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D Rehder

Publications and source records attributed to D Rehder.

12 recordsLinked to original sources
PubMed2001

A potentiometric and 51V NMR study of the aqueous H+/H2VO4-/H2O2/L-alpha-alanyl-L-histidine system.

The speciation in the quaternary aqueous H+/H2VO4-/H2O2/L-alpha-alanyl-L-histidine (Ah) system has been determined from quantitative 51V NMR measurements and potentiometric data (glass electrode). The study was performed in 0.150 M Na(Cl) medium at 25 degrees C. Data were evaluated with the computer program LAKE, which is able to treat combined EMF and NMR data. The pKa values for Ah were determined as 8.06, 6.72 and 2.64. In the ternary H+/H2VO4-/Ah system, two complexes, (H+)p(H2VO4-)q(Ah)r, for which (p, q, r) values of (0, 1, 1) and (1, 1, 1) with log beta(0,1,1) = 2.52 +/- 0.03 and log beta(1,1,1) = 9.40 +/- 0.05 (pKa = 6.88), respectively, explain all data. The errors given are 3sigma. In the quaternary H+/H2VO4-/H2O2/Ah system, eight complexes were determined in addition to all binary and ternary complexes, four with a V/X/Ah ratio 1:1:1 and four with a ratio 1:2:1 (X = peroxo ligand). VX2Ah2- and VX2Ah- (pKa = 8.19) are the main complexes and predominate in the pH range 5 to 9. Three additional minor species have also been found but their compositions could not be determined owing to their small amounts. Equilibria are slow, significant decomposition of peroxide occurs only in acidic solutions. Data in the pH range 5 to 10 have been used for the LAKE calculations. Chemical shifts, compositions, and formation constants for the eight quaternary complexes are given, and equilibrium conditions are illustrated in distribution diagrams. Structural proposals for VX2Ah2- and VX2Ah- are made from 1H and 13C NMR measurements.

Dipeptides↗
PubMed2000

Speciation in the vanadate-alanylhistidine-peroxide system.

The speciation in the quaternary aqueous H+-H2VO4(-)-H2O2-L-alpha-alanyl-L-histidine (Ah) system has been determined from quantitative 51V NMR measurements and potentiometric data (glass electrode). The study was performed in 0.150 M Na(Cl) medium at 25 degrees C. Data were evaluated with the computer program LAKE, which is able to treat combined potentiometric and NMR data. In the ternary H+-H2VO4(-)-Ah system, two complexes, (H+)p(H2VO4-)q(Ah)r, having (p, q, r) values (0, 1, 1) and (1, 1, 1) (pKa = 6.88) explain all data. In the quaternary H+-H2VO4(-)-H2O2-Ah system, seven complexes were determined in addition to all binary and ternary complexes, four with a V/X/Ah ratio 1:1:1 and three with a ratio 1:2:1 (X = peroxo ligand). VX2Ah2- and VX2Ah- (pKa = 8.26) are the main quaternary complexes and predominate in the pH range 5 to 9. Chemical shifts, compositions and formation constants for all the quaternary complexes are given, and equilibrium conditions are illustrated in distribution diagrams.

Alanine↗
PubMed2000

Water and bromide in the active center of vanadate-dependent haloperoxidases.

Two aqua-oxovanadium complexes, viz. [A-VO(H2O)(sal-L-Leu)] (1) and [VO(H2O)2(5-Br-sal-Gly)] x H2O(2 x H2O), containing the water ligands in cis- and trans-positions to the oxo group at V-OH2 distances ranging from 2.008 to 2.228 A, have been structurally characterized in order to model the apical electron density feature found in the structures of fungal and algal vanadate-dependent peroxidases. Br K-edge XAS of bromide-treated bromoperoxidase from Ascophyllum nodosum and model compounds (including 2 x H2O) has been used to show that the substrate bromide does not bind to active site vanadium but to a light atom, possibly carbon, in its vicinity.

Binding Sites↗
PubMed2000

Vanadium nitrogenase.

The topic, vanadium nitrogenase, is reviewed with respect to biological characteristics and findings on its structure and functions. Structural models (vanadium complexes containing ligands related to the active center in the iron-vanadium cofactor) and functional models for the reductive protonation of dinitrogen, the activation of alkynes and reductive C-C coupling of isocyanides are addressed.

Binding Sites↗
PubMed1999

Bromine K-edge EXAFS studies of bromide binding to bromoperoxidase from Ascophyllum nodosum.

Bromine K-edge EXAFS studies have been carried out for bromide/peroxidase samples in Tris buffer at pH 8. The results are compared with those of aqueous (Tris-buffered) bromide and vanadium model compounds containing Br-V, Br-C(aliphatic) and Br-C(aromatic) bonds. It is found that bromide does not coordinate to the vanadium centre. Rather, bromine binds covalently to carbon. A possible candidate is active site serine.

Bromides↗
PubMed1992

(Model) studies on vanadate-dependent bromo/iodoperoxidase from Ascophyllum nodosum. VO2+ is not incorporated into the active site.

Vanadate-dependent peroxidase A.n.I, the main isoenzyme (M(r) = 100 kDa) from the seaweed, Ascophyllum nodosum, contains 2 V per enzyme molecule (as shown by ICP-MS metal analysis) after complete reconstitution with vanadate (V), possibly distributed in a 1:1 ratio between the surface and active site. VO2+ is only weakly associated to the surface of A.n.I. There is no transport channel for VO2+. The EPR spectrum of the reduced holoenzyme is anisotropic (axial) already at room temperature, with EPR parameters similar to those of VO2+ complexes of small model peptides such as Ala-His, Gly-Tyr, Gly-Ser, Gly-Glu, Ser-Gly and Phe-Glu. The complex formation between Ala-His and H2VO4- in water has also been investigated (by 51V NMR); the formation constant at pH 7.2 amounts to 266(28) M-1.

Binding Sites↗
PubMed1992

Structure and function of vanadium compounds in living organisms.

Vanadium has been recognized as a metal of biological importance only recently. In this mini-review, its main functions uncovered during the past few years are addressed. These encompass (i) the regulation of phosphate metabolizing enzymes (which is exemplified for the inhibition of ribonucleases by vanadate), (ii) the halogenation of organic compounds by vanadate-dependent non-heme peroxidases from seaweeds, (iii) the reductive protonation of nitrogen (nitrogen fixation) by alternative, i.e. vanadium-containing, nitrogenases from N2-fixing bacteria, (iv) vanadium sequestering by sea squirts (ascidians), and (v) amavadine, a low molecular weight complex of V(IV) accumulated in the fly agaric and related toadstools. The function of vanadium, while still illusive in ascidians and toadstools, begins to be understood in vanadium-enzyme interaction. Investigations into the structure and function of model compounds play an increasingly important role in elucidating the biological significance of vanadium.

Animals↗
PubMed1989

Binding of vanadate (V) to ribonuclease-T1 and inosine, investigated by 51V NMR spectroscopy.

Ribonuclease T1 (RNase-T1) from Aspergillus Oryzae cleaves ribonucleic acid specifically at guanosine to yield oligonucleotides with terminal guanosine-3'-phosphate. It forms a complex with vanadate (association constant K approximately 145 +/- 30 M-1; delta (51V) = -514 ppm) with spectral features similar to the less stable complexes obtained with di- and tripeptides (Gly-His, Pros-His-Ala, Gly-His-Lys, Val-Glu) containing amino acids that are constituents at the active site of the enzyme. Guanosine also forms a (sparingly soluble) complex with vanadate. Its role is mimicked by inosine, which yields two soluble complexes with vanadate, characterized by delta values of -511 (K = 94 M-1) and -523 ppm (K = 305 M-1 in TRIS buffer and 685 m-1 in buffer-free solution). Comparison with literature values leads to an assignment of the delta = -523 signal to a complex where monovanadate, possibly in a trigonal bipyramidal geometry suggested for the transition state of the phosphate analogue, is coordinated to the 2'- and 3'-oxygens of the ribose ring. A drastic increase of complex stability is observed in the ternary vanadate (12-16 mM)/inosine(10.5 mM)/RNase-T1(5.4 mM) system. An approximate lower limit for the association constant is 1.5.10(5) M-2. The spectral characteristics of the main component of the binary vanadate/inosine complex are essentially maintained (delta = -525 ppm, half-width = 960 Hz), suggesting vanadate binding to the enzyme through hydrogen bonds.

Endoribonucleases↗
PubMed1988

Vanadium K-edge absorption spectrum of bromoperoxidase from Ascophyllum nodosum.

With synchrotron radiation from the Bonn 2.5 GeV synchrotron, high-resolution absorption spectra have been measured at the vanadium K-edge of bromoperoxidase from the marine brown alga Ascophyllum nodosum and several model compounds. The near-edge structure (XANES) of these spectra was used to determine the charge state and the coordination geometry around the vanadium atom. For the active enzyme a coordination charge of 2.7 was found which is compatible with a formal valence of +5, assuming coordination by atoms with a high electronegativity such as oxygen or nitrogen. For the reduced enzyme the coordination charge value of 2.15 indicates the reduction of the valency by 1 unit. Our results suggest that the coordination sphere of the vanadium atom in the native enzyme consists of at least seven oxygen atoms in a distorted octahedral environment with an average bond length of about 2 A. Through the reduction process, the coordination sphere of the vanadium atom changes with a simultaneous decrease of the coordination cage. These results agree with those deduced from previous EPR and 51V-NMR measurements.

Eukaryota↗