Spectroscopic identification of a dinuclear metal centre in manganese(II)-activated aminopeptidase P from Escherichia coli: implications for human prolidase

Electron paramagnetic resonance (EPR) spectra and X-ray absorption (EXAFS and XANES) data have been recorded for the manganese enzyme aminopeptidase P (AMPP, PepP protein) from Escherichia coli. The biological function of the protein, a tetramer of 50-kDa subunits, is the hydrolysis of N-terminal Xaa-Pro peptide bonds. Activity assays confirm that the enzyme is activated by treatment with Mn²⁺. The EPR spectrum of Mn²⁺–activated AMPP at liquid-He temperature is characteristic of an exchange-coupled dinuclear Mn(II) site, the Mn-Mn separation calculated from the zero-field splitting D of the quintet state being 3.5?(±0.1)?Å. In the X-ray absorption spectrum of Mn²⁺–activated AMPP at the Mn K edge, the near-edge features are consistent with octahedrally coordinated Mn atoms in oxidation state +2. EXAFS data, limited to k≤12?Å^–1 by traces of Fe in the protein, are consistent with a single coordination shell occupied predominantly by O donor atoms at an average Mn-ligand distance of 2.15?Å, but the possibility of a mixture of O and N donor atoms is not excluded. The Mn-Mn interaction at 3.5?Å is not detected in the EXAFS, probably due to destructive interference from light outer-shell atoms. The biological function, amino acid sequence and metal-ion dependence of E. coli AMPP are closely related to those of human prolidase, an enzyme that specifically cleaves Xaa-Pro dipeptides. Mutations that lead to human prolidase deficiency and clinical symptoms have been identified. Several known inhibitors of prolidase also inhibit AMPP. When these inhibitors are added to Mn²⁺–activated AMPP, the EPR spectrum and EXAFS remain unchanged. It can be inferred that the inhibitors either do not bind directly to the Mn centres, or substitute for existing Mn ligands without a significant change in donor atoms or coordination geometry. The conclusions from the spectroscopic measurements on AMPP have been verified by, and complement, a recent crystal structure analysis.     

Composition,Characteristic and Activity of Rare Earth Element-Bound Polysaccharide from Tea

The compositions and structural characteristics of rare earth elements-bound polysaccharides from tea (REE-TPS) were studied with the methods of Inductively Coupled Plasma Mass Spectrometry (ICP-MS), Gas Chromatography (GC) and Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy. The results show that polysaccharide from tea (TPS) was a sort of glycoprotein and coordinated with Rare Earth Elements (REE) closely. The sugar fraction was composed of Rha, Ara, Xyl, Fuc, Glc, and Gal. There existed almost all natural amino acids with Glx, Asx, and Hyp as the major parts in the protein fraction. The REEs in REE-TPS were mainly composed of La, Ce, and Nd, especially, more than 75% of them was La. The coordination atom of the first coordination shell of La in REE-TPS was oxygen, the coordination number of which was 6, and the average distance between the atoms was 2.52 Å. The second shell was formed from sulfur atoms, the coordination number and the average distance were 3 and 2.91 Å, respectively. The bio-experiments show that REE-TPS could decrease the content of blood glucose in mice significantly.     

An EXAFS study of the zinc sites in sheep liver metallothionein

Measurement and interpretation of the EXAFS associated with the K-absorption edge of zinc atoms in sheep liver metallothionein indicate that the primary coordination shell of each of these metal atoms comprises four sulphur atoms, with the Zn-S distance being 2.29 ± 0.02 Å.     

X-ray absorption spectroscopy of dimethylsulfoxide reductase from Rhodobacter capsulatus

Mo K-edge X-ray absorption spectroscopy (XAS) has been used to probe the environment of Mo in dimethylsulfoxide (DMSO) reductase from Rhodobacter capsulatus in concert with protein crystallographic studies. The oxidised (Mo^VI) protein has been investigated in solution at 77?K; the Mo K-edge position (20006.4?eV) is consistent with the presence of Mo^VI and, in agreement with the protein crystallographic results, the extended X-ray absorption fine structure (EXAFS) is also consistent with a seven-coordinate site. The site is composed of one oxo-group (Mo=O 1.71?Å), four S atoms (considered to arise from the dithiolene groups of the two molybdopterins, two at 2.32?Å and two at 2.47?Å, and two O atoms, one at 1.92?Å (considered to be H-bonded to Trp 116) and one at 2.27?Å (considered to arise from Ser 147). The Mo K-edge XAS recorded for single crystals of oxidised (Mo^VI) DMSO reductase at 77?K showed a close correspondence to the data for the frozen solution but had an inferior signal:noise ratio. The dithionite-reduced form of the enzyme and a unique form of the enzyme produced by the addition of dimethylsulfide (DMS) to the oxidised (Mo^VI) enzyme have essentially identical energies for the Mo K-edge, at 20004.4?eV and 20004.5?eV, respectively; these values, together with the lack of a significant presence of Mo^V in the samples as monitored by EPR spectroscopy, are taken to indicate the presence of Mo^IV. For the dithionite-reduced sample, the Mo K-edge EXAFS indicates a coordination environment for Mo of two O atoms, one at 2.05?Å and one at 2.51?Å, and four S atoms at 2.36?Å. The coordination environment of the Mo in the DMS-reduced form of the enzyme involves three O atoms, one at 1.69?Å, one at 1.91?Å and one at 2.11?Å, plus four S atoms, two at 2.28?Å and two at 2.37?Å. The EXAFS and the protein crystallographic results for the DMS-reduced form of the enzyme are consistent with the formation of the substrate, DMSO, bound to Mo^IV with an Mo-O bond of length 1.92?Å.     

Spectroscopic studies for identifying the chemical states of the periostracum of the Corbicula species in Lake Biwa

Corbicula clam shells consist of thin periostracum and calcareous layers made of calcium carbonate (CaCO₃). Depending on habitat conditions, the shell exhibits various colorations, such as yellow, brown, and black. The chemical state of the periostracum of the Corbicula species in Lake Biwa was studied by X-ray absorption fine structure (XAFS) and Raman scattering spectroscopies. Fe K-edge X-ray absorption near edge structure (XANES) revealed that the Fe³⁺ intensity increases as the color of the shell changes from yellow to black. Raman spectra suggested that quinone-based polymers cover the yellow shell, and the black shell is further covered by dihydroxyphenylalanine (DOPA) rings of amino acid derivatives. From Fe K-edge extended X-ray absorption fine structure (EXAFS), it was found that Fe³⁺ in the periostracum was surrounded by five to six oxygen atoms with an average Fe-O ligand distance of 2.0 Å. Accordingly, a tris-DOPA-Fe³⁺ complex is formed, which is responsible for the periostracum’s black color.     

Synthesis,properties and structure of hexaaquo-tris(N,N-dimethylformamide)-lanthanide trifluoromethanesulfonates

The compounds of general formula [Ln(DMF)₃- (H₂O)₆](CF₃SO₃)₃ (Ln = LaEu, Tb, Dy) were synthesized and characterized by microanalysis, conductance measurements, IR absorption (Nd³⁺) and emission (Eu³⁺) spectra. The crystal structure of the neodymium compound was determined by X-ray diffraction techniques. The compound crystallizes in the triclinic system, space group P1, a = 8.589(4), b = 11.222(2), c = 12.271(2) Å, α = 56.83(2), β = 62.13(2), γ = 75.14(2)°, V = 875.2 ų, M = 918.4, Z = 1, D_c = 1.73 g cm⁻³, λ(Mo Kα) = 0.71073 Å, μ = 1.65 mm⁻¹, F(000) = 456, R = 0.056, R_w = 0.057, for 2979 independent reflections with I > 3σ(I). Nd³⁺ is coordinated to the oxygen atoms of six independent water molecules at a mean distance NdO = 2.52(1) Å, and to the oxygen atoms of three independent DMF groups at a mean distance NdO = 2.40(2) Å. The coordination polyhedron is a tricapped trigonal prism of point symmetry C_3v.     

EXAFS and Mössbauer characterization of the Diiron(III) site in stearoyl-acyl carrier protein Δ9– desaturase

Stearoyl-acyl carrier protein (ACP) Δ⁹-desaturase (Δ9D) from the castor plant is the best characterized soluble acyl-ACP desaturase. This enzyme utilizes a diiron center to catalyze the O₂- and NADPH-dependent introduction of a cis double bond between carbons 9 and 10 of stearoyl-ACP, yielding oleoyl-ACP. In the present study, we have used X-ray absorption spectroscopy to provide the first metrical information for the diferric oxidation state. These studies reveal distinct diiron clusters that have Fe-Fe distances of either 3.12 or 3.41?Å. The species having the 3.12?Å Fe-Fe distance also exhibits a 1.8?Å Fe-O bond and is thus proposed to represent Δ9D molecules containing a (μ-oxo)bis(μ-carboxylato)diiron(III) cluster. The species having the 3.41?Å Fe-Fe distance exhibits no short Fe-O bond, and thus likely represents Δ9D molecules containing a (μ-hydroxo)diiron(III) cluster. Mössbauer studies of the extended X-ray absorption fine structure (EXAFS) samples revealed three quadrupole doublets (ΔE _Q(1)=1.53?mm/s, 72%;ΔE _Q(2)=0.72?mm/s, 21%;ΔE _Q(3)=2.20?mm/s, 7%) that originate from three distinct dinuclear clusters. From analysis of spectral intensities and by comparison with previous studies of (μ-oxo)- and (μ-hydroxo)diiron(III) clusters in both model complexes and proteins, doublet 1, the Mössbauer majority species, is likely associated with the EXAFS majority species having a 3.12?Å Fe-Fe separation and a 1.8?Å Fe-μ-oxo bond, while doublet 2 likely results from one iron site (or both) of a cluster associated with the EXAFS species having a 3.41?Å Fe-Fe separation. The presence of multiple diiron center conformations in diferric Δ9D may reflect the necessity for the active site to allow access of the substrate stearoyl-ACP (～9?kDa) during desaturation catalysis.     

Mercury(II) complex formation with glutathione in alkaline aqueous solution

The structure and speciation of the complexes formed between mercury(II) ions and glutathione (GSH = L-glutamyl-L-cysteinyl-glycine) have been studied for a series of alkaline aqueous solutions (\( C_{{{\text{Hg}}^{{2 + }}}}\,{\sim18\,{\rm{mmol}}\,{\rm{{dm^{-3}}}}}\) and C _GSH = 40–200 mmol dm^?3 at pH ～10.5) by means of extended X-ray absorption fine structure (EXAFS) and ¹⁹⁹Hg NMR spectroscopy at ambient temperature. The dominant complexes are [Hg(GS)₂]^4? and [Hg(GS)₃]^7?, with mean Hg–S bond distances of 2.32(1) and 2.42(2) Å observed in digonal and trigonal Hg–S coordination, respectively. The proportions of the Hg²⁺–glutathione complexes were evaluated by fitting linear combinations of model EXAFS oscillations representing each species to the experimental EXAFS spectra. The [Hg(GS)₄]^10? complex, with four sulfur atoms coordinated at a mean Hg–S bond distance of 2.52(2) Å, is present in minor amounts (<30%) in solutions containing a large excess of glutathione (C _GSH ≥ 160 mmol dm^?3). Comparable alkaline mercury(II) cysteine (H₂Cys) solutions were also investigated and a reduced tendency to form higher complexes was observed, because the deprotonated amino group of Cys^2? allows the stable [Hg(S,N-Cys)₂]^2? chelate to form. The effect of temperature on the distribution of the Hg²⁺–glutathione complexes was studied by comparing the EXAFS spectra at ambient temperature and at 25 K of a series of glycerol/water (33/67, v/v) frozen glasses with \( C_{{{\text{Hg}}^{{2 + }} }} \,{\sim7\,{\rm{mmol}}\,{\rm{{dm^{-3}}}}} \) and C _GSH = 16–81 mmol dm^?3. Complexes with high Hg–S coordination numbers, [Hg(GS)₃]^7? and [Hg(GS)₄]^10?, became strongly favored when just a moderate excess of glutathione (C _GSH ≥28 mmol dm^?3) was used in the glassy samples, as expected for a stepwise exothermic bond formation. Addition of glycerol had no effect on the Hg(II)–glutathione speciation, as shown by the similarity of the EXAFS spectra obtained at room temperature for two parallel series of Hg(II)-glutathione solutions with \( C_{{{\text{Hg}}^{{2 + }} }} \,{\sim7\,{\rm{mmol}}\,{\rm{{dm^{-3}}}}},\) with and without 33% glycerol. Also, the ¹⁹⁹Hg NMR chemical shifts of a series of ～18 mmol dm^?3 mercury(II) glutathione solutions with 33% glycerol were not significantly different from those of the corresponding series in aqueous solution.     

Effects of ionization on stability of 1-methylcytosine — DFT and PCM studies

This report present the results of natural energy decomposition analysis (NEDA), natural bond orbital (NBO), and quantum theory of atoms in molecules (QTAIM) calculations of three derivatives of biphenyl-1-aza-18-crown-6 ether and their 1:1 complexes with Cd²⁺. All calculations used the B3LYP density functional theory in combination with the 6-311G and WTBS basis sets for ligands and Cd²⁺ ion, respectively. Ligands 1 and 3 have a single 1-aza-18-crown-6, substituent; ligand 2 has two such substituents. The results show that, in the optimized geometries of the complexes, the distance between N and Cd²⁺ is greater than the distance between O and Cd²⁺. NBO and QTAIM data confirm these results. There was no stabilization energy or bond critical point for N · · · Cd²⁺ in NBO or QTAIM, respectively. Data show that the O · · · Cd²⁺ interaction is a kind of closed shell interaction. The trend of the calculated stabilization energy was similar to the experimental data. Different contributions of interaction energies for complex formation were analyzed by NEDA, and the results show that the main component of the interactions is accounted for by polarization.

     

Protein structure refinement: Streptomyces griseus serine protease A at 1.8 A resolution.

The crystal structure of the bacterial serine protease from Streptomyces griseus (SGPA) has been refined at 1.8 Å resolution by a restrained parameter least-squares procedure (Konnert, 1976) to a conventional R factor of 0.139 for 12662 statistically significant reflections [I > 3σ(I)]. The number of variable parameters in the final model was 5912 which included positional and individual thermal parameters of the enzyme, and positions, B factors and occupancies of 175 solvent molecules. The algorithm used for this refinement allows for the simultaneous restraint on bond distances and distances related to interbond angles, the coplanarity of atoms in planar groups, the conservation of chirality of asymmetric centres, non-bonded contact distances, conformational torsional angles and individual isotropic temperature factors.The refined structure of SGPA differs from ideal bond lengths by an overall root-mean-square deviation of 0.02 Å; the corresponding value for angle distances is 0.038 Å. Comparison of the phase angles for the shell of data, 8.0 to 2.8 Å, between the multiple isomorphous replacement phases (Brayer et al., 1978a) and the refined phases, indicates an overall difference (r.m.s.) of 56.6 °. The average conformational angle of the peptide bond (ω) is 179.7 ° (root-mean-square deviation ± 2.5 °) for the 180 peptide bonds of SGPA. Of the 175 solvent molecules included during the course of the refinement, 22 with occupancies ranging from 1.00 to 0.38 are located in the active site and the substrate binding region. It was not until these water molecules were included in the refinement process that the active Ser195 adopted its final conformation (χ¹ = ?77 °). The resulting distance from O^γ of Ser195 to N^ε2 of His57 is 3.1 Å, which, when taken with the observed distortion from linearity (50 °), indicates a rather weak interaction.     

Diaquabis(3,6-dioxaheptanoato)copper(II): crystal structure and EPR characteristics

The structure of the title compound has been determined by X-ray diffraction at 190 K. The complex has an all trans configuration with an elongated tetragonally distorted octahedral CuO6 chromophore. The elongated axis corresponding to the trans-Cu–O(ether) bonds. The ligand molecules are bidentate via the carboxyl and the 3-ether O atoms; the 6-ether O atoms are not coordinated and are remote from the Cu centres. The bond lengths to the Cu centres are Cu–O(ether) 2.355 Å, Cu–O(Carboxyl) 1.933 Å and Cu–OH2 1.995 Å.The EPR spectrum of both the powder and frozen solution forms is typical of a rhombic system with a dx2−y21 electronic configuration. There were no significant differences in spectra recorded over the temperature range 77 K to room temperature. These results are discussed in relation to earlier published results on closely related oxa-carboxyl complexes.     

Crystal Structure of Organolanthanide Complexes [Li(THF)2]2 (μt-Cl)4(η5-C5 H5 )Ln·THF (Ln = La,Nd)

The crystal and molecular structures of the title compounds have been determined from single crystal X-ray diffraction. The two complexes crystallize in the orthorhombic space group Pna2₁ with Z = 4. Lattice parameters are: a = 10.504(2) [10.522(2)], ¹b = 16.816(4) [16.927(3)] and c = 18.931(4) [18.969(3)] Å. The two crystals are isomorphous. The structures were solved by Patterson and Fourier techniques and refined by least-squares techniques to R = 0.0430 for 1508 reflections. The Nd³⁺. ion is eight-coordinate, being bonded to five carbons of the cyclopentadiene ring, to four chloride atoms and to the one oxygen atom of the THF ring. The NdC distances are in the range 2.67–2.85 Å (average: 2.77 Å) and Nd-Cl distances are in the range 2.76–2.80 Å (average: 2.78 Å). The Nd-O distance is 2.52 A. The Li⁺ ion is four-coordinate, being bonded to the two chloride atoms and to the two oxygen atoms of the two THF rings. The other Li⁺ ion is the same as the above. The Li-Cl distances are in the range 2.17– 2.55 Å (average: 2.35 A) and LiO distances are in the range 1.89–1.98 Å (average: 1.91 Å). The Nd atom and the two Li atoms are bridged asymmetrically by the chloride ions, respectively.     

Mechanistic Studies on Metal-pyrophosphate Hydrolysis. Structure of Tetraammine(chloroaquo)cobait(III) dichloride ([CO(NH3)4ClH20]2+•-2Cl−), an Acid Hydrolysis Product of Co(NH3)4HP2O7 and Co(NH3)4PO4

Abstract

The octahedral complex tetraammine(chloroaquo)cobalt(III) dichloride is shown to be the HCl hydrolysis product of both P¹,^2-bidentate tetraammine(pyrophosphato)cobalt(III) [CO(NH₃)₄HP₂0₇ or CoPP] and bidentate tetraammine(phosphato)cobalt(III) [Co(NH₃)₄P0₄or CoP]. The complex crystallizes in the orthorhombic space group Pna2¹ with cell dimensions α=13.033(2)Å, b=6.710(1) Å, and c=10.318(2)Å; the crystal structure was refined to a final disagreement index of 0.033. The average of the four Co-N distances is 1.944±6Å. The Co-Cl distance is 2.257(2)Å and the Co-O(W) distance is 1.971(4)Å. Both protons of the coordinated water molecule are engaged in strong hydrogen bonds to the two nonbonded chloride counterions with 0(W)-C1 distances of 3.087(6)Å and 3.123(6)Å. Each nonbonded chloride is engaged in seven hydrogen bonding interactions resulting from the high ratio of hydrogen bond donors to acceptors in the CoP structure. Cobalt bisphosphate (CoP₂) is the final enzyme hydrolysis product when CoPP is used as substrate in the yeast inorganic pyrophosphatase reaction. The bridge oxygen atom is the site of initial CoPP cleavage both, for HCl catalyzed hydrolysis as well as for enzyme catalyzed hydrolysis.     

The reconstruction of side-chain conformations from protein Cα coordinates

A model of nine proteins including side-chain atoms have been built from the known C^α coordinates and amino acid sequences using a Monte Carlo Protein Building Annealing method. The Cartesian coordinates for the side-chain atoms were established with bond lengths and angles selected randomly from within previously determined ranges. A simulated annealing technique is used to generate some 300 structures with differing side-chain conformations. The atomic coordinates of the backbone atoms are fixed during the simulated annealing process. The coordinates of the side-chain atoms of 300 low energy conformations are averaged to obtain a mean structure that is minimized with the C^α atoms constrained to their position in the x-ray structure using the OPLS/AMBER force field with the GB/SA water model. The rms deviation of the main-chain atoms (without C^β) compared with the corresponding crystal structures is in the range 0.20–0.64 Å. The rms deviation of the side-chain atoms is between 1.72 and 2.71 Å and for all atoms is between 1.19 and 1.99 Å. The method is insensitive to random errors in the C^α positions and the computational requirement is modest. © 1997 John Wiley & Sons, Inc.     

Mechanism of lanthanum effect on chlorophyll of spinach

The mechanism of La3+ effect on chlorophyll (chl) of spinach in solution culture has been studied. The results show that La3+ can obviously promote growth, increase chlorophyll contents and photosynthetic rate of spinach. La3+ may substitute Mg2+ for chlorophyll formation of spinach when there is no Mg2+ in solution. La3+ improves significantly PSII formation and enhances electron transport rate of PSII. By ICP-MS and atom absorption spectroscopy methods, it has been revealed that rare earth elements (REEs) can enter chloroplasts and increase Mg2+-chl contents; and REEs bind to chlorophyll and also form REE-chl. REE-chl is about 72% in total chlorophyll with La3+ treatment and without Mg2+ in solution. By UV-Vis, FT-IR and extended X-ray absorption fine structure spectroscopy (EXAFS) methods, it has been found that La3+ coordinates with nitrogen of porphyrin rings with the average La-N bond length of 0.253 nm.     

Mechanism of lanthanum effect on chlorophyll of spinach

The mechanism of La3+ effect on chlorophyll (chl) of spinach in solution culture has been studied. The results show that La3+ can obviously promote growth, increase chlorophyll contents and photosynthetic rate of spinach. La3+ may substitute Mg2+ for chlorophyll formation of spinach when there is no Mg2+ in solution. La3+ improves significantly PSII formation and enhances electron transport rate of PSII. By ICP-MS and atom absorption spectroscopy methods, it has been revealed that rare earth elements (REEs) can enter chloroplasts and increase Mg2+-chl contents; and REEs bind to chlorophyll and also form REE-chl. REE-chl is about 72% in total chlorophyll with La3+ treatment and without Mg2+in solution. By UV-Vis, FT-IR and extended X-ray absorption fine structure spectroscopy (EXAFS) methods, it has been found that La3+ coordinates with nitrogen of porphyrin rings with the average La-N bond length of 0.253 nm.     

Water structure in a protein crystal: rubredoxin at 1.2 A resolution

The model for rubredoxin based on X-ray diffraction data has been extensively refined with a 1.2 Å resolution data set. Water oxygen atoms were deleted from the model if B exceeded 50 Ų and occupancy was less than 0.3 eÅ^?3. The final water model consists of 127 sites with B values ranging from 15 to $\text{6}\text{?}$0 Ų and occupancies from unity down to 0.3, the most tightly bound water oxygen atoms being hydrogen bonded to two or more main-chain nitrogen or oxygen atoms. The water forms extensive hydrogen bond networks bridging the crevices on the molecular surfaces and between adjacent molecules. The minimum distances of the water sites from the protein surface are distributed about two distinct maxima, the major one at 2.5 to 3 Å and a minor one at 4 to 4.5 Å. Beyond $\text{5}\text{?}$ to 6 Å from the protein surface, the discrete water merges into the aqueous continuum.     

Structure of iron saturated C‐lobe of bovine lactoferrin at pH 6.8 indicates a weakening of iron coordination

The bilobal lactoferrin is an approximately 76 kDa glycoprotein. It sequesters two Fe³⁺ ions together with two ions. The C‐terminal half (residues, Tyr342–Arg689, C‐lobe) of bovine lactoferrin (BLF) (residues Ala1–Arg689) was prepared by limited proteolysis using trypsin. Both C‐lobe and intact BLF were saturated to 100%. Both of them retained up to nearly 85% of iron at pH 6.5. At pH 5.0, C‐lobe retained 75% of iron whereas intact protein could retain only slightly more than 60%. At pH 4.0 both contained 25% iron and at pH 2.0 they were left with iron concentration of only 10%. The structure of iron saturated C‐lobe was determined at 2.79 Å resolution and refined to R_cryst and R_free factors of 0.205 and 0.273, respectively. The structure contains two crystallographically independent molecules, A and B. They were found to have identical structures with an r.m.s. shift of 0.5 Å for their C^α atoms. A high solvent content of 66% was observed in the crystals. The average value of an overall B‐factor was 68.0 Ų. The distance of 2.9 Å observed for the coordination bond between Fe³⁺ ion and N^e2 of His595 appeared to be considerably longer than the normally observed values of 1.9–2.2 Å. This indicated that the coordination bond involving His595 may be absent. Other coordination distances were observed in the range of 2.1–2.3 Å. Based on the present structure of iron saturated C‐lobe, it may be stated that His595 is the first residue to dissociate from ferric ion when the pH is lowered. Proteins 2016; 84:591–599. © 2016 Wiley Periodicals, Inc.     

Composition, characteristic and activity of rare earth element-bound polysaccharide from tea.

The compositions and structural characteristics of rare earth elements-bound polysaccharides from tea (REE-TPS) were studied with the methods of Inductively Coupled Plasma Mass Spectrometry (ICP-MS), Gas Chromatography (GC) and Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy. The results show that polysaccharide from tea (TPS) was a sort of glycoprotein and coordinated with Rare Earth Elements (REE) closely. The sugar fraction was composed of Rha, Ara, Xyl, Fuc, Glc, and Gal. There existed almost all natural amino acids with Glx, Asx, and Hyp as the major parts in the protein fraction. The REEs in REE-TPS were mainly composed of La, Ce, and Nd, especially, more than 75% of them was La. The coordination atom of the first coordination shell of La in REE-TPS was oxygen, the coordination number of which was 6, and the average distance between the atoms was 2.52 A. The second shell was formed from sulfur atoms, the coordination number and the average distance were 3 and 2.91 A, respectively. The bio-experiments show that REE-TPS could decrease the content of blood glucose in mice significantly.     

Binding of VIVO2+ to the Fe binding sites of human serum transferrin. A theoretical study

The binding of V^IVO²⁺ to human serum transferrin (hTF) at the Fe^III binding sites is addressed. Geometry optimization calculations were performed for the binding of V^IVO²⁺ to the N-terminal lobe of hTF (hTF_N), and indicate that in the presence of CO₃ ^2? or HCO₃ ^?, V^IV is bound to five atoms in a distorted geometry. The structures of V^IVO–hTF_N species optimized at the semiempirical level were also used to calculate the ⁵¹V and ¹⁴N A tensors by density functional theory methods, and were compared with the reported experimental values. Globally, of all the calculated V^IVO–hTF structures, the one that yields the lowest calculated heats of formation and minimum deviations from the experimental values of the ⁵¹V and ¹⁴N A tensor components is the structure that includes CO₃ ^2? as a synergistic anion. In this structure the V=O bond length is approximately 1.6 Å, and the vanadium atom is also coordinated to the phenolate oxygen atom of Tyr188 (at approximately 1.9 Å), the aspartate oxygen atom of Asp63 (at approximately 1.9 Å), the His249 Nτ atom (at approximately 2.1 Å), and a carbonate oxygen atom (at approximately 1.8 Å). The Tyr95 phenolic ocygen atom is approximately 3.3 Å from the metal center, and thus is very weakly bound to V^IV. All of these oxygen atoms are able to establish dipolar interactions with groups of the protein.