Nuclear relaxation in the magnetic coupled system Cu2Co2SOD. Histidine-44 is detached upon anion binding

Molecular Dynamics and Structural Studies of Zinc Chloroquine Complexes

Chloroquine (CQ) is a first-choice drug against malaria and autoimmune diseases. It has been co-administered with zinc against SARS-CoV-2 and soon dismissed because of safety issues. The structural features of Zn-CQ complexes and the effect of CQ on zinc distribution in cells are poorly known. In this study, state-of-the-art computations combined with experiments were leveraged to solve the structural determinants of zinc-CQ interactions in solution and the solid state. NMR, ESI-MS, and X-ray absorption and diffraction methods were combined with ab initio molecular dynamics calculations to address the kinetic lability of this complex. Within the physiological pH range, CQ binds Zn²⁺ through the quinoline ring nitrogen, forming [Zn(CQH)Cl_x(H₂O)_3-x]^(3+)-x (x = 0, 1, 2, and 3) tetrahedral complexes. The Zn(CQH)Cl₃ species is stable at neutral pH and at high chloride concentrations typical of the extracellular medium, but metal coordination is lost at a moderately low pH as in the lysosomal lumen. The pentacoordinate complex [Zn(CQH)(H₂O)₄]³⁺ may exist in the absence of chloride. This in vitro/in silico approach can be extended to other metal-targeting drugs and bioinorganic systems.

Coordination chemistry and solution structure of Fe(II)-peplomycin. Two possible coordination geometries

The solution structure of Fe(II)-peplomycin was determined from NMR data collected for this molecule. As found previously for Fe(II)- and Co(II)-bound bleomycin; the coordination sphere of the metal is composed of the primary and secondary amines in β-aminoalanine, the pyrimidine and imidazole rings in the pyrimidinylpropionamide, and β-hydroxyhistidine moieties, respectively, the amine nitrogen in β-hydroxyhistidine, and either the carbamoyl group in mannose or a solvent molecule. The two most discussed coordination geometries for the aforementioned ligands in metallo-bleomycins have been tested against the NMR data generated for Fe(II)-peplomycin. The interpretation of the experimental evidence obtained through molecular dynamics indicates that both geometries are equally likely in solution for this compound in the absence of DNA, but arguments are offered to explain why one of these geometries is preferred in the presence of DNA.

A study on the flexibility of enzyme active sites

BACKGROUND

A common assumption about enzyme active sites is that their structures are highly conserved to specifically distinguish between closely similar compounds. However, with the discovery of distinct enzymes with similar reaction chemistries, more and more studies discussing the structural flexibility of the active site have been conducted.

RESULTS

Most of the existing works on the flexibility of active sites focuses on a set of pre-selected active sites that were already known to be flexible. This study, on the other hand, proposes an analysis framework composed of a new data collecting strategy, a local structure alignment tool and several physicochemical measures derived from the alignments. The method proposed to identify flexible active sites is highly automated and robust so that more extensive studies will be feasible in the future. The experimental results show the proposed method is (a) consistent with previous works based on manually identified flexible active sites and (b) capable of identifying potentially new flexible active sites.

CONCLUSIONS

This proposed analysis framework and the former analyses on flexibility have their own advantages and disadvantage, depending on the cause of the flexibility. In this regard, this study proposes an alternative that complements previous studies and helps to construct a more comprehensive view of the flexibility of enzyme active sites.

Insights into Hofmeister Mechanisms: Anion and Degassing Effects on the Cloud Point of Dioctanoylphosphatidylcholine/Water Systems

Water dispersions of dioctanoylphosphatidylcholine (diC8PC) exhibit upper consolute curves. How they are affected by some salts and other additives (D2O, urea) has already been explored and the phase separation has been interpreted within the framework of the Blankschtein-Thurston-Benedek (BTB) model. This deduces the chemical potential gain in micellar growth (Deltamu) and the intermicellar interaction coefficient (C) as fitting parameters from the coexistence curves. But, the specific mechanisms that drive such phenomena have remained obscure. To identify these mechanisms, we investigate the effects of a range of anions on the cloud points of diC8PC/H2O systems and extract the phenomenological parameters of the BTB model. We show how these parameters, for micellar growth, i.e., surface, intramolecular interactions, and for interaggregate interactions, i.e., bulk solute effects, are connected to anionic polarizabilities. Nonelectrostatic (NES) quantum mechanical fluctuation (Lifshitz or dispersion) forces missing from conventional theories are then shown to regulate lyotropic Hofmeister effects, both explicitly and implicitly.

Polyethylene glycol-superoxide dismutase, a conjugate in search of exploitation

Without a doubt PEG-SOD has been the enzyme most studied in PEGylation. One can say that it represents the preferred model to assess chemistries for PEG activation, analytical procedures suitable for conjugate characterization, the influence of PEG size in conjugate removal from circulation and elimination of immunogenicity and antigenicity, and the effect of route of administration. The effect of PEG conjugation was studied in vitro and in vivo models in comparison with the free enzyme and the following conclusions may be drawn: (1) At the blood vessel level, PEG-SOD has been shown to provide a greater resistance to oxidant stress, to improve endothelium relaxation and inhibit lipid oxidation. (2) In the heart, PEG-SOD proved to be at least as effective as native SOD in treatment of reperfusion-induced arrhythmias and myocardial ischemia. (3) In the lung, PEG-SOD appeared to be able to reduce oxygen toxicity and E. coli-induced lung injury, but not in the treatment of lung physiopathology associated with endotoxin-induced acute respiratory failure and in the reduction of asbestos-induced cell damage. (4) On cerebral ischemia/reperfusion injuries the effect of PEG-SOD was uncertain, also due to the difficulty of cerebral cell penetration. (5) In kidney and liver ischemia both enzyme forms were found to ameliorate reperfusion damage. In view of so much positive research on PEG-SOD, it is surprising that no approved application in human therapy has been established and approved.

Coordination chemistry of co(II)-bleomycin: its investigation through NMR and molecular dynamics

Previous studies on the coordination chemistry of Co-bleomycin have suggested the secondary amine in beta-aminoalanine, the N5 and N1 nitrogens in the pyrimidine and imidazole rings, respectively, and the amide nitrogen in beta-hydroxyhistidine as equatorial ligands to the cobalt ion. The primary amine in beta-aminoalanine and the carbamoyl group of the mannose have been proposed alternatively as possible axial ligands. The first coordination sphere of Co(II) in Co(II)BLM has been investigated in the present study through the use of NMR and molecular dynamics calculations. The data collected from the NMR experiments are in agreement with the equatorial ligands previously proposed, and also support the participation of the primary amine as an axial ligand. The paramagnetic shifts of the gulose and mannose protons could suggest the latter as a second axial ligand. This possibility was investigated by way of molecular dynamics, with distance restraints derived from the relaxation times measured through NMR. The molecular dynamics results indicate that the most favorable structure is six-coordinate, with the primary amine and either the carbamoyl oxygen or a solvent molecule occupying the axial sites. The analysis of the structures previously derived for HOO-Co(III)-bleomycin and HOO-Co(III)-pepleomycin led us to propose the six-coordinate structure with only endogenous ligands, as the one held in solution by the Co(II) derivative of bleomycin.

Synthesis, Structure, Magnetic Properties, and (1)H NMR Studies of a Moderately Antiferromagnetically Coupled Binuclear Copper(II) Complex

A binuclear Cu(II) complex of [(Cu(2)(HAP)(2)IPA)(OH)(H(2)O)](ClO(4))(2).H(2)O (HAP = 3-amino-1-propanol; IPA = 2-hydroxy-5-methylisophthalaldehyde) has been synthesized and characterized by X-ray crystallography, by solid state magnetic susceptibility, and in solution by (1)H NMR studies. The binuclear copper(II) complex crystallizes in the orthorhombic system, space group Pbcn, a = 27.9972(9) Å, b = 8.8713(3) Å, c = 19.5939(6) Å, and Z = 8. The two copper(II) atoms in this binuclear Cu(II) complex are bridged by the oxygen atoms of the phenolate and hydroxy groups. The axial position at one Cu atom is occupied by a water molecule while another Cu has weak interaction with a perchlorate group. The coordination geometries around the two Cu atoms are distorted square pyramid and square planar. The solid state magnetic susceptibility measurement reveals a moderate antiferromagnetic exchange interaction between the two Cu atoms with a -2J value of 113 +/- 9 cm(-)(1). The variable-temperature (1)H NMR studies in CD(3)CN solution show that the observed relatively sharp hyperfine shifted signals follow a Curie behavior. The exchange coupling constant (-2J) obtained in solution by using chemical shift as a function of temperature also reveals a moderate antiferromagnetic exchange interaction between two Cu(II) ions. An analysis of the relaxation data shows that the reorientational correlation time (tau(c)) is dominated probably by a combination of electronic relaxation time tau(s) and rotational correlation time (tau(r)) due to an exchange-modulated dipolar mechanism for this moderately antiferromagnetically coupled binuclear copper(II) system.

1H NMR spectroscopy of the binuclear Cu(II) active site of Streptomyces antibioticus tyrosinase

The 600 MHz 1H NMR spectrum of tyrosinase (31 kDa) of Streptomyces antibioticus in the oxidized, chloride-bound form is reported. The downfield part of the spectrum (15-55 ppm) exhibits a large number of paramagnetically shifted signals. The paramagnetism is ascribed to a thermally populated triplet state. The signals derive from six histidines binding to the metals through their Nepsilon atoms. There is no evidence for endogenous bridges. The exchange coupling, -2J, amounts to 298 cm(-1). In the absence of chloride the peaks broaden. This is ascribed to a slowing down of the electronic relaxation. The exchange coupling decreases to -2J=103 cm(-1).

Synthesis of Defective Phospholipids

Phospholipids have been synthesized that possess a normal 16-carbon chain plus a "defective" chain only 8 or 12 carbons long and terminated with methoxyl, hydroxyl, or carboxyl groups. In addition, dimeric phospholipids have been prepared in which two phospholipid units are joined at position-1 with chains of 22 or 32 carbons while unconnected chains at position-2 are, once again, short and functionalized. These phospholipids are potentially useful for constructing membranes that contain cavities or irregularities and, therefore, are capable of serving as self-assembled host systems in which drugs and other guest molecules are retained and, perhaps, eventually released.

A spectroscopic characterization of a monomeric analog of copper, zinc superoxide dismutase

A mutated protein of human Cu(II)2Zn(II)2 SOD in which residues Phe50 and Gly51 at the dimer interface were substituted by Glu's, thus producing a monomeric species, has been characterized by electronic absorption spectroscopy, EPR, relaxivity and 1H NMR techniques. Such substitutions and/or accompanying remodeling and exposure of the dimer interface to solvent, alter the geometry of the active site: increases in the axiality of the copper chromophore and the Cu-OH2 distance have been observed. The affinity of both metal binding sites for Co(II) is also altered. The observed NMR parameters of the Co(II) substituted derivative have been interpreted as a function of the decrease of rotational correlation time as a consequence of the lower molecular weight of the mutated protein. Sharper NMR signals are also obtained for the reduced diamagnetic enzyme. Results are consistent with an active site structure similar to that observed for the dimeric analog. Thr137Ile characterized elsewhere. An observed proportional decrease in enzymatic activity and affinity for the N3-anion suggests the importance of electrostatic forces during substrate docking and catalysis.

An essential role for the conserved Glu-133 in the anion interaction with superoxide dismutase

Negatively charged glutamic acid residues at positions 132 and 133 in human Cu2Zn2SOD are located at the entrance to the active site cavity and affect electrostatic interactions with the negatively charged substrate. The mutants in which these residues have been neutralized separately and together by conversion to glutamine residues or changed to a positive group on position 133 have been characterized through a variety of biophysical techniques. The structure around the metal ions, as monitored by spectroscopic measurements, is the same in the mutants and native enzyme. The mutants have been characterized with respect to the affinity for the anion N3-. The mutants have larger affinity for azide than the WT, as a result of the removal of one or two negative charges or of the introduction of a positive group. The pattern of the azide affinity constants parallels that of the rate of O2- dismutation. The substitution of the negative Glu-133 with a positive group does not induce a larger increase in the affinity as well as in the catalytic rates with respect to its neutralization. These patterns cannot, therefore, be rationalized only in terms of electrostatic interactions. The behavior of the mutants towards the substrate (O2-) and substrate analogue (N3-) is discussed on the basis of theoretical predictions available in the literature.

The role of the active site amino acid residues on the catalytic activity of Cu2Zn2SOD

Copper, zinc superoxide dismutase, the enzyme that catalyzes the dismutation of the superoxide ion, is a key enzyme for neurodegenerative processes caused by the formation into tissues of this reactive free radical. Although the structure of enzyme is known, the catalytic mechanism has not yet fully elucidated. The copper ion represents the center, which exchanges electrons with the superoxide ion, but other residues present in the active site seem to play a relevant role. The most recent advancements in clarifying the structure-function relationship in this enzyme are discussed.

Sequential resonance assignments of oxidized high-potential iron-sulfur protein from Chromatium vinosum

2D NMR spectra of the high-potential iron-sulfur protein (HiPIP) from Chromatium vinosum have been used to obtain partial resonance assignments for the oxidized paramagnetic redox state of the protein. Sequence-specific assignments were made using NOESY and COSY spectra in H2O and D2O of the following backbone segments: Asn-5-Arg-33, Glu-39-Asp-45, Gly-55-Cys-63, Gly-68-Ala-78, and Leu-82-Gly-85. NOESY spectra with a spectral width wide enough to include the hyperfine-shifted resonances revealed numerous NOE contacts between these signals and those in the main envelope of the proton spectrum. With the aid of the X-ray crystal structure [Carter, C.W., Kraut, J., Freer, S. T., Xuong, N. H., Alden, R. A., & Bartsch, R. G. (1974) J. Biol. Chem. 249, 4212], these NOEs permitted seven of the nine hyperfine-shifted signals to be assigned to three of the cysteine residues liganded to the metal cluster (Cys-43, Cys-46, and Cys-77). The other two hyperfine-shifted signals produced no detectable NOEs to other resonances in the spectrum and were tentatively assigned to the remaining cysteinyl ligand (Cys-63). These assignments, in conjunction with recent theoretical models of the electronic structure of the Fe4S4 cluster [Noodleman, L. (1988) Inorg. Chem. 27, 3677; Bertini, I., Briganti, F., Luchinat, C., Scozzafava, A., & Sola, M. (1991) J. Am. Chem. Soc. 113, 1237], indicate that the iron atoms coordinated to Cys-63 and Cys-77 are those of the mixed-valence Fe(3+)-Fe2+ pair whereas Cys-43 and Cys-46 are ligands to the Fe(3+)-Fe3+ metal pair.

Assignment of active-site protons in the 1H-NMR spectrum of reduced human Cu/Zn superoxide dismutase

600-MHz 1H-NMR and nuclear Overhauser enhancement spectroscopy (NOESY) spectra in 2H2O and H2O, as well as truncated driven NOE difference spectra in H2O of reduced human Cu(II)2Zn(II)2 superoxide dismutase (Cu/Zn SOD) have been recorded and used to assign the active-site proton signals. A derivative with histidines selectively deuteriated in the C2 position has been used for the detection of the HC2 histidine protons, 16 out of 17 observed signals of the 18 active-site histidine ring protons have been assigned. The results are compared with previous proposals based on more limited data sets. The numerous cross peaks confirm that the structure in solution is essentially similar to the crystallographic data obtained on the oxidized form. Probably this holds also for His63 which in the reduced form is not bridging any more the two metal ions. The effects of azide binding on the exchangeable 1H-NMR signals of the reduced protein are also reported.

NMR evidence for perturbation of the copper coordination sphere upon chemical modification of arginine 141 in bovine Cu,Zn superoxide dismutase

The reaction of the Cu,Co derivative of bovine Cu,Zn superoxide dismutase with phenylglyoxal or butanedione, which are known to inactivate the enzyme by selectively binding to Arg 141, has been studied by 1H NMR. Several 1H NMR lines of the copper-liganding histidine residues were perturbed, reproducing an effect so far observed only in the case of binding of anions to this protein. The room temperature EPR spectrum of the modified Cu,Zn protein was altered very slightly, indicating that the geometry of the copper site was not grossly affected by the modification. NMR and EPR changes were reversed by dialysis in the case of the reversible butanedione adduct. These data show that the coordination of the copper in Cu,Zn superoxide dismutase can be destabilized by modifications occurring at a neighboring but not a metal-liganding residue. It is suggested that part of the NMR effects seen on copper ligands in the case of anion binding are produced by interaction of anions with Arg 141, rather than by direct ligand replacement.

Advances in the understanding of the structure-function relationship in Cu,Zn superoxide dismutase

The structure-function relationship in Cu,Zn superoxide dismutase has been partially elucidated by the combined use of many spectroscopic techniques (electronic spectroscopy, circular dichroism, EPR and NMR) and site-directed mutagenesis techniques. The comparison of the spectroscopic and catalytic properties of various mutants, in which some active site residues have been substituted through site-directed mutagenesis, allowed us to establish that the activity is in general more sensitive to electrostatic effects rather than to steric effects or changes in the copper hydration or coordination geometry.

Mechanism and atomic structure of superoxide dismutase

The active site Cu ion in Cu,Zn superoxide dismutase is alternately oxidized and reduced during the enzymatic dismutation of superoxide to hydrogen peroxide and molecular oxygen. For oxidized Cu,Zn superoxide dismutase, an atomic structure has been determined for the human enzyme at 2.5 A resolution. The resolution of the bovine enzyme structure has been extended to 1.8 A. Atomic resolution data has been collected for reduced and inhibitor-bound Cu,Zn superoxide dismutases, and the interpretation of the electron density difference maps is in progress. The geometry and molecular surfaces of the active sites in these structures, together with biochemical data, suggest a specific model for the enzyme mechanism. Similarities in the active site geometry of the Mn and Fe superoxide dismutases with the Cu,Zn enzyme suggest that dismutation in these enzymes may follow a similar mechanism.

Spectroscopic characterization of polyethyleneglycol modified superoxide dismutase: 1H NMR studies on its Cu2Co2 derivative

Spectroscopic methods have been employed in order to understand the molecular basis of the decrease in enzymatic activity of the antiinflammatory enzyme copper-zinc superoxide dismutase (SOD) following the covalent binding of polyethyleneglycol (PEG) chains to the protein amino-groups. The PEG modification is a general method recently proposed to improve the therapeutic index of enzymes. 1H NMR spectra on the cobalt substituted PEG-modified SOD, Cu2Co2-PEG-SOD, have been recorded. The signals are quite broad with respect to the unmodified enzyme. This has been interpreted on the basis of the effect of molecular weight on the linewidth. The analysis has shown that the histidine hydrogens involved in metal binding at the enzyme active site are the same in both native and PEG-modified SOD. Similarly, circular dichroism and absorption spectra indicate that the overall conformation of the metal clusters is not perturbed upon modification. On the other hand, azide titration shows that the affinity constant of N-3 for SOD is largely reduced upon PEG modification (K = 154 M-1 and 75 M-1 for the native and modified SOD, respectively). These results indicate that the decrease in enzymatic activity upon surface modification with PEG is not caused by a perturbation of the active site geometry, but to a decrease in the channeling of the O2- ion towards the enzyme active site.

Identification of localized redox states in plant-type two-iron ferredoxins using the nuclear Overhauser effect

The homonuclear Overhauser effect (NOE), in conjunction with nonselective spin-lattice relaxation measurements, has been employed to assign the contact-shifted resonances for the reduced form of two typical plant-type two-iron ferredoxins from the algae Spirulina platensis and Porphyra umbilicalis. These results demonstrate that the NOE should have broad general applicability for the assignments and electronic structural elucidation of diverse subclasses of paramagnetic iron-sulfur cluster proteins. NOE connectivities were detected only among sets of resonance exhibiting characteristically different deviations from Curie behavior, providing strong support for the applicability of the spin Hamiltonian formulation for the NMR properties of the antiferromagnetically coupled iron clusters [Dunham, W. R., Palmer, G., Sands, R. H., & Bearden, A. J. (1971) Biochim. Biophys. Acta 253, 373-384; Banci, L., Bertini, I., & Luchinat, C. (1989) Struct. Bonding (in press)]. The geminal beta-methylene protons for the two cysteines bound to the iron(II) center were clearly identified, as well as the C alpha H and one C beta H for each of the cysteines bound to the iron(III). The identification of the iron bound to cysteines 41 and 46 as the iron(II) in the reduced protein was effected on the basis of dipolar contacts between the bound cysteines, as predicted by crystal coordinates of S. platensis Fd [Tsukihara, T., Fukuyama, K., Nakamura, M., Katsube, Y., Tanaka, N., Kakudo, M., Wada, K., Hase, T., & Matsubara, H. (1981) J. Biochem. (Tokyo) 90, 1763-1773].(ABSTRACT TRUNCATED AT 250 WORDS)

Mapping the copper ligands of Cu,Zn superoxide dismutase by nuclear Overhauser enhancement of the isotropically shifted 1H-NMR lines of the Cu,Co derivative

Transient nuclear Overhauser effect (NOE) experiments were made with bovine Cu,Co superoxide dismutase on the hyperfine shifted resonances of protons of the imidazole groups bound to the Cu,Zn cluster of the native enzyme. Assignments of lines based on the observed magnetization transfers were satisfactorily obtained only by altering the arrangement of the ligands around the catalytically active copper shown by X-ray crystallography.

Water in the active cavity of copper/zinc superoxide dismutase. A water 1H-nuclear-magnetic-relaxation-dispersion study

Water 1H-nuclear-magnetic-relaxation-dispersion (NMRD) measurements of solutions of several copper/zinc superoxide dismutase isoenzymes as well as mutants of the human isoenzyme have been performed in order to monitor the presence of exchangeable water at the copper(II) center. The results have been compared with other spectroscopic features of the various derivatives and with their catalytic efficiency. The decrease in the amount of water in the first coordination sphere, detected through NMRD, parallels, in most cases, the increase in the tetragonal nature of the copper ion. On the other hand, the catalytic activity seems unrelated to the presence of water. Most strikingly, the Ile137 mutant of the human isoenzyme, approximately equal to 90% active, has no water in the copper coordination sphere; this is taken as evidence that the electron transfer is not a water-mediated process. The variation in the pH dependence of NMRD data between the wild-type enzyme and the human Ile137 mutant has been found to parallel the variation in the pH dependence of activity.

Active-site modification of superoxide dismutase by H2O2 studied through 1H NMR of the cobalt derivatives

It is known that H2O2 at pH 10, inactivates copper(II)-zinc(II)-SOD although not much information is available on what happens at the ligands coordinated to the two metal ions. We have reinvestigated the system through the electronic and 1H NMR spectra of the cobalt(II) and copper(II)-cobalt(II) derivatives. Such studies indicate that the coordinated residues are maintained although there is evidence of some flexibility of the donor groups. The coordination around copper is slightly more tetragonal. Azide binding to the copper ion does not cause the complete detachment of one of the histidines from the copper coordination sphere, as happens with the untreated enzyme.

An ENDOR study of human and bovine erythrocyte superoxide dismutase: 1H and 14N interactions

Hyperfine interactions (1H and 14N) with the paramagnetic Cu(II)-site obtained from frozen solutions of human and bovine erythrocyte superoxide dismutase (superoxide:superoxide oxidoreductase, EC 1.15.1.1) as well as from their derivatives produced by anion binding (N3-, CN-) and by depletion of the Zn(II) site were studied using electron nuclear double resonance (ENDOR) spectroscopy at about 15 K. Both interactions were found to be identical in human and bovine erythrocyte superoxide dismutase. In all compounds, an anisotropic, exchangeable 1H interaction with a nearly constant coupling value (approximately 3 MHz along g perpendicular ) was observed which is due to either histidine NH- or water protons. Other proton interactions were tentatively assigned to H beta 1 of His-44, H delta 2 of His-46 and to H beta 2 of His-44. Depletion of the Zn(II) site did not alter appreciably the pattern of the proton interactions. The 14N couplings of the native specimen indicated equivalent coordination, whereas Zn(II) depletion and CN- addition were found to produce either some or drastic inequivalences, respectively. For N3- addition to either the native or the Zn(II)-depleted sample only minor effects on the respective 14N coupling pattern were observed.

1H and 31P NMR studies of the binding of low-affinity anions to Cu,Zn superoxide dismutase

Anions that do not coordinate to the catalytically active copper ion of Cu,Zn superoxide dismutase, but still affect the activity of the enzyme by weaker interactions with the protein moiety surrounding the active site (low affinity anions), uniformly perturbed the 1H NMR line of the NH group of the copper ligand His 46. This effect was detected on the enzyme having Co(II) substituted for the native Zn(II), in which the resonances of residues bound to the copper are detected because of the antiferromagnetic coupling between Cu(II) and Co(II). The interaction with the enzyme of phosphate, a good representative of low-affinity anions, was also studied by 31P NMR of the native enzyme and of enzyme samples covalently modified at all lysines or at the Arg 141, which is 5 A away from the copper. The results obtained indicate that Arg 141 is a likely candidate for binding of low-affinity anions in the vicinity of the copper and that the 1H NMR line of His 46 NH is diagnostic for such an interaction.