The equilibria in the systems VO2+ + L (L = Gly-L-Asp, L-Asp-Gly, N-acetyl-L-aspartic acid or succinic acid) have been studied at 25 degrees C and 0.2 mol dm(3) K(CI) medium by a combination of potentiometric and spectroscopic methods (ESR, circular dichroism and visible absorption). Formation constants were calculated from pH-metric data with total oxovanadium(Iv) concentrations of(0.6-4) x 10(-3) mol dm(-3) and ligand-to-metal (L:M) ratios of 2-8 (AspGly) or 4-15: 1 (other systems). The position of the Asp residue in the peptide chain affects the co-ordination mode of the ligands: while in the GlyAsp system bis complexes start to form at pH less than 2, for AspGly only 1 : 1 complexes form, with relatively high CD signal. The co-ordination behaviour of N-acetyl-L-aspartic and succinic acids is similar. The results of potentiometric and spectroscopic methods are self consistent. Isomeric structures are discussed for each stoichiometry proposed and the results compared with those for L-aspartic acid and dipeptides with non-coordinating side chains.

According to the model proposed in previous papers [Pettigrew, G. W., Prazeres, S., Costa, C., Palma, N., Krippahl, L., and Moura, J. J. (1999) The structure of an electron-transfer complex containing a cytochrome c and a peroxidase, J. Biol. Chem. 274, 11383-11389; Pettigrew, G. W., Goodhew, C. F., Cooper, A., Nutley, M., Jumel, K., and Harding, S. E. (2003) Electron transfer complexes of cytochrome c peroxidase from Paracoccus denitrificans, Biochemistry 42, 2046-2055], cytochrome c peroxidase of Paracoccus denitrificans can accommodate horse cytochrome c and Paracoccus cytochrome c(550) at different sites on its molecular surface. Here we use (1)H NMR spectroscopy, analytical ultracentrifugation, molecular docking simulation, and microcalorimetry to investigate whether these small cytochromes can be accommodated simultaneously in the formation of a ternary complex. The pattern of perturbation of heme methyl and methionine methyl resonances in binary and ternary solutions shows that a ternary complex can be formed, and this is confirmed by the increase in the sedimentation coefficient upon addition of horse cytochrome c to a solution in which cytochrome c(550) fully occupies its binding site on cytochrome c peroxidase. Docking experiments in which favored binary solutions of cytochrome c(550) bound to cytochrome c peroxidase act as targets for horse cytochrome c and the reciprocal experiments in which favored binary solutions of horse cytochrome c bound to cytochrome c peroxidase act as targets for cytochrome c(550) show that the enzyme can accommodate both cytochromes at the same time on adjacent sites. Microcalorimetric titrations are difficult to interpret but are consistent with a weakened binding of horse cytochrome c to a binary complex of cytochrome c peroxidase and cytochrome c(550) and binding of cytochrome c(550) to the cytochrome c peroxidase that is affected little by the presence of horse cytochrome c in the other site. The presence of a substantial capture surface for small cytochromes on the cytochrome c peroxidase has implications for rate enhancement mechanisms which ensure that the two electrons required for re-reduction of the enzyme after reaction with hydrogen peroxide are delivered efficiently.

The implications of the dimeric state of cytochrome c550 for its binding to Paracoccus cytochrome c peroxidase and its delivery of the two electrons required to restore the active enzyme during catalysis have been investigated. The amino acid sequence of cytochrome c550 of Paracoccus denitrificans strain LMD 52.44 was determined and showed 21 differences from that of strain LMD 22.21. Based on the X-ray structure of the latter, a structure for the cytochrome c550 monomer from strain 52.44 is proposed and a dipole moment of 945 debye was calculated with an orientation close to the exposed haem edge. The behaviour of the cytochrome on molecular-exclusion chromatography is indicative of an ionic strength-dependent monomer (15 kDa)/dimer (30 kDa) equilibrium that can also be detected by 1H-NMR spectroscopy. The apparent mass of 50 kDa observed at very low ionic strength was consistent with the presence of a strongly asymmetric dimer. This was confirmed by cross-linking studies, which showed that a cross-linked species of mass 30 kDa on SDS behaved with an apparent mass of 50 kDa on molecular-exclusion chromatography. A programme which carried out and evaluated molecular docking of two monomers to give a dimer generated a most probable dimer in which the monomer dipoles lay almost antiparallel to each other. The resultant dipole moment of the dimer is therefore small. Although this finding calls into question the possibility of preorientation of a strongly asymmetrically charged cytochrome as it collides with a redox partner, the stoichiometry of complex formation with cytochrome c peroxidase as studied by 1H-NMR spectroscopy shows that it is the monomer that binds.

Efficient biological electron transfer may require a fluid association of redox partners. Two noncrystallographic methods (a new molecular docking program and 1H NMR spectroscopy) have been used to study the electron transfer complex formed between the cytochrome c peroxidase (CCP) of Paracoccus denitrificans and cytochromes c. For the natural redox partner, cytochrome c550, the results are consistent with a complex in which the heme of a single cytochrome lies above the exposed electron-transferring heme of the peroxidase. In contrast, two molecules of the nonphysiological but kinetically competent horse cytochrome bind between the two hemes of the peroxidase. These dramatically different patterns are consistent with a redox active surface on the peroxidase that may accommodate more than one cytochrome and allow lateral mobility.

A nitrate reductase was solubilized with Triton X-100 from the membranes of Pseudomonas chlororaphis DSM 50135 grown microaerobically in the presence of nitrate. Like other membrane-bound nitrate reductases, it contains three subunits, of 129, 66 (64) and 24 kDa, referred to in the literature as alpha, beta and gamma, respectively. Electrocatalytic studies revealed that only the membrane-bound, not the solubilized form of the enzyme, can accept electrons from a menaquinone analog, menadione, whereas both forms can accept electrons from methylviologen. The isolated enzyme possesses several iron-sulfur clusters and a molybdopterin guanine dinucleotide active center. The iron-sulfur clusters can be grouped in two classes according to their redox properties, the high-potential and low-potential clusters. In the as-isolated enzyme, two forms of the molybdenum center, high- and low-pH, are detectable by electron paramagnetic resonance spectroscopy. The low-pH form shows a hyperfine splitting due to a proton, suggesting the presence of an -OHx ligand. Dithionite reduces the Mo(V) center to Mo(W) and subsequent reoxidization with nitrate originates a new Mo(V) signal, identical to the oxidized low-pH form but lacking its characteristic hyperfine splitting. The isolated preparation also contains heme c (in a sub-stoichiometric amount) with the ability to relay electrons to the molybdenum center, suggesting that this nitrate reductase may contain heme c instead of the heme b usually found in this class of enzymes. (c) 2005 Elsevier B.V. All rights reserved.

Two azurins (Az624 and Az626) were isolated from the soluble extract of two strains of Pseudomonas chlororaphis, DSM 50083(T) and DSM 50135, respectively, grown under microaerobic conditions with nitrate as final electron acceptor. The azurins, purified to electrophoretic homogeneity in three chromatographic steps, exhibit several peculiar properties. They have high reduction potentials and lower pI than most azurins described in the literature. As previously observed for Pseudomonas aeruginosa azurin, their reduction potentials are pH-dependent, but the pK values of their oxidized forms are lower, which suggests that deeper structural changes are associated with the oxidation process of these novel azurins. A hitherto undescribed pH-dependence of the diffusion coefficient was observed in Az624, that could be caused either by conformational changes, or by the formation of supramolecular aggregates associated with a protonation process. Both azurins exhibit axial X-band electron paramagnetic resonance spectra in frozen solution showing a typical hyperfine with the copper nucleus (I = 3/2) and a well-resolved superhyperfine structure with two equivalent N-14 nucleus (I = 1), which is not usually observed for azurins from other species. (C) 2003 Elsevier Inc. All rights reserved.

The nitrite reductase (Nir) isolated from Pseudomonas chlororaphis DSM 50135 is a blue enzyme, with type 1 and type 2 copper centers, as in all copper-containing Nirs described so far. For the first time, a direct determination of the reduction potentials of both copper centers in a Cu-Nir was performed: type 2 copper (T2Cu), 172 mV and type 1 copper (T1Cu), 298 mV at pH 7.6. Although the obtained values seem to be inconsistent with the established electron-transfer mechanism, EPR data indicate that the binding of nitrite to the T2Cu center increases its potential, favoring the electron-transfer process. Analysis of the EPR spectrum of the turnover form of the enzyme also suggests that the electron-transfer process between T1Cu and T2Cu is the fastest of the three redox processes involved in the catalysis: (a) reduction of T1Cu; (b) oxidation of T1Cu by T2Cu; and (c) reoxidation of T2Cu by NO2-. Electrochemical experiments show that azurin from the same organism can donate electrons to this enzyme.

Cytochrome c peroxidase from Paracoccus denitrificans LMD 52.44 was recently identified. The enzyme contains two c-type haems: one is reducible physiologically by cytochrome c550 from the same organism or non-physiologically by ascorbate (high-potential haem) and the other by dithionite (low-potential haem). The enzymatically active form of the peroxidase is the half-reduced enzyme state, in which the high-potential haem is in the iron(II) state and the low-potential haem is in the iron(III) state. It was found that the two haems interact and that the enzyme binds calcium ions near the haem sites which are necessary to promote its activation. In the oxidized form, the high-potential haem is in a high-spin and the low-potential haem is in a low-spin state. The half-reduction of the enzyme with ascorbate-diaminodurol changes the high-potential haem (high-spin) into a low-spin state and the low-potential haem converts from a low- into a high-spin state. This high-spin conversion of the low-potential haem is induced by the presence of calcium ions. These processes of reduction and spin state change can be easily resolved in time by removing the calcium from the enzyme using EDTA, facilitating the observation of the intermediate form by NMR.

Mossbauer and electron paramagnetic resonance (EPR) spectroscopies were used to characterize the diheme cytochrome c peroxidase from Paracoccus denitrificans (L.M.D. 52.44). The spectra of the oxidized enzyme show two distinct spectral components characteristic of low spin ferric hemes (S = 1/2), revealing different heme environments for the two heme groups. The Paracoccus peroxidase can be non-physiologically reduced by ascorbate. Mossbauer investigation of the ascorbate-reduced peroxidase shows that only one heme (the high potential heme) is reduced and that the reduced heme is diamagnetic (S = 0). The other heme (the low potential heme) remains oxidized, indicating that the enzyme is in a mixed valence, half-reduced state. The EPR spectrum of the half-reduced peroxidase, however, shows two low spin ferric species with gmax = 2.89 (species I) and gmax = 2.78 (species II). This EPR observation, together with the Mossbauer result, suggests that both species are arising from the low potential heme. More interestingly, the spectroscopic properties of these two species are distinct from that of the low potential heme in the oxidized enzyme, providing evidence for heme-heme interaction induced by the reduction of the high potential heme. Addition of calcium ions to the half-reduced enzyme converts species II to species I. Since calcium has been found to promote peroxidase activity, species I may represent the active form of the peroxidatic heme.

A two cluster (4Fe-4S) ferredoxin and a rubredoxin have been isolated from the sulfur-reducing bacterium Desulfuromonas acetoxidans. Their amino acid compositions are reported and compared to those of other iron-sulfur proteins. The ferredoxin contains 8 cysteine residues, 8 atoms of iron and 8 atoms of labile sulfur per molecule; its minimum molecular weight is 6163. The protein exhibits an abosrbance ratio of A385/A283 = 0.74. Storage results in a bleaching of the chromophore; the denatured ferredoxin is reconstitutable with iron and sulfide. The instability temperature is 52 degrees C. The rubredoxin does not differ markedly from rubredoxins from other anaerobic bacteria.

The aerobic purification of Pseudomonas nautica 617 nitrous oxide reductase yielded two forms of the enzyme exhibiting different chromatographic behaviors. The protein contains six copper atoms per monomer, arranged in two centers named Cu(A) and Cu(Z). Cu(Z) could be neither oxidized nor further reduced under our experimental conditions, and exhibits a 4-line EPR spectrum (g(x)=2.015, A(x)=1.5 mT, g(y)=2.071, A(y)=2 mT, g(z)=2.138, A(z)=7 mT) and a strong absorption at approximately 640 nm. Cu(A) can be stabilized in a reduced EPR-silent state and in an oxidized state with a typical 7-line EPR spectrum (g(x)=g(y)= 2.021, A(x) = A(y)=0 mT, g(z) = 2.178, A(z)= 4 mT) and absorption bands at 480, 540, and approximately 800 nm. The difference between the two purified forms of nitrous oxide reductase is interpreted as a difference in the oxidation state of the Cu(A) center. In form A, Cu(A) is predominantly oxidized (S = (1)/(2), Cu(1.5+)-Cu(1.5+)), while in form B it is mostly in the one-electron reduced state (S = 0, Cu(1+)-Cu(1+)). In both forms, Cu(Z) remains reduced (S = 1/2). Complete crystallographic data at 2.4 A indicate that Cu(A) is a binuclear site (similar to the site found in cytochrome c oxidase) and Cu(Z) is a novel tetracopper cluster [Brown, K., et al. (2000) Nat. Struct. Biol. (in press)]. The complete amino acid sequence of the enzyme was determined and comparisons made with sequences of other nitrous oxide reductases, emphasizing the coordination of the centers. A 10.3 kDa peptide copurified with both forms of nitrous oxide reductase shows strong homology with proteins of the heat-shock GroES chaperonin family.