Preparation and spectroscopic studies of cobalt(II) derivatives of blue copper proteins

Effect of solvent inclusion on the structures and solid-state fluorescence of coordination compounds of naphthalimide derivatives and metal halides

The structural characteristics and solid-state fluorescence of coordination compounds formed between the fluorophore N-((3-pyridyl)methyl)-1,8-naphthalimide (3-pn) and a range of divalent metal halides, were investigated. Six new crystal structures, of the formula ML 2 X 2.solv, were obtained through the combination of the organic ligand L = 3-pn with the divalent metal halides CoX2 (X = Cl− or Br−) or ZnX2 (X = Cl− or Br−) in different solvents. It was found that solvent molecules are incorporated into all the structures, and that the same complex can incorporate different solvent molecules. The conformation of the complex molecule is determined by the solvent molecule incorporated into the structure. The fluorescence intensity is affected by the identity of the metal and halido ligand in the complex molecule, while the identity of the solvent molecule and the metal ion affects the emission wavelength. Thus, the solvent of crystallisation impacts on the solid-state fluorescence of the compounds, hence the same complex can be made to fluoresce at different wavelengths depending on the solvent of crystallisation.

Oxidation Protection in Metal-Binding Peptide Motif and Its Application to Antibody for Site-Selective Conjugation

Here, we demonstrate that a metal ion binding motif could serve as an efficient and robust tool for site-specific conjugation strategy. Cysteine-containing metal binding motifs were constructed as single repeat or tandem repeat peptides and their metal binding characteristics were investigated. The tandem repeats of the Cysteine-Glycine-Histidine (CGH) metal ion binding motif exhibited concerted binding to Co(II) ions, suggesting that conformational transition of peptide was triggered by the sequential metal ion binding. Evaluation of the free thiol content after reduction by reducing reagent showed that metal-ion binding elicited strong retardation of cysteine oxidation in the order of Zn(II)>Ni(II)>Co(II). The CGH metal ion binding motif was then introduced to the C-terminus of antibody heavy chain and the metal ion-dependent characteristics of oxidation kinetics were investigated. As in the case of peptides, CGH-motif-introduced antibody exhibited strong dependence on metal ion binding to protect against oxidation. Zn(II)-saturated antibody with tandem repeat of CGH motif retains the cysteine reactivity as long as 22 hour even with saturating O₂ condition. Metal-ion dependent fluorophore labeling clearly indicated that metal binding motifs could be employed as an efficient tool for site-specific conjugation. Whereas Trastuzumab without a metal ion binding site exhibited site-nonspecific dye conjugation, Zn(II) ion binding to antibody with a tandem repeat of CGH motif showed that fluorophores were site-specifically conjugated to the heavy chain of antibody. We believe that this strong metal ion dependence on oxidation protection and the resulting site-selective conjugation could be exploited further to develop a highly site-specific conjugation strategy for proteins that contain multiple intrinsic cysteine residues, including monoclonal antibodies.

Methionine Ligand Interaction in a Blue Copper Protein Characterized by Site-Selective Infrared Spectroscopy

The reactivity of metal sites in proteins is tuned by protein-based ligands. For example, in blue copper proteins such as plastocyanin (Pc), the structure imparts a highly elongated bond between the Cu and a methionine (Met) axial ligand to modulate its redox properties. Despite extensive study, a complete understanding of the contribution of the protein to redox activity is challenged by experimentally accessing both redox states of metalloproteins. Using infrared (IR) spectroscopy in combination with site-selective labeling with carbon-deuterium (C-D) vibrational probes, we characterized the localized changes at the Cu ligand Met97 in the oxidized and reduced states, as well as the Zn(II) or Co(II)-substituted, the pH-induced low-coordinate, the apoprotein, and the unfolded states. The IR absorptions of (d3-methyl)Met97 are highly sensitive to interaction of the sulfur-based orbitals with the metal center and are demonstrated to be useful reporters of its modulation in the different states. Unrestricted Kohn-Sham density functional theory calculations performed on a model of the Cu site of Pc confirm the observed dependence. IR spectroscopy was then applied to characterize the impact of binding to the physiological redox partner cytochrome (cyt) f. The spectral changes suggest a slightly stronger Cu-S(Met97) interaction in the complex with cyt f that has potential to modulate the electron transfer properties. Besides providing direct, molecular-level comparison of the oxidized and reduced states of Pc from the perspective of the axial Met ligand and evidence for perturbation of the Cu site properties by redox partner binding, this study demonstrates the localized spatial information afforded by IR spectroscopy of selectively incorporated C-D probes.

A novel blue luminescent material Na2[Co(C2O4)2(H2O)2]·6H2O: synthesis, structure, luminescence and magnetic properties

The synthesis, crystal structure and physical properties of new disodium trans-diaquabis(oxalato)cobaltate(ii)hexahydrate {Na₂[Co(C₂O₄)₂(H₂O)₂]·6H₂O} crystals have been investigated.

Azurin as a protein scaffold for a low-coordinate nonheme iron site with a small-molecule binding pocket

The apoprotein of Pseudomonas aeruginosa azurin binds iron(II) to give a 1:1 complex, which has been characterized by electronic absorption, Mössbauer, and NMR spectroscopies, as well as X-ray crystallography and quantum-chemical computations. Despite potential competition by water and other coordinating residues, iron(II) binds tightly to the low-coordinate site. The iron(II) complex does not react with chemical redox agents to undergo oxidation or reduction. Spectroscopically calibrated quantum-chemical computations show that the complex has high-spin iron(II) in a pseudotetrahedral coordination environment, which features interactions with side chains of two histidines and a cysteine as well as the C═O of Gly45. In the (5)A(1) ground state, the d(z(2)) orbital is doubly occupied. Mutation of Met121 to Ala leaves the metal site in a similar environment but creates a pocket for reversible binding of small anions to the iron(II) center. Specifically, azide forms a high-spin iron(II) complex and cyanide forms a low-spin iron(II) complex.

Palladium(II) and platinum(II) bind strongly to an engineered blue copper protein

Studies of palladium(II) and platinum(II) binding to well-characterized proteins contribute to understanding the influence of these metals in the environment and body. The well-characterized apoprotein of azurin has a soft-metal binding site that may be exposed to solvent by mutation of a coordinating His-117 residue to glycine (H117G). Palladium(II) and platinum(II) form strong 1:1 adducts with the apo form of H117G azurin. A combination of UV-vis, circular dichroism, and inductively coupled plasma mass spectrometry techniques suggests that the metal binds specifically at His-46 and Cys-112 of the protein.

Proteins as electronic materials: electron transport through solid-state protein monolayer junctions

Electron transfer (ET) through proteins, a fundamental element of many biochemical reactions, is studied intensively in aqueous solutions. Over the past decade, attempts were made to integrate proteins into solid-state junctions in order to study their electronic conductance properties. Most such studies to date were conducted with one or very few molecules in the junction, using scanning probe techniques. Here we present the high-yield, reproducible preparation of large-area monolayer junctions, assembled on a Si platform, of proteins of three different families: azurin (Az), a blue-copper ET protein, bacteriorhodopsin (bR), a membrane protein-chromophore complex with a proton pumping function, and bovine serum albumin (BSA). We achieve highly reproducible electrical current measurements with these three types of monolayers using appropriate top electrodes. Notably, the current-voltage (I-V) measurements on such junctions show relatively minor differences between Az and bR, even though the latter lacks any known ET function. Electron Transport (ETp) across both Az and bR is much more efficient than across BSA, but even for the latter the measured currents are higher than those through a monolayer of organic, C18 alkyl chains that is about half as wide, therefore suggesting transport mechanism(s) different from the often considered coherent mechanism. Our results show that the employed proteins maintain their conformation under these conditions. The relatively efficient ETp through these proteins opens up possibilities for using such biomolecules as current-carrying elements in solid-state electronic devices.

Further perspectives on the charge transfer transitions of blue copper proteins and the ligand moieties in stellacyanin

The charge transfer spectra of plastocyanin and azurin are analyzed in detail, taking into account the structural information that is available. The number and relative energies of the low-lying charge transfer bands attributable to cysteine and methionine sulfur are considered with reference to the energy levels associated with the donor centers and to experimental work on relevant complexes. Two cysteine --> Cu(II) charge transfer transitions are assigned to bands that occur near 620 nm and near 770 nm. Energy considerations suggest that the methionine --> Cu(II) transition occurs in the vicinity of 550 nm for both plastocyanin and azurin. The existence of a disulfide linkage in stellacyanin is established by thiol titrations, and the possibility exists that the fourth ligand in stellacyanin may involve disulfide sulfur. If so, the cystine --> Cu(II) transition is also expected to appear around 550 nm.

Coordination environment and fluoride binding of type 2 copper in the blue copper protein ascorbate oxidase

The coordination environment of the type 2 (nonblue) copper in native ascorbate oxidase (L-ascorbate:oxygen oxidoreductase, EC 1.10.3.3) and of a derivative of the enzyme having the type 1 (blue) copper reversibly bleached has been examined by electron paramagnetic resonance (EPR) spectroscopy. In the g[unk] region of the spectrum of bleached ascorbate oxidase, a seven-line superhyperfine pattern is seen that is attributed to the presence of three nitrogen-donor ligands to a type 2 copper having tetragonal geometry. The superhyperfine splitting patterns in the g parallel region of the EPR spectra of native and bleached ascorbate oxidase show that as many as two fluorides may bind to type 2 copper. Because fluoride inhibits the enzyme competitively with respect to ascorbic acid, it is proposed that the type 2 copper is part of the ascorbate binding site.

Blue copper proteins: Synthesis, spectra, and structures of CuN(3)(SR) and CuN(3)(SR) active site analogues

The reaction of Cu(SR) or [Cu(SR)][ClO(4)] derivatives (SR = p-nitrobenzenethiolate or O-ethylcysteinate) with potassium hydrotris(3,5-dimethyl-1-pyrazolyl)borate produces redox pairs of the stoichiometry Cu(I)N(3)(SR) and Cu(II)N(3)(SR). These complexes are well-defined synthetic approximations to the proposed N(3)S binding sites of blue (type 1) copper electron transfer proteins. The compounds were investigated by a variety of chemical and spectral (optical, resonance Raman, and electron paramagnetic resonance) techniques; the complex K[Cu(hydrotris(3,5-dimethyl-1-pyrazolyl)borate)(p- NO(2)C(6)H(4)S]-2 acetone was also studied by single-crystal x-ray diffraction methods. The spectrochemical characteristics of the Cu(II)N(3)(SR) species are in large part similar to the native system and thus provide some perspective regarding the origin of the unique type 1 spectral parameters and electron transfer properties.

The role of ligand-containing loops at copper sites in proteins

Many approaches are being used to engineer metalloproteins, with most of these informed by, and aiming to further elucidate, the basic structural requirements for biological metal centers. Cupredoxins are type 1 (T1) copper-containing electron transfer (ET) proteins with a -barrel fold that is thought to constrain metal site structure. The T1 copper ion is bound by ligands mainly originating from a single active site loop whose length and structure varies. This Highlight article will focus on protein engineering studies which have investigated the role of the metal-binding loop for active site integrity and functionality. Scaffold differences are present within the cupredoxin family and their influence has also been assessed. Given the widespread occurrence of -barrel domains in nature, and the array of metal sites in proteins composed of loop regions, the studies described on this model system have implications for a variety of metalloproteins.

Metal–thiolate bonds in bioinorganic chemistry

Metal-thiolate active sites play major roles in bioinorganic chemistry. The M--S(thiolate) bonds can be very covalent, and involve different orbital interactions. Spectroscopic features of these active sites (intense, low-energy charge transfer transitions) reflect the high covalency of the M--S(thiolate) bonds. The energy of the metal-thiolate bond is fairly insensitive to its ionic/covalent and pi/sigma nature as increasing M--S covalency reduces the charge distribution, hence the ionic term, and these contributions can compensate. Thus, trends observed in stability constants (i.e., the Irving-Williams series) mostly reflect the dominantly ionic contribution to bonding of the innocent ligand being replaced by the thiolate. Due to high effective nuclear charges of the Cu(II) and Fe(III) ions, the cupric- and ferric-thiolate bonds are very covalent, with the former having strong pi and the latter having more sigma character. For the blue copper site, the high pi covalency couples the metal ion into the protein for rapid directional long range electron transfer. For rubredoxins, because the redox active molecular orbital is pi in nature, electron transfer tends to be more localized in the vicinity of the active site. Although the energy of hydrogen bonding of the protein environment to the thiolate ligands tends to be fairly small, H-bonding can significantly affect the covalency of the metal-thiolate bond and contribute to redox tuning by the protein environment.

Spectroscopic and DFT investigation of [M{HB(3,5-iPr2pz)3}(SC6F5)] (M = Mn, Fe, Co, Ni, Cu, and Zn) model complexes: periodic trends in metal-thiolate bonding

A series of metal-varied [ML(SC6F5)] model complexes (where L = hydrotris(3,5-diisopropyl-1-pyrazolyl)borate and M = Mn, Fe, Co, Ni, Cu, and Zn) related to blue copper proteins has been studied by a combination of absorption, MCD, resonance Raman, and S K-edge X-ray absorption spectroscopies. Density functional calculations have been used to characterize these complexes and calculate their spectra. The observed variations in geometry, spectra, and bond energies are interpreted in terms of changes in the nature of metal-ligand bonding interactions. The metal 3d-ligand orbital interaction, which contributes to covalent bonding in these complexes, becomes stronger going from Mn(II) to Co(II) (the sigma contribution) and to Cu(II) (the pi contribution). This change in the covalency results from the increased effective nuclear charge of the metal atom in going from Mn(II) to Zn(II) and the change in the 3d orbital populations (d5-->d10). Ionic bonding also plays an important role in determining the overall strength of the ML(+)-SC6F5(-) interaction. However, there is a compensating effect: as the covalent contribution to the metal-ligand bonding increases, the ionic contribution decreases. These results provide insight into the Irving-Williams series, where it is found that the bonding of the ligand being replaced by the thiolate makes a major contribution to the observed order of the stability constants over the series of metal ions.

Modulation of zinc- and cobalt-binding affinities through changes in the stability of the zinc ribbon protein L36

Cysteine-rich Zn(II)-binding sites in proteins serve two distinct functions: to template or stabilize specific protein folds, and to facilitate chemical reactions such as alkyl transfers. We are interested how the protein environment controls metal site properties, specifically, how naturally occurring tetrahedral Zn(II) sites are affected by the surrounding protein. We have studied the Co(II)- and Zn(II)-binding of a series of derivatives of L36, a small zinc ribbon protein containing a (Cys)(3)His metal coordination site. UV-vis spectroscopy was used to monitor metal binding by peptides at pH 6.0. For all derivatives, the following trends were observed: (1) Zn(II) binds tighter than Co(II), with an average K (A) (Zn) /K (A) (Co) of 2.8(+/-2.0)x10(3); (2) mutation of the metal-binding ligand His32 to Cys decreases the affinity of L36 derivatives for both metals; (3) a Tyr24 to Trp mutation in the beta-sheet hydrophobic cluster increases K (A) (Zn) and K (A) (Co) ; (4) mutation in the beta-hairpin turn, His20 to Asn generating an Asn-Gly turn, also increases K (A) (Zn) and K (A) (Co) ; (5) the combination of His20 to Asn and Tyr24 to Trp mutations also increases K (A) (Zn) and K (A) (Co) , but the increments versus C(3)H are less than those of the single mutations. Furthermore, circular dichroism, size-exclusion chromatography, and 1D and 2D (1)H NMR experiments show that the mutations do not change the overall fold or association state of the proteins. L36, displaying Co(II)- and Zn(II)-binding sensitivity to various sequence mutations without undergoing a change in protein structure, can therefore serve as a useful model system for future structure/reactivity studies.

Structural and Spectroscopic Characterization of First-Row Transition Metal(II) Substituted Blue Copper Model Complexes with Hydrotris(pyrazolyl)borate

[CuL(SC(6)F(5))] (1) (L = hydrotris(3,5-diisopropyl-1-pyrazolyl)borate anion) has been reported as a good model for blue copper proteins [Kitajima, N.; Fujisawa, K.; Tanaka, M.; Moro-oka, Y. J. Am. Chem. Soc. 1992, 114, 9232-9233]. To obtain more structural and spectroscopic insight, the first-row transition metal(II) substituted complexes of Cu(II) (1) to Mn(II) (2), Fe(II) (3), Co(II) (4), Ni(II) (5), and Zn(II) (6) were synthesized and their crystal structures were determined. These model complexes have a distorted tetrahedral geometry arising from the tripodal ligand L. The d value, which is defined by the distance from the N(2)S basal plane to the metal(II) ion, and the bond angles such as N-M-N and S-M-N are good indicators of these structural distortions. The obtained complexes were characterized by UV-vis absorption, EPR, NMR, far-IR, and FT-Raman spectroscopies and electrochemical and magnetic properties. In UV-vis absorption spectra, the sulfur-to-metal(II) CT bands and the d-d transition bands are observed for 1 and 3-5. For 1, the strong sulfur to Cu(II) CT band at 663 nm, which is one of the unique properties of blue copper proteins, is observed. The CT energies of the Fe(II) (3), Co(II) (4), and Ni(II) (5) complexes are shifted to higher energy (308 and 355 nm for 3, 311 and 340 nm for 4, 357 and 434 nm for 5) and are almost the same as the corresponding Co(II)- and Ni(II)-substituted blue copper proteins. In the far-IR spectra, three far-IR absorption bands for 2-6 at ca. 400, ca. 350, and ca. 310 cm(-1) are also observed similar to those for 1. Other properties are consistent with their distorted tetrahedral geometries.

Molecular characterization of DnaK from the halotolerant cyanobacterium Aphanothece halophytica for ATPase, protein folding, and copper binding under various salinity conditions

Previously, it was found that the dnaK1 gene of the halotolerant cyanobacterium Aphanothece halophytica encodes a polypeptide of 721 amino acids which has a long C-terminal region rich in acidic amino acid residues. To understand whether the A. halophytica DnaK1 possesses chaperone activity at high salinity and to clarify the role of the extra C-terminal amino acids, a comparative study examined three kinds of DnaK molecules for ATPase activity as well as the refolding activity of other urea-denatured proteins under various salinity conditions. DnaK1s from A. halophytica and Synechococcus sp. PCC 7942 and the C-terminal deleted A. halophytica DnaK1 were expressed in Escherichia coli and purified. The ATPase activity of A. halophytica DnaK1 was very high even at high salinity ( 1.0 M NaCl or KCl), whereas this activity in Synechococcus PCC 7942 DnaK1 decreased with increasing concentrations of NaCl or KCl. The salt dependence on the refolding activity of urea-denatured lactate dehydrogenase by DnaK1s was similar to that of ATPase activity of the respective DnaK1s. The deletion of the C-terminal amino acids of A. halophytica DnaK had no effect on the ATPase activity, but caused a significant decrease in the refolding activity of other denatured proteins. These facts indicate that the extra C-terminal region of A. halophytica DnaK1 plays an important role in the refolding of other urea-denatured proteins at high salinity. Furthermore, it was shown that DnaK1 could assist the copper binding of precursor apo-plastocyanin as well as that of mature apo-plastocyanin during the folding of these copper proteins.

The dynamic properties of the M121H azurin metal site as studied by NMR of the paramagnetic Cu(II) and Co(II) metalloderivatives

The M121H azurin mutant in solution presents various species in equilibrium that can be detected and studied by 1H NMR of the Cu(II) and Co(II) paramagnetic metalloderivatives. In both cases up to three species are observed in slow exchange, the proportions of which are different for the two metalloderivatives. Above pH 5 the major species displays a tetrahedral coordination in which the His121 can be observed as a coordinated residue. Its metal site corresponds to a new type of site that is defined as a type 1.5 site. The second and third species resemble the wild type (type 1) azurin and, above pH 4.5, they are present only at a low concentration. At low pH a protonation process increases the proportion of both type 1 species at the expense of the type 1.5 species. This process, characterized by a pKa = 4.3, is assigned to the protonation of His121. At high pH the NMR spectrum of the Co(II)-M121H azurin experiences an additional transition, which is not observed in the case of the Cu(II) protein. The dynamic properties of the M121H metal site appear to be related to changes in the coordination geometry and the strength of the axial interaction between the Ndelta1 (His121) and the metal.

The metal site of Pseudomonas aeruginosa azurin, revealed by a crystal structure determination of the Co(II) derivative and Co-EPR spectroscopy

The crystal structure of cobalt-substituted azurin from Pseudomonas aeruginosa has been determined to final crystallographic R value of 0.175 at 1.9 A resolution. There are four molecules in the asymmetric unit in the structure, and these four molecules are packed as a dimer of dimers. The dimer packing is very similar to that of the wild-type Pseudomonas aeruginosa azurin dimer. Replacement of the native copper by the cobalt ion has only small effects on the metal binding site presumably because of the existence of an extensive network of hydrogen bonds in its immediate neighborhood. Some differences are obvious, however. In wild-type azurin the copper atom occupies a distorted trigonal bipyramidal site, while cobalt similar to zinc and nickel occupy a distorted tetrahedral site, in which the distance to the Met121,S(delta) atom is increased to 3.3-3.5 A and the distance to the carbonyl oxygen of Gly45 has decreased to 2.1-2.4 A. The X-band EPR spectrum of the high-spin Co(II) in azurin is well resolved (apparent g values gx' = 5.23; gy' = 3.83; gz' = 1.995, and hyperfine splittings Ax' = 31; Ay' = 20-30; Az' = 53 G) and indicates that the ligand field is close to axial.

Ligand perturbation effects on a pseudotetrahedral Co(II)(His)3-ligand site. A magnetic circular dichroism study of the Co(II)-substituted insulin hexamer

Magnetic circular dichroism (MCD) spectra of a series of adducts formed by the Co(II)-substituted R-state insulin hexamer are reported. The His-B10 residues in this hexamer form tris imidazole chelates in which pseudotetrahedral Co(II) centers are completed by an exogenous fourth ligand. This study investigates how the MCD signatures of the Co(II) center in this unit are influenced by the chemical and steric characteristics of the fourth ligand. The spectra obtained for the adducts formed with halides, pseudohalides, trichloroacetate, nitrate, imidazole, and 1-methylimidazole appear to be representative of near tetrahedral Co(II) geometries. With bulkier aromatic ligands, more structured spectra indicative of highly distorted Co(II) geometries are obtained. The MCD spectrum of the phenolate adduct is very similar to those of Co(II)-carbonic anhydrase (alkaline form) and Co(II)-beta-lactamase. The MCD spectrum of the Co(II)-R6-CN- adduct is very similar to the CN- adduct of Co(II)-carbonic anhydrase. The close similarity of the Co(II)-R6-pentafluorophenolate and Co(II)-R6-phenolate spectra demonstrates that the Co(II)-carbonic anhydrase-like spectral profile is preserved despite a substantial perturbation in the electron withdrawing nature of the coordinated phenolate oxygen atom. We conclude that this type of spectrum must arise from a specific Co(II) coordination geometry common to each of the Co(II) sites in the Co(II)-R6-phenolate, Co(II)-R6-pentafluorophenolate, Co(II)-beta-lactamase, and the alkaline Co(II)-carbonic anhydrase species. These spectroscopic results are consistent with a trigonally distorted tetrahedral Co(II) geometry (C3v), an interpretation supported by the pseudotetrahedral Zn(II)(His)3(phenolate) center identified in a Zn(II)-R6 crystal structure (Smith, G. D., and Dodson, G. G. (1992) Biopolymers 32, 441-445).

Enforced interaction of one molecule of plastocyanin with two molecules of cytochrome c and an electron-transfer reaction involving the hydrophobic patch on the plastocyanin surface

Laser flash photolysis is used to study the photoinduced electron-transfer reaction cyt(III)//pc(II) + 3Zncyt --> cyt(III)//pc(I) + Zincyt+ at pH 7.0 and 25 degrees. In the covalent (symbol//) complex cyt(III)//pc(II) the acidic patch in cupriplastocyanin is directly cross-linked to the basic patch in ferricytochrome c. The triplet state of zinc cytochrome c reduces the pc(II) moiety, not the cyt(III) moiety, of the covalent complex. The reaction is strictly bimolecular in the entire range of ionic strength studied, from 1.25 mM to 1.00 M. The two reactants interact only transiently, in a collisional complex, and do not form a persistent complex cyt(III)//pc(II)/Zncyt. Because noncovalent (symbol/) association of three separate protein molecules is far less probable than association of the covalent complex and another protein molecule, we conclude that, without the aid of covalent cross-links, one molecule of plastocyanin will not form a ternary complex with two molecules of cytochrome c, cyt/pc/cyt. Dependence of the rate constant on ionic strength is analyzed in terms of van Leeuwen theory of electrostatic interactions, which recognizes the importance of dipole moments of the proteins. This analysis shows that 3Zncyt reacts with the hydrophobic patch in the pc(II) moiety of the covalent complex cyt(III)//pc(II). At high ionic strength, at which electrostatic interactions are practically abolished, the blue copper site is reduced with approximately equal rates via the hydrophobic patch in the pc(II) moiety of the complex and via the acidic patch in free pc(II). This is evidence that the two distinct patches on the plastocyanin surface are comparable in their intrinsic "conductivity" for electrons coming to the copper site. Positively charged and electroneutral redox partners tend to react at the acidic patch (although not necessarily at the initial docking site in this broad patch) for electrostatic, not electronic, reasons. Earlier theorectical studies disagreed about the relative electronic conductivities of the two patches. This experimental study corroborates very recent theoretical studies that found the two patches to be comparable in the efficiency of electron transfer.

1H-NMR study of a cobalt-substituted blue copper protein: Pseudomonas aeruginosa Co(II)-azurin

Substitution of copper by cobalt in blue copper proteins gives a paramagnetic metalloderivative suitable for paramagnetic NMR studies. A thorough analysis of the 1H-NMR spectrum of Pseudomonas aeruginosa Co(II)-azurin is presented here. All the observable contact-shifted signals as well as many other paramagnetic signals from protons placed up to about 1.0 nm around the metal center, including some residues belonging to functionally important parts of the protein like the hydrophobic patch and the His35 region, have been assigned. The results obtained permit the detection and study of structural variations like those originated by the His35 ionization, and allow us to draw a feasible picture of the metal coordination site. Contact-shifted signals correspond to the same five residues which are found in the coordination sphere of the native Cu(II)-azurin, i.e. His46, His117, Cys112, Met121 and Gly45. Among them, the histidine residues present a pattern of resonances typical for histidines coordinated to cobalt in other cobalt protein derivatives, and the cysteine signals clearly indicate a strong interaction with the paramagnetic Co(II) ion. In contrast, the Met121 signals indicate a weak but still existent contact interaction with the metal center. On the other hand, the very weak copper ligand, Gly45, appears here as clearly coordinated to cobalt. Results are consistent with a distorted tetrahedral metal site with the cobalt deviated from the N2S plane towards the Gly45 O axial position and weakly interacting with the Met121 sulfur.

1H-, 13C-, and 113Cd-NMR study of the Cd(II) complex of a blocked peptide, Z-Cys-Ala-Pro-His-OMe, in organic solvents

The Cd(II) complex of a peptide, Z-Cys-Ala-Pro-His-OMe was prepared and characterized by absorption, CD, 1H-, 13C-, and 113Cd-nmr, and nuclear Overhauser effect spectroscopy (NOESY) spectra to show the coordination of cysteine thiolate and histidine imizazole to Cd(II) ion. The NOESY spectra in dimethyl formamide showed that the cysteine residue was in proximity to the histidine residue. These results reveal the chelation of Z-Cys-Ala-Pro-His-OMe to Cd(II) ion in solution. Temperature-dependent dissociation equilibrium of histidine imidazole in solution was observed in this complex. Structural features of the chelating peptide are discussed.

A 1H NMR NOE study on Co(II) stellacyanin: some clues about the structure of the metal site

The 1H NMR spectrum of Co(II) stellacyanin is reported, in which four signals not previously observed have been detected. NOE experiments were performed to assign the hyperfine shifted signals corresponding to a Cys and two His residues. Both His residues are solvent-accessible and are shown to bind the metal ion through their N delta 1 atoms. The beta-CH2 Cys proton shifts indicate the presence of a strong axial ligand.

Magnetic circular dichroism studies on the mononuclear ferrous active site of phthalate dioxygenase from Pseudomonas cepacia show a change of ligation state on substrate binding

Phthalate dioxygenase from Pseudomonas cepacia contains a mononuclear ferrous center that is strictly required for catalytic oxygen activation. The spectroscopic characterization of this iron site and its ligand interactions has been complicated in the past by interference from a Rieske-type binuclear (2Fe-2S) cluster in the enzyme, which dominates the absorption spectra and is superimposed in X-ray absorption spectra for the mononuclear site. We have used low-temperature, variable magnetic field circular dichroism spectroscopy to selectively detect the ligand field spectra of the paramagnetic mononuclear ferrous active site in the presence of the diamagnetic exchange-coupled Rieske center and observe spectral changes associated with substrate binding. The perturbations of the d-->d spectra for the mononuclear ferrous site reflect a decrease in coordination number from six to five on binding substrate. This structural change suggests that displacement of an iron ligand prepares the ferrous center for dioxygen activation.

Low frequency EPR of Pseudomonas aeruginosa azurin. Analysis of ligand superhyperfine structure from a type 1 copper site

The type 1 copper in Pseudomonas aeruginosa azurin was studied by electron paramagnetic resonance (EPR) spectroscopy at low microwave frequencies. Partially resolved ligand hyperfine structure was observed in the perpendicular region of the spectra at both S-band (2.4 GHz) and L-band (1.1 GHz). A trial and error method, requiring several hundred simulations, has been used to simulate the low frequency EPR data and yield an optimum value of 30 MHz for ACUx, more than one half that previously reported. The fit between the simulated and experimental data is sensitive to changes in the Euler angles and, in particular, to the angle alpha which rotates the Cu A-tensor about the z-axis. Thus, the A- and g-tensors for copper in P. aeruginosa azurin do not appear to be coincident. A value for the Euler angle beta of at least 10 degrees does not disturb the fit between the simulated and experimental data. These studies demonstrate the advantage of evaluating EPR parameters from simulations at more than one frequency, especially at low frequencies where ligand superhyperfine structure may be resolved for type 1 copper.