113Cd NMR spectroscopy of coordination compounds and proteins

Single-Crystal, Solid-State, and Solution (113)Cd and (77)Se NMR and X-ray Single-Crystal Study of a [Cd(SeR)(2)(N-donor)(2)] Complex

[Cd(Se-2,4,6-i-Pr(3)-C(6)H(2))(2)(bpy)] (1) has been characterized by X-ray crystallography and studied by solution and solid-state (113)Cd and (77)Se NMR to serve as an analog for biologically occurring [M(S-Cys)(2)(His)(2)] centers. The unit cell parameters for 1 are as follows: a = 21.99(2) Å, b = 21.43(4) Å, c = 16.72(3) Å, V = 7881.3(4) Å(3), Z = 8, orthorhombic space group, Pccn. Two chemically inequivalent cadmiums and seleniums are found per unit cell. The principal values of the cadmium chemical shift tensors are (delta(11) = 752 ppm, delta(22) = 570 ppm, delta(33) = 93 ppm) and (delta(11)= 733 ppm, delta(22) = 547 ppm, delta(33) = 100 ppm) with respect to 0.1 M aq. Cd(ClO(4))(2). Those for selenium are (delta(11) = -950 ppm, delta(22) = -1040 ppm, delta(33) = -1599 ppm) and (delta(11) = -933 ppm, delta(22) = -1069 ppm, delta(33) = -1586 ppm) with respect to solid (NH(4))(2)SeO(4). The orientation of the cadmium chemical shift tensor is similar to that of the previously published sulfur analog.

Correlation of a Solution-State Conformational Change between Mercuric Chloride Complexes of Tris[(2-(6-methylpyridyl))methyl]amine with X-ray Crystallographic Structures

Solution-state NMR and X-ray crystallography were used to investigate the complexation of HgCl(2) by the potentially tetradentate ligand tris[(6-methyl-2-pyridyl)methyl]amine (TLA) in acetonitrile. A change in the ligand conformation as a function of the metal-to-ligand ratio could be indirectly monitored through large changes in (3)J((1)H(199)Hg) to the methylene protons at -40 degrees C. The solution-state NMR were correlated with two solid-state structures. The five-coordinate complex [Hg(TLA)Cl(2)] (1) crystallizes in the triclinic space group P&onemacr; with a = 8.663(3) Å, b = 11.539(4) Å, c = 13.739(3) Å, alpha = 80.81(2) degrees, beta = 75.84(2) degrees, gamma = 80.97(3) degrees, and Z = 2. The Hg-N(amine) distance of 2.505(7) Å for the tridentate ligand is the same as the average Hg-N(lutidyl) distance of 2.50(3) Å for the two bound lutidyl nitrogens. [Hg(TLA)Cl](2)(Hg(2)Cl(6)) (2) also crystallizes in P&onemacr; with a = 10.606(2) Å, b = 15.104(3) Å, c = 17.785(4) Å, alpha = 67.46(3) degrees, beta = 83.52(3) degrees, gamma = 80.29(3) degrees, and Z = 2. The ligand is tetradentate in the two crystallographically unique cations which are arranged in a dimer-like orientation. The average Hg-Cl distance is 2.37 (1) Å, and the average interionic Hg-Cl distance is 3.51(1) Å. The Hg- N(lutidyl) distances are of two types: two have an average distance of 2.36(3) Å, nearly the same as the Hg-N(amine) distance of 2.35(2) Å. The remaining four N(lutidyl) distances have an average distance of 2.56(5) Å.

Methylation of Tethered Thiolates in [(bme-daco)Zn](2) and [(bme-daco)Cd](2) as a Model of Zinc Sulfur-Methylation Proteins

The dimeric dithiolate complex [1,5-bis(mercaptoethyl)-1,5-diazacyclooctanato]zinc(II), [(bme-daco)Zn](2) or Zn-1, and its cadmium analogue, Cd-1, were investigated as models for the active site of zinc-dependent methylation proteins. The key issue addressed was whether alkylation of a thiolate in a relatively rigid tetradentate ligand would result in coordination of the thioether product to the metal. On the basis of (1)H and (13)C NMR spectroscopy and similar reactivity toward alkylating agents, the newly synthesized cadmium complex, Cd-1, is proposed to be isostructural with the previously reported Zn-1 complex, which is known from X-ray crystallography to be dimeric in the solid state (Tuntulani, T.; Reibenspies, J. H.; Farmer, P. J.; Darensbourg, M. Y. Inorg. Chem.1992, 31, 3497). Iodomethane reacts with Zn-1 in hot CH(3)OH/CH(3)CN to produce a thioether which dissociates, replaced by coordination of iodide in the pseudotetrahedral complex, (Me(2)bme-daco)ZnI(2) or Zn-2. Complex Zn-2 crystallizes in the triclinic P&onemacr; space group with a = 7.911(2) Å, b = 10.675(2) Å, c = 12.394(2) Å, alpha = 75.270(10) degrees, beta = 75.270(10) degrees, gamma = 82.12(2) degrees, V = 998.270 Å(3), and Z = 2. An analogous reaction was observed for the cadmium derivative, Cd-1, which displays a (1)H NMR spectrum identical to that of Zn-2. In attempts to promote thioether binding, the iodide was displaced by addition of AgBF(4) to solutions of Zn-2 or the BF(4)(-) analogue was synthesized directly from Zn(BF(4))(2) and methylated ligand, Me(2)bme-daco, to yield Zn-3. Similar reactions with the cadmium analogue yielded a product identified as Cd-3 that was indistinguishable from Zn-3 by (1)H NMR. The (113)Cd NMR spectra of Cd-3 displayed a single resonance at 88 ppm consistent with a hard donor environment and inconsistent with sulfur binding. As a further attempt to induce thioether binding to zinc, the macrocyclization reagent 1,3-dibromopropane was added to Zn-1. The resulting product, [BrZn(macrocycle)](+), was only slightly soluble in pyridine and identified by +FAB/MS as the desired macrocyclic product with a large amount of free macrocycle ligand. Recrystallization from pyridine/ether resulted in loss of the zinc as Zn(py)(2)Br(2), which was obtained as colorless crystals and characterized by X-ray crystallography. Complex Zn(py)(2)Br(2) crystallizes in the monoclinic P2(1)/c space group with a = 8.534(2) Å, b = 18.316(4) Å, c = 8.461(2) Å, beta = 101.07(3) degrees, V = 1297.9(5) Å(3), and Z = 4.

NMR determination of the global structure of the 113Cd derivative of desulforedoxin: investigation of the hydrogen bonding pattern at the metal center

Desulforedoxin (Dx) is a simple homodimeric protein isolated from Desulfovibrio gigas (Dg) containing a distorted rubredoxin-like center with one iron coordinated by four cysteinyl residues (7.9 kDa with 36 amino acids per monomer). In order to probe the geometry and the H-bonding at the active site of Dx, the protein was reconstituted with 113Cd and the solution structure determined using 2D NMR methods. The structure of this derivative was initially compared with the NMR solution structure of the Zn form (Goodfellow BJ et al., 1996, J Biol Inorg Chem 1:341-353). Backbone amide protons for G4, D5, G13, L11 NH, and the Q14 NH side-chain protons, H-bonded in the X-ray structure, were readily exchanged with solvent. Chemical shift differences observed for amide protons near the metal center confirm the H-bonding pattern seen in the X-ray model (Archer M et al., 1995, J Mol Biol 251:690-702) and also suggest that H-bond lengths may vary between the Fe, Zn, and 113Cd forms. The H-bonding pattern was further probed using a heteronuclear spin echo difference (HSED) experiment; the results confirm the presence of NH-S H-bonds inferred from D2O exchange data and observed in the NMR family of structures. The presence of "H-bond mediated" coupling in Dx indicates that the NH-S H-bonds at the metal center have significant covalent character. The HSED experiment also identified an intermonomer "through space" coupling for one of the L26 methyl groups, indicating its proximity to the 113Cd center in the opposing monomer. This is the first example of an intermonomer "through space" coupling. Initial structure calculations produced subsets of NMR families with the S of C28 pointing away from or toward the L26 methyl: only the subset with the C28 sulfur pointing toward the L26 methyl could result in a "through space" coupling. The HSED result was therefore included in the structure calculations. Comparison of the Fe, Zn, and 113Cd forms of Dx suggests that the geometry of the metal center and the global fold of the protein does not vary to any great extent, although the H-bond network varies slightly when Cd is introduced. The similarity between the H-bonding pattern seen at the metal center in Dx, Rd (including H-bonded and through space-mediated coupling), and many zinc-finger proteins suggests that these H-bonds are structurally vital for stabilization of the metal centers in these proteins.

Effect of excess cadmium ion on the metal binding site of cabbage histidinol dehydrogenase studied by 113Cd-NMR spectroscopy

The enzymatic reaction of histidinol dehydrogenase (HDH) was stimulated by about maximally 75% on the addition of Cd2+ ion to the reaction mixture. 113Cd-substituted HDH in the presence of excess Cd2+ has been studied by 113Cd-NMR. 113Cd2+ less than 1 equiv. per subunit preferentially binds to the catalytic metal binding site of the apoenzyme. Further addition of the metal ions causes the structural change of the enzyme including the catalytic metal binding site. HDH takes at least three discernible states, which may correspond to the more or less active forms of the enzyme induced by metal ions.

Zinc ejection as a new rationale for the use of cystamine and related disulfide-containing antiviral agents in the treatment of AIDS

The highly conserved and mutationally intolerant retroviral zinc finger motif of the HIV-1 nucleocapsid protein (NC) is an attractive target for drug therapy due to its participation in multiple stages of the viral replication cycle. A literature search identified cystamine, thiamine disulfide, and disulfiram as compounds that have been shown to inhibit HIV-1 replication by poorly defined mechanisms and that have electrophilic functional groups that might react with the metal-coordinating sulfur atoms of the retroviral zinc fingers and cause zinc ejection. 1H NMR studies reveal that these compounds readily eject zinc from synthetic peptides with sequences corresponding to the HIV-1 NC zinc fingers, as well as from the intact HIV-1 NC protein. In contrast, the reduced forms of disulfiram and cystamine, diethyl dithiocarbamate and cysteamine, respectively, were found to be ineffective at zinc ejection, although cysteamine formed a transient complex with the zinc fingers. Studies with HIV-1-infected human T-cells and monocyte/macrophage cultures revealed that cystamine and cysteamine possess significant antiviral properties at nontoxic concentrations, which warrant their consideration as therapeutically useful anti-HIV agents.

Syntheses, Structures, and Binding Constants of Cyclic Ether and Thioether Adducts of Soluble Cadmium(II) Carboxylates. Intermediates in the Homopolymerization of Oxiranes and Thiiranes and in Carbon Dioxide Coupling Processes

A synthetic methodology for the preparation of a large variety of eta(3)-HB(3-Phpz)(3)Cd(acetate) adducts is presented which involves replacement of toluene in the eta(3)-HB(3-Phpz)(3)Cd(acetate) solvate complex by the appropriate cyclic ether or cyclic thioether. In this manner, adducts of THF, dioxane, propylene oxide, cyclohexene oxide, and propylene sulfide were isolated. The solid-state structures of several of these complexes were determined by X-ray crystallography, revealing a six-coordinate complex where the acetate ligand is shown to be fairly symmetrically bonded to the cadmium center. In methylene chloride solution, the cyclic ether or thioether readily dissociates to afford the five-coordinate complex, as demonstrated by (113)Cd NMR. A quantitative assessment of the binding of these base adducts of eta(3)-HB(3-Phpz)(3)Cd(acetate) was determined by measuring the temperature dependence of the equilibrium constants for the five- and six-coordinate derivatives. The presence of one sharp (113)Cd resonance in this equilibrium mixture is indicative of rapid intermolecular exchange between the five- and six-coordinate complexes when compared to the chemical shift differences in these two species ( approximately 6600 Hz at 89 MHz). The order established for ether binding is THF > dioxane > propylene sulfide > cyclohexene oxide >/= propylene oxide, with DeltaH degrees and DeltaS degrees spanning the ranges -27.7 to 24.3 kJ/mol and -89.7 to -94.1 J/(mol K). The epoxide and thioepoxide adducts were shown to serve as models for the initiation step in the copolymerization of epoxides with carbon dioxide catalyzed by metal carboxylates. That is, the carboxylate ligand was shown to ring-open the epoxide or thioepoxide, subsequently affording polyethers or polythioethers with ester end groups. By way of contrast, in the presence of CO(2) and epoxides, this system led to cyclic carbonate production.

Alkyl Transfer to Metal Thiolates: Kinetics, Active Species Identification, and Relevance to the DNA Methyl Phosphotriester Repair Center of Escherichia coli Ada

The Ada protein of Escherichia coli employs a [Zn(S-cys)(4)](2)(-) site to repair deoxyribonucleic acid alkyl phosphotriester lesions. The alkyl group is transferred to a cysteine thiolate in a stoichiometric reaction. We describe a functional model for this chemistry in which a thiolate of [(CH(3))(4)N](2)[Zn(SC(6)H(5))(4)] accepts a methyl group from (CH(3)O)(3)PO. The thiolate salt (CH(3))(4)N(SC(6)H(5)) is also active in methyl transfer, but the thiol C(6)H(5)SH fails to react. Conductivity measurements and kinetic studies demonstrate that [(CH(3))(4)N](2)[Zn(SC(6)H(5))(4)] forms ion pairs in dimethyl sulfoxide (DMSO) solution (K(IP) = 13 +/- 4 M(-)(1)) which exhibit diminished reactivity. The reaction of [Zn(SC(6)H(5))(4)](2)(-) with (CH(3)O)(3)PO is first order with respect to each reagent. A second-order rate constant for this reaction, k(Zn), was determined to be (1.6 +/- 0.3) x 10(-)(2) M(-)(1) s(-)(1). From kinetic data and equilibria studies, all reactivity of [(CH(3))(4)N](2)[Zn(SC(6)H(5))(4)] toward (CH(3)O)(3)PO could be attributed to dissociated thiolate. Metal complexes representing alternative protein sites were prepared and displayed the following kinetic trend of methyl transfer ability: [(CH(3))(4)N](2)[Zn(SC(6)H(5))(4)] > [(CH(3))(4)N](2)[Co(SC(6)H(5))(4)] approximately [(CH(3))(4)N](2)[Cd(SC(6)H(5))(4)] > [(CH(3))(4)N][Zn(SC(6)H(5))(3)(MeIm)] > [Zn(SC(6)H(5))(2)(MeIm)(2)], where MeIm = 1-methylimidazole. These results are consistent with a dissociated thiolate being the active species and suggest that a similar mechanism might apply to alkyl phosphotriester repair by Ada.

A multiphase 113Cd NMR investigation of metalloporphyrin reorientation in cadmium-substituted myoglobin

113Cd NMR spectroscopy in both the solution and solid state has been used to investigate the role of the metal ion and the proximal histidine on metalloporphyrin reorientation in myoglobin. Heme disorder has been known to exist for many years but understanding its mechanism has proved difficult due to the short-lived nature of the minor porphyrin isomer in native myoglobin. Cadmium-substituted myoglobin can be generated in one form which contains different insertion isomers or in another form which contains predominantly only one of these species. This allows for direct investigation of heme disorder in metal-substituted myoglobin.

113Cd Shielding Tensors of Monomeric Cadmium Compounds Containing Nitrogen Donor Atoms. 3. CP/MAS Studies on Five-Coordinate Cadmium Complexes Having N(3)X(2) (X = H, N, O, and S) Donor Atoms

The principal elements of the (113)Cd shielding tensor for a set of five- coordinate compounds having mixed donor atoms coordinating to the cadmium were determined via CP/MAS NMR experiments. The first complex, [HB(3,5-Me(2)pz)(3)]CdBH(4) (where pz = pyrazolyl), has a CdN(3)H(2) inner coordination sphere. The isotropic chemical shift in the solid state is 355.1 ppm, and its chemical shift anisotropy (CSA, Deltasigma) is -596 ppm with an asymmetry parameter (eta) of 0.64. The second complex, [HB(3,5-Me(2)pz)(3)]Cd[H(2)B(pz)(2)], has five nitrogen donor atoms bonded to the cadmium. This N(5) or N(3)N(2) compound was the only material of this study to manifest dipolar splitting of the cadmium resonance from the quadrupolar (14)N. The isotropic chemical shift, CSA, and the value of eta for this material were therefore determined at higher field where the dipolar splitting was less than the linewidth, yielding values of 226.6 ppm, -247 ppm, and 0.32, respectively. A second N(5) material, [HB(3-Phpz)(3)]Cd[H(2)B(3,5-Me(2)pz)(2)], was also investigated and has an isotropic shift of 190.2 ppm, a CSA of 254 ppm, and an eta of 0.86. Also studied was [HB(3-Phpz)(3)]Cd[(Bu(t)CO)(2)CH], which has an CdN(3)O(2) inner core. The isotropic chemical shift of this complex is 173.6 ppm, and the values of Deltasigma and eta were determined to be -258 ppm and 0.38, respectively. The final compound, [HB(3,5-Me(2)pz)(3)]Cd[S(2)CNEt(2)], with N(3)S(2) donor atoms, has an isotropic shift of 275.8 ppm, an eta of 0.51, and a CSA of +375 ppm. Utilizing previous assignments, the most shielded tensor element was determined to be oriented normal to the plane of the tridentate ligand. The shielding tensor information is used to speculate on the coordination geometry of the CdN(3)O(2) inner core complex.

111Cd NMR studies of the domain specificity of Ag+ and Cu+ binding to metallothionein

Metal displacement reactions of Cd7MT with Ag+ or Cu+ and interprotein metal exchange reactions between Cd7MT and Ag12MT or Cu12MT were studied by 111Cd NMR. Titration of 111Cd7MT with Ag+ indicates that Ag+ binds preferentially to the beta-domain of the protein to form the metal hybrid species, (Cd4)alpha(Ag6)beta MT. Once the beta-domain is filled, additional Ag+ ions displace Cd2+ from the alpha-domain to form (Ag6)alpha(Ag6)beta MT. The metal displacement reaction is cooperative and the two domains react independently of one another. The (Cd4)alpha(Ag6)beta MT hybrid protein is also formed as the major product of direct interprotein metal exchange between Cd7MT and Ag12MT. Cu+ reacts with Cd7MT in a manner similar to Ag+, with addition of 6 equiv of Cu+ leading to preferential formation of (Cd4)alpha(Cu6)beta-MT, and 12 equiv of Cu+ to formation of (Cu6)alpha(Cu6)beta MT. However, unlike Ag+, Cu+ appears to produce intermediate species that may contain mixed-metal clusters. Interprotein metal exchange between Cu12-MT and Cd7MT leads to the net transfer of Cd2+ into the alpha-domain and Cu+ into the beta-domain. The differential affinities of the two domains for monovalent and divalent metal ions plus the availability of facile pathways for metal exchange may be features that enable MT to function simultaneously in the metabolism of different metal ions.

Syntheses of Cationic, Six-Coordinate Cadmium(II) Complexes Containing Tris(pyrazolyl)methane Ligands. Influence of Charge on Cadmium-113 NMR Chemical Shifts

Treating a thf (thf = tetrahydrofuran) suspension of Cd(acac)(2) (acac = acetylacetonate) with 2 equiv of HBF(4).Et(2)O results in the immediate formation of [Cd(2)(thf)(5)](BF(4))(4) (1). Crystallization of this complex from thf/CH(2)Cl(2) yields [Cd(thf)(4)](BF(4))(2) (2), a complex characterized in the solid state by X-ray crystallography. Crystal data: monoclinic, P2(1)/n, a = 7.784(2) Å, b = 10.408(2) Å, c = 14.632(7) Å, beta = 94.64(3) degrees, V = 1181.5(6) Å(3), Z = 2, R = 0.0484. The geometry about the cadmium is octahedral with a square planar arrangement of the thf ligands and a fluorine from each (BF(4))(-) occupying the remaining two octahedral sites. Reactions of [Cd(2)(thf)(5)](BF(4))(4) with either HC(3,5-Me(2)pz)(3) or HC(3-Phpz)(3) yield the dicationic, homoleptic compounds {[HC(3,5-Me(2)pz)(3)](2)Cd}(BF(4))(2) (3) and {[HC(3-Phpz)(3)](2)Cd}(BF(4))(2) (4) (pz = 1-pyrazolyl). The solid state structure of 3 has been determined by X-ray crystallography. Crystal data: rhombohedral, R&thremacr;, a = 12.236(8) Å, c = 22.69(3) Å, V = 2924(4) Å(3), Z = 3, R = 0.0548. The cadmium is bonded to the six nitrogen donor atoms in a trigonally distorted octahedral arrangement. Four monocationic, mixed ligand tris(pyrazolyl)methane-tris(pyrazolyl)borate complexes {[HC(3,5-Me(2)pz)(3)][HB(3,5-Me(2)pz)(3)]Cd}(BF(4)) (5), {[HC(3,5-Me(2)pz)(3)][HB(3-Phpz)(3)]Cd}(BF(4)) (6), {[HC(3-Phpz)(3)][HB(3,5-Me(2)pz)(3)]Cd}(BF(4)) (7), and {[HC(3-Phpz)(3)][HB(3-Phpz)(3)]Cd}(BF(4)) (8) are prepared by appropriate conproportionation reactions of 3or 4 with equimolar amounts of the appropriate homoleptic neutral tris(pyrazolyl)borate complexes [HB(3,5-Me(2)pz)(3)](2)Cd or [HB(3-Phpz)(3)](2)Cd. Solution (113)Cd NMR studies on complexes 3-8 demonstrate that the chemical shifts of the new cationic, tris(pyrazolyl)methane complexes are very similar to the neutral tris(pyrazolyl)borate complexes that contain similar substitution of the pyrazolyl rings.

1H and (113)Cd NMR Investigations of Cd(2+) and Zn(2+) Binding Sites on Serum Albumin: Competition with Ca(2+), Ni(2+), Cu(2+), and Zn(2+)

1H and (113)Cd NMR studies are used to investigate the Cd(2+) binding sites on serum albumin (67 kDa) in competition with other metal ions. A wide range of mammalian serum albumins possess two similar strong Cd(2+) binding sites (site A 113-124 ppm; site B 24-28 ppm). The two strong sites are shown not to involve the free thiol at Cys34. Ca(2+) influences the binding of Cd(2+) to isolated human albumin, and similar effects due to endogenous Ca(2+) are observed for intact human blood serum. (1)H NMR studies show that the same two His residues of human serum albumin are perturbed by Zn(2+) and Cd(2+) binding alike. Zn(2+) displaces Cd(2+) from site A which leads to Cd(2+) occupation of a third site (C, 45 ppm). The N-terminus of HSA is not the locus of the two strong Cd(2+) binding sites, in contrast to Cu(2+) and Ni(2+). After saturation of the N-terminal binding site, Cu(2+) or Ni(2+) also displaces Cd(2+) from site A to site C. The effect of pH on Cd(2+) binding is described. A common Cd(2+)/Zn(2+) binding site (site A) involving interdomain His residues is discussed.

Probe of cadmium(II) binding on soil fulvic acid investigated by113Cd NMR spectroscopy

Binding of cadmium(II) on soil fulvic acid (FA) was investigated over a range of fulvate-to-cadmium concentration ratios (8 – 59 equiv. mol−1) using113Cd NMR spectroscopy. The113Cd chemical shift of cadmium bound on fulvate was observed in a more downfield region (δ −20.4 to −15.6) than that bound on synthetic polymers, poly(acrylic acid) (PAA: δ −36.6 to −38.2), poly(methacrylic acid) (PMAA: δ −34.0 to −25.4), and poly(vinyl benzoic acid) (PVBA: δ −34.7 to −31.2). The calculated values of individual chemical shifts for the species CdL+and CdL2(L: carboxylate) formed in Cd(II)–carboxylate systems (e.g., acetate, benzoate) are δ −22 to −24 and δ −39 to −40, respectively. The relative downfield shift of cadmium(II)–fulvate suggests that functional groups (e.g., hydroxyl and neutral N donor) other than carboxylates may be involved in cadmium coordination. The chemical shifts of cadmium complexes of hydroxycarboxylates (e.g., glycolate) or carboxylates containing neutral N donor (e.g., picolinate) were generally observed in more downfield regions than their carboxylate counterparts. Key words: fulvic acid, polyfunctionality, binding sites, chemical shift,113Cd NMR.

113Cd nuclear magnetic resonance studies of cabbage histidinol dehydrogenase

Histidinol dehydrogenase (HDH), a dimeric protein, catalyzes two sequential oxidation reactions to yield L-histidine from L-histidinol via L-histidinal. HDH contains 1 mol of Zn(II) per mol of subunit, and removal of this metal abolishes the enzymatic activity. On substitution of Zn(II) with 113Cd(II), the enzyme ([113Cd]HDH) showed similar catalytic activity. The 113Cd NMR spectra of [113Cd]HDH were measured under various conditions. The 113Cd NMR spectrum of [113Cd]HDH showed a resonance at 110 ppm, which indicates that the metal ion is bound to the protein by a combination of nitrogen and oxygen ligands. 113Cd NMR spectra of [113Cd]HDH were measured as complexes with two substrates (L-histidinol and DL-histidinal) and four inhibitors (imidazole, histamine, L-histidine, and DL-4-(4-imidazolyl)-3-amino-2-butanone) in the absence and presence of NAD+. Significant shifts of [113Cd]-HDH resonance in the presence of the ligand indicate that the metal ion is located in the catalytic site of HDH and that substrates and inhibitors interact with the metal ion. The role of the metal ion in the HDH reaction is discussed.

Metal coordination environment and dynamics in 113cadmium bleomycin: relationship to zinc bleomycin

The 13C chemical shifts of Cd- and ZnBlm A2 are almost identical throughout the entire molecule, suggesting that these structures adopt similar conformations. Nuclear magnetic resonance experiments with 113Cd-bleomycin have defined part of the metal-ligand environment of the molecule. Nitrogen atoms from the primary amine, pyrimidine, and imidazole are bound to 113Cd according to 13C spectra showing 113Cd-13C spin-spin couplings. Bound and free forms of the secondary amine nitrogen may be in equilibrium, as suggested by temperature-dependent 13C studies with Cd-bleomycin. In addition, a number of other carbon resonances are in chemical exchange over the temperature range 5-54 degrees C. The temperature dependence of the line widths of carbon atoms of Zn-bleomycin strongly resembles that of Cd-bleomycin. Examination of the 113Cd resonance as a function of temperature also supports the presence of at least two differently coordinated forms of cadmium in the molecule. According to the position of the 113Cd chemical shift, at most four nitrogen atoms are bound to Cd at low temperature. Titrations of 113Cd-bleomycin with chloride or acetate demonstrate that these anions can bind to major and minor forms of the structure and that a minor species exists which does not associate with chloride.

Mercury-199 NMR of the metal receptor site in MerR and its protein-DNA complex

Structural insights have been provided by mercury-199 nuclear magnetic resonance (NMR) into the metal receptor site of the MerR metalloregulatory protein alone and in a complex with the regulatory target, DNA. The one- and two-dimensional NMR data are consistent with a trigonal planar Hg-thiolate coordination environment consisting only of Cys side chains and resolve structural aspects of both metal ion recognition and the allosteric mechanism. These studies establish 199Hg NMR techniques as useful probes of the metal coordination environment of regulatory proteins, copper enzymes, and zinc transcription factor complexes as large as 50 kilodaltons.

Characterization of the calcium-binding sites of calcineurin B

Calcineurin (CaN) is a calcium- and calmodulin-dependent serine/threonine phosphatase whose inhibition by the immunosuppressant-immunophilin complexes (cyclosporin-cyclophilin and FK506-FKBP) is considered key to the mechanism of immunosuppression. CaN is a heterodimer, consisting of a 59 kDa catalytic subunit (A) and a 19 kDa calcium-binding regulatory subunit (B). The latter is postulated to harbor four calcium binding domains of the EF hand type. The titration of the CaN B apoprotein with the isomorphic Cd2+ was followed by 113Cd NMR and these data support one high-affinity metal binding site and three lower-affinity ones. Flow dialysis data with Ca2+ indicate one high affinity calcium binding site with Kd approximately 2.4 x 10(-8) M and three other sites with Kd approximately 1.5 x 10(-5) M. The chemical shifts of all four 113Cd resonances (-75, -93, -106 and -119 ppm) are in the same range as found in other 113Cd substituted calcium-binding proteins, and are indicative of all-oxygen coordination of pentagonal bipyramidal geometry.

Cadmium-113 and magnesium-25 NMR study of the divalent metal binding sites of isocitrate dehydrogenases from pig heart

The metal activator sites of NAD(+)-dependent and NADP(+)-dependent isocitrate dehydrogenases from pig heart have been probed using 113Cd- and 25Mg-NMR. In the presence of isocitrate and ADP, a broad resonance for cadmium bound to NAD-dependent isocitrate dehydrogenase was observed (-8 ppm) arising from exchange with isocitrate (-20 ppm) and/or ADP (27 ppm) complexes. The Cd shift with ADP suggests interaction of the metal with the nucleotide ring nitrogen. Increasing shifts with excess ADP are indicative of macrochelate formation. 25Mg-NMR demonstrates that, unlike manganese, magnesium has a similar dissociation constant (1.8 mM) from NADP-dependent isocitrate dehydrogenase as from the enzyme-isocitrate complex (1.1 mM). The extrapolated line width of bound magnesium increases from 674 Hz in the binary complex to 10,200 Hz in the ternary complex. The quadrupole coupling constant, calculated from relaxation rates, is larger in the ternary complex, indicative of greater distortion in the magnesium coordination sphere. The line widths of magnesium complexed to NAD-dependent isocitrate dehydrogenase are broader, as expected for the larger octamer. 113Cd- and 25Mg-NMR both show that the metal sites have anisotropic octahedral symmetry. 25Mg relaxation rates yield correlation times corresponding to motions of a domain with motion independent of the enzyme multimers.

The DNA-binding domain of the yeast Saccharomyces cerevisiae CYP1(HAP1) transcription factor possesses two zinc ions which are complexed in a zinc cluster

Various fragments of the N-terminal, DNA-binding domain of the yeast Saccharomyces cerevisiae transcriptional activator CYP1(HAP1) have been cloned and expressed in Escherichia coli. The corresponding polypeptides have been analysed biochemically and we have undertaken a more extensive physical study of a fragment consisting of amino acids 49-126 [CYP1(49-126)]. We show that this CYP1(49-126) peptide requires zinc or cadmium in the growth medium in order to maintain a stable structure. A method to purify CYP1(49-126) is presented. We demonstrate that the purified CYP1(49-126) fragment contains two zinc ions/fragment or two cadmium ions/fragment, which are necessary for DNA binding. 113Cd one-dimensional NMR data suggest that CYP1(HAP1) has a tetrahedral coordination, and that it forms a zinc-cluster complex like GAL4.

Metal-coordination sphere in the methylated Ada protein-DNA co-complex

BACKGROUND

The Ada protein of Escherichia coli repairs methyl phosphotriesters in DNA by direct, irreversible methyl transfer to one of its own cysteine residues. This residue, Cys69, is ligated to a tightly bound zinc ion in the protein. After methyl transfer, Ada can bind DNA sequence-specifically, inducing the transcription of genes that confer resistance to the toxic effects of methylating agents. Coordination of zinc via a thioether-S is exceedingly rare. We therefore investigated whether methylation causes ligand exchange of Cys69, replacing the thioether with a new zinc ligand with higher affinity for the metal.

RESULTS

We added a 13C-labeled methyl group to Cys69 of Ada and used isotope-edited NMR to observe the behavior of its protons. Comparison of the spectra for the Zn- and 112Cd-bound forms of the methylated protein with that of the 113Cd-bound form provided clear evidence that S-Me-Cys69 is coordinated to the metal in Ada when Ada is bound specifically to DNA.

CONCLUSIONS

The transcriptionally competent form of Ada, in which Cys69 is methylated and the protein is bound to DNA, maintains the coordination of S-Me-Cys69 to the metal ion. Thus, ligand exchange is not responsible for switching Ada from a DNA-repair protein to a transcriptional activator. We propose that the lability of the thioether-zinc coordinate bond may provide a mechanism for down-regulation of the adaptive response by inactivation of the Ada DNA-binding domain.

Phosphophoryn, an "acidic" biomineralization regulatory protein: conformational folding in the presence of Cd(II)

The divalent cation-induced protein folding properties of the template macromolecule, bovine dentine phosphophoryn (BDPP), have been examined by 1H/31P/13C/113Cd-nmr spectroscopy. Cd(II) was employed, exploiting the sensitivity of 113Cd-nmr to ligand-binding interactions and kinetics. Cation binding was studied over the stoichiometric range of 0-50: 1 Cd(II): protein (mole ratio), well below the range of Cd(II) concentration required to induce protein precipitation. The stepwise titration of divalent cation-depleted phosphophoryn at pH 7.2 in H2O/D2O with 113CdCl2 revealed that (PSer)n, (PSerAsp)n, and (Asp)n polyelectrolyte cation-binding domains undergo two major transitions in their secondary and tertiary structures: the first transition, occurring between 1:10 and 1:1 Cd(II): protein stoichiometry, and the second, between 10:1 and 50:1. By monitoring the amide NH intensities, 31P-nmr chemical shift, and 13C Asp-C, resonances, it was concluded that Cd(II) ions exhibit a binding-site preference for polyelectrolyte cation-binding domains, in the order (PSer)n > (PSerAsp)n > (Asp)n This preference correlates with the degree of negative charge density for each sequence motif. Accompanying the backbone conformational transitions at the polyelectrolyte regions were conformational transitions in the flanking hinge domains, indicating that the hinge domains participate in the folding of the phosphophoryn molecule as divalent cation binding occurs at the polyelectrolyte domains. We were unsuccessful in detecting phosphophoryn-bound Cd(II) species by 113Cd-nmr because of chemical exchange modulation. However, using a smaller 21-residue peptide mimetic of phosphophoryn, we have observed three stoichiometric-dependent 113Cd resonances that differ in terms of the oxoanion coordination number. Our observation of multiple Cd(II) species in the presence of the peptide supports our contention that Cd(II) has many chemically distinct coordination sites on phosphophoryn, each in multiple equilibria with H2O, Cl-, and side-chain oxoatoms.

Molecular tuning of ion binding to calcium signaling proteins

Intracellular calcium plays an essential role in the transduction of most hormonal, neuronal, visual, and muscle stimuli. (Recent reviews include Putney, 1993; Berridge, 1993a,b; Tsunoda, 1993; Gnegy, 1993; Bachset al.1992; Hanson & Schulman, 1992; Villereal & Byron, 1992; Premack & Gardner, 1992; Meanset al.1991).