal cations in diethylene glycol solutions revealed several key features that made it possible to develop a new method for synthesis of the nanocrystalline transition metal ferrites. The 3-7 nm particles of [MFe2O4]n[O2CR]m, where M = Mn, Fe, Co, Ni, and Zn, ligated on their surface with long-chain carboxylate anions, have been obtained in an isolated yield of 75-90%. The key features are the following. Complexation of the first-row transition metal cations with diethylene glycol at a presence of alkaline hydroxide is sufficient to enable control over the rate of their hydrolysis. The reaction of hydrolysis leads to the formation of metal oxide nanocrystals in colloidal solution. The nanoparticles growth is terminated by an added long-chain carboxylic acid, which binds to their surface and acts as a capping ligand. The isolated nanocrystalline powders are stable against agglomeration and highly soluble in nonpolar organic solvents.

Journal Article↗
PubMed2001

Separation of critical radioactive and non-radioactive species from aqueous waste streams.

The separation of radioactive and non-radioactive species from the simulated DOE neutralized current acid waste (NCAW) stream was studied. Cation and anion species were referred to their possible basic compounds, and divided into seven groups (nitrate, phosphate, sulfate, fluoride, nitrite, carbonate, and hydroxide). The nitrate group (the major anion in the DOE waste streams) contains several cations species, while the rest of the groups are only in the form of sodium. The precipitation measurements were conducted in three experimental stages. In the first stage, the precipitation of sodium sulfate, sodium phosphate, sodium-sulfate-phosphate, and aluminum nitrate systems were studied using isopropylamine (IPA) as a precipitation solvent. The objectives of this stage were to evaluate the precipitation ability of IPA in precipitating these compounds individually, and to validate the consistency of the analytical instruments and the employed experimental procedure. Tests performed on the acquired data indicated a high level of experimental consistency. The removal of phosphate, sulfate and aluminum were very high. In the second stage, the precipitation studies were conducted on the: (1) nitrate group alone; (2) binary groups containing the groups of nitrate-phosphate, nitrate-sulfate, nitrate-fluoride, nitrate-nitrite, and nitrate-carbonate; (3) combined nitrate, phosphate, sulfate, and fluoride groups and (4) combined nitrate, phosphate, sulfate, fluoride, nitrite, and carbonate groups. IPA was used as a precipitation solvent. The objectives of this stage were to evaluate the interactions of these groups in the absence of the hydroxide group (e.g. DOE acid-dissolved sludge and acidified supernate streams), and the influence of such interactions on the individual removal of the targeted species. The removal of the aluminum, phosphate, fluoride, and alkaline cations were significantly high (reached 99.9%). The removal of sulfate were moderately high (reached 87%), and the removal of nitrate and alkali cations including cesium were to some extent low (reached about 50%). In the third stage, the precipitation of inorganic species from the simulated NCAW stream was studied using IPA and ethylamine (EA). The precipitation process is very feasible for reducing the radioactivity contents of alkaline cations. However, the process is less effective in separating alkali cations including cesium. The removal of polyvalent transition metals such as aluminum ion is negatively influenced by the significant presence of hydroxide. While the process is effectively capable of separating phosphate, fluoride, and sulfate, it is significantly less effective in separating nitrate and nitrite. A previously derived thermodynamics framework was used to model the precipitation measurements. The framework provided two predictive equations (the 2-Suffix and 3-Suffix equations). Both equations were reasonably adequate for predicting the solubility phase behavior of tested inorganic species in a mixed-solvents mixture as well as for estimating optimum interaction parameters. However, the 3-Suffix equation was better than the 2-Suffix equation. The parameters were useful for estimating the: (1) precipitation fractions (%P) of the studied species, for instance, at different concentration levels of similar targeted species, or in different waste streams with similar or approximate abundance of species, or at different solvents volume ratio (V(r)) where no experimental data are available.

Chemical Precipitation↗
PubMed1991

Structural basis for the 3'-5' exonuclease activity of Escherichia coli DNA polymerase I: a two metal ion mechanism.

The refined crystal structures of the large proteolytic fragment (Klenow fragment) of Escherichia coli DNA polymerase I and its complexes with a deoxynucleoside monophosphate product and a single-stranded DNA substrate offer a detailed picture of an editing 3'-5' exonuclease active site. The structures of these complexes have been refined to R-factors of 0.18 and 0.19 at 2.6 and 3.1 A resolution respectively. The complex with a thymidine tetranucleotide complex shows numerous hydrophobic and hydrogen-bonding interactions between the protein and an extended tetranucleotide that account for the ability of this enzyme to denature four nucleotides at the 3' end of duplex DNA. The structures of these complexes provide details that support and extend a proposed two metal ion mechanism for the 3'-5' editing exonuclease reaction that may be general for a large family of phosphoryltransfer enzymes. A nucleophilic attack on the phosphorous atom of the terminal nucleotide is postulated to be carried out by a hydroxide ion that is activated by one divalent metal, while the expected pentacoordinate transition state and the leaving oxyanion are stabilized by a second divalent metal ion that is 3.9 A from the first. Virtually all aspects of the pretransition state substrate complex are directly seen in the structures, and only very small changes in the positions of phosphate atoms are required to form the transition state.

Amino Acid Sequence↗
PubMed1999

Catalysis by metal-activated hydroxide in zinc and manganese metalloenzymes.

The metal-activated hydroxide ion is a critical nucleophile in metalloenzymes that catalyze hydrolysis or hydration reactions. The most common metal used is zinc; occasionally, other transition metals such as manganese are required. Human carbonic anhydrase II and rat liver arginase serve as well-studied paradigms of zinc and manganese metalloenzymes, respectively. Comparative structure-function relationships between these two metalloenzymes highlight parallels in the chemistry of metal-activated hydroxide: (a) the protein environment of metal-bound hydroxide modulates its reactivity; (b) a hydrogen bond with metal-bound hydroxide holds it in the proper orientation for catalysis; (c) nonmetal substrate-binding sites are implicated in both enzyme mechanisms; and (d) regeneration of metal-bound hydroxide ion from a metal-bound water molecule requires proton transfer to bulk solvent mediated by a histidine proton shuttle residue. Interestingly, the electrostatics of catalysis differ between the two enzymes, in that the first step of catalysis requires formation of a negatively charged transition state in the carbonic anhydrase II mechanism, whereas a neutral transition state is approached in the arginase mechanism. This electrostatic feature may contribute to the differences in the chemistry, the metal binding sites, and the metal specificity between these two enzymes.

Animals↗