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

Novel catalytic potential of a hyperthermostable mono‑copper oxidase (LPMO-AOAA17) for the oxidation of lignin monomers and depolymerisation of lignin dimer in aqueous media

Lytic polysaccharide monooxygenase (LPMO) are mono‑copper enzymes known for the oxidative cleavage of recalcitrant polysaccharides with their intriguing and unique catalytic chemistry. Such impeccable oxidation potential has made them highly valuable in the enzymatic consortia for the degradation of ligno-cellulosic biomass. Bioinformatic analysis has revealed an unannotated LPMO gene in the genome of A. oryzae. Multiple sequence alignment showed the presence of conserved "histidine brace" of LPMO in the amino acid sequence of the enzyme. The enzyme, named as LPMO-AOAA17 was recombinantly expressed in E. coli BL21 and characterised for its substrate specificity. Recombinant enzyme did not show any characteristic cleavage of polysaccharides. However, it was found to be oxidising broad range of phenolic and non-phenolic monomers of lignin. Biochemical study revealed the optimum activity of LPMO-AOAA17 at pH 7 and was highly stable and active at 100 °C. The enzyme LPMO-AOAA17 was also observed to be stable after autoclaving at 121 °C and 15 psi. Thermal stability of the LPMO-AOAA17 was further confirmed through differential scanning calorimetry. GC-MS analysis has confirmed the catalysis of LPMO-AOAA17 for the depolymerisation of lignin dimer, guaicyl glycerol β-guaicyl ether into guaiacol. This study has first time documented the identification of a hyperthermostable LPMO for oxidative cleavage of β-O-4 linkage of lignin compounds to form aromatic products in aqueous media.

Synthesis, characterization and xanthine oxidase inhibition of Cu(II)-chrysin complex

Xanthine oxidase (XO) is a key enzyme catalyzing hypoxanthine to xanthine and then uric acid causing hyperuricemia. A Cu(II) complex of chrysin was synthesized and characterized by UV-vis absorption, Fourier transform infrared, nuclear magnetic resonance (¹H NMR) and mass spectroscopy studies. The interaction of Cu(II)-complex with XO was investigated by spectroscopic methods and molecular simulation. The Cu(II)-chrysin complex exhibited a better inhibitory ability (IC₅₀=0.82±0.034μM) against XO than its corresponding ligands chrysin and Cu²⁺ in a mix-competitive manner. The binding affinity of Cu(II)-chrysin complex with XO was much higher than that of chrysin. The hydrogen bonds and van der Waals forces played main roles in the binding. Analysis of circular dichroism spectra indicated that the complex induced the conformational change of XO. The molecular simulation found that the Cu(II)-chrysin complex inserted into the active cavity of XO with Cu acting as a bridge, occupying the catalytic center of the enzyme to avoid entry of the substrate xanthine, leading to the inhibition of XO. This study may provide new insights into the inhibition mechanism of the Cu(II)-chrysin complex as a promising XO inhibitor and its potential application for the treatment of hyperuricemia.

Blue copper protein analogue: synthesis and characterization of copper complexes of the N2S2 macrocycle 1,8-dithia-4,11-diazacyclotetradecane

To improve understanding of copper at the active site of Type 1 copper proteins, Cu(I) and Cu(II) complexes of 1,8-dithia-4,11-diazacyclotetradecane, shown in , have been successfully isolated and structurally characterized by X-ray crystallography. In these compounds, both Cu(I) and Cu(II) are centered in the plane of the macrocycle containing two sulphur and two nitrogen heteroatoms comprising the distorted tetrahedral/square planar coordination geometry. The UV/VIS spectra, electrochemistry and EPR properties have been obtained for the Cu(II) complex 2. Three absorption bands at 295 nm, 354 nm, and 545 nm are observed in aqueous solution at a pH of 5. These bands have been assigned to the N → Cu(II) and S → Cu(II) charge transfer bands and the d-d transitions respectively. The Cu(I/II) redox midpoint potential of complex 2 in CH3CN is +403 mV versus NHE.

Spectroscopic, thermodynamic, kinetic studies and oxidase/antioxidant biomimetic catalytic activities of tris(3,5-dimethylpyrazolyl)borate Cu(II) complexes

A series of copper(ii) complexes, viz. [Tp(MeMe)Cu(Cl)(H2O)] (), [Tp(MeMe)Cu(OAc)(H2O)] (), [Tp(MeMe)Cu(NO3)] () and [Tp(MeMe)Cu(ClO4)] () containing tris(3,5-dimethylpyrazolyl)borate (KTp(MeMe)), have been synthesized and fully characterized. The substitution reaction of with thiourea was studied under pseudo-first-order conditions as a function of concentration, temperature and pressure in methanol and acetonitrile as solvents. Two reaction steps that both depended on the nucleophile concentration were observed for both solvents. Substitution of coordinated methanol is about 40 times faster than the substitution of chloride. In acetonitrile, the rate constant for the displacement of coordinated acetonitrile was more than 20 times faster than the substitution of chloride. The reported activation parameters indicate that both reaction steps follow a dissociative mechanism in both solvents. On going from methanol to acetonitrile, the rate constant for the displacement of the solvent becomes more than 200 times faster due to the more labile acetonitrile, but the substitution mechanism remained to have a dissociative character. The antioxidant activities of were evaluated for superoxide dismutase (SOD), glutathione-s-transferase (GST0 and glutathione reduced (GSH-Rd) activity. and were found to show (p < 0.05) the highest antioxidant activity in comparison to and , which can be ascribed to the geometric configuration as well as the nature of the co-ligand. showed catechol oxidase activity with turnover numbers of 20 min(-1) and a coordination affinity for 3,5-DTBC of K1, = 31 mM(-1). K1 is rather large and seems to be typical for faster biomimetic models, and also for the enzyme itself (25 mM(-1)). The reaction rate depended linearly on the complex concentration, indicating a first-order dependence on the catalyst concentration.

(N,N-diethyldithiocarbamato)[tris(3,5-dimethylpyrazol-1-yl)hydroborato]copper(II): a new copper(II) dithiocarbamate compound with the classic Tp(Me2) scorpionate

The title complex, [Cu(C5H10NS2)(C15H22BN6)] or (Tp(Me2))Cu(S2CNEt2), incorporating the classic Tp(Me2) scorpionate, is relevant to blue copper protein models and to Cu extraction from waste treatment and mine-tailing leachate. The IR and UV-Vis spectra are consistent with the crystal structure.

Characterization of the blue copper site in oxidized azurin by extended x-ray absorption fine structure: Determination of a short Cu-S distance

The primary coordination environment of the "blue" copper ion in oxidized azurin has been elucidated by x-ray absorption spectroscopy. The most striking feature is the unambiguous presence of a very short copper-sulfur distance at 2.10 +/- 0.02 A. Nitrogen ligands, presumed to be from imidazoles, are found at 1.97 A. There is some evidence that the copper coordination sphere may be completed by a second sulfur, the distance of which is determined with much less certainty.

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.

Catalytic and spectroscopic characterisation of a copper-substituted alcohol dehydrogenase from yeast

Yeast alcohol dehydrogenase (Y-ADH) is widely studied for its biotechnological importance and various attempts to improve its catalytic properties have been made. In this paper, a catalytically active metal-substituted Y-ADH was prepared in vitro by substituting one zinc atom with copper. EPR and Raman spectroscopy suggest that copper maintains the same co-ordination geometry as zinc in native Y-ADH. The active Cu-ADH shows lower substrate affinity and lower specific activity (SA) than native ADH, but greater than a previously obtained Co-ADH. Furthermore, Cu-ADH maintains its catalytic efficiency in a wider pH range than native enzyme.

Interaction of Cu2+ ion with milk xanthine oxidase

The interaction of Cu2+ ion with milk xanthine oxidase (XO) has been studied by optical spectroscopy, circular dichroism, ESR and transient kinetic techniques. It is observed that XO forms optically observable complexes with Cu2+ ion. The pH dependence studies of the formation of Cu2+-XO complex by optical spectroscopy and circular dichroism show that at least one ionizable group may be responsible for the formation of the complex. The EPR studies show that Cu2+ ion binds to XO with sulfur and nitrogenous ligands. The transient kinetic study of the interaction of Cu2+ with XO shows the existence of two Cu2+ bound XO complexes formed at two different time scales of the interaction, one at < or =5 ms and the other one at around 20 s. The complex formed at longer time scale may be responsible for the inhibition of the enzyme activity.

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.

Design of metal ion binding peptides

Two cyclic and branched peptides (PLA and AZU) were synthesized with the aim of reproducing the active site of the blue copper proteins plastocyanin and azurin. Both peptides, designed on the basis of the x-ray structures of Poplar plastocyanin and Alcaligenes denitrificans azurin, contain the same coordinating residues of the parent native proteins. The visible spectra of PLA in the presence of equimolar amount of Cu(II) strongly support the interaction between the peptide and copper(II) ion. The CD titration of AZU with the Hg(II) ion indicates for the formation of two species, [AZUHg]+ and [AZUHg2]3+ having binding constants (Keq) of 3.10(6) and 2.10(4) M-1, respectively.

Stellacyanin. Studies of the metal-binding site using x-ray absorption spectroscopy

Stellacyanin is a mucoprotein of molecular weight approximately 20,000 containing one copper atom in a blue or type I site. The metal ion can exist in both the Cu(II) and Cu(I) redox states. The metal binding site in plastocyanin, another blue copper protein, contains one cysteinyl, one methionyl, and two imidazoyl residues (Colman et al. 1978. Nature [Lond.]. 272:319-324.), but an exactly analogous site cannot exist in stellacyanin as it lacks methionine. The copper coordination in stellacyanin has been studied by x-ray edge absorption and extended x-ray absorption fine structure (EXAFS) analysis. A new, very conservative data analysis procedure has been introduced, which suggests that the there are two nitrogen atoms in the first coordination shell of the oxidized [Cu(II)] protein and one in the reduced [Cu(I)] protein; these N atoms have normal Cu--N distances: 1.95-2.05 A. In both redox states there are either one or two sulfur atoms coordinating the copper, the exact number being indeterminable from the present data. In the oxidized state the Cu--S distance is intermediate between the short bond found in plastocyanin and those found in near tetragonal copper model compounds. Above -140 degree C, radiation damage of the protein occurs. At room temperature the oxidized proteins is modified in the x-ray beam at a rate of 0.25%/s.

Modeling coordination sites in metallobiomolecules

Synthetic metal complexes can closely approach the properties of metal ions in proteins and yield useful information concerning biological structure and function.

Resonance Raman spectroscopy in biochemistry and biology

Resonance Raman (RR) spectroscopy provides detailed information on the vibrational and electronic properties of biochemical and biological chromophores. The analysis of RR spectra, using for example model compounds or a group frequency approach, enables us to form an accurate structural picture of the chromophore in its natural biological site. Moreover, the insight gained into the electronic states of a biological chromophore can be crucial to our understanding of its function. Thus the RR technique represents a powerful means of eliciting precise structural and electronic data from a coloured species and of focusing upon key aspects of its function. It has even been possible to obtain RR spectra from some natural chromophores invivo, giving spectra detailed and informative enough to please a spectroscopist from a system complex enough to satisfy a biologist.