Fe XAS data were collected on the oxidized and reduced forms of desulforedoxin from Desulfovibrio gigas, the oxidized form of rubredoxin from Clostridium pasteurianum, and the reduced form of rubredoxin from Pyrococcus furiosus. Analysis of these data is consistent with tetrahedral FeS(4) coordination in both oxidation states, and an expansion of the Fe-S distances from 2.27 to 2.33 Angstrom upon reduction. (C) 1998 Elsevier Science S.A. All rights reserved.

Mossbauer and EPR spectroscopies were used to characterize the Fe clusters in an Fe-S protein isolated from Desulfovibrio desulfuricans (ATCC 27774). This protein was previously thought to contain hexanuclear Fe clusters, but a recent X-ray crystallographic measurement on a similar protein isolated from Desulfovibrio vulgaris showed that the protein contains two tetranuclear clusters, a cubane-type [4Fe-4S] cluster and a mixed-ligand cluster of novel structure [Lindley et al. (1997) Abstract, Chemistry of Metals in Biological Systems, European Research Conference, Tomar, Portugal]. Three protein samples poised at different redox potentials (as-purified, 40 and 320 mV) were investigated. In all three samples, the [4Fe-4S] cluster was found to be present in the diamagnetic 2+ oxidation state and exhibited typical Mossbauer spectra. The novel-structure cluster was found to be redox active. In the 320-mV and as-purified samples, the cluster is at a redox equilibrium between its fully oxidized and one-electron reduced states. In the 40-mV sample, the cluster is in a two-electron reduced state. Distinct spectral components associated with the four Fe sites of cluster 2 in the three oxidation states were identified. The spectroscopic parameters obtained for the Fe sites reflect different ligand environments, making it possible to assign the spectral components to individual Fe sites. In the fully oxidized state, all four iron ions are high-spin ferric and antiferromagnetically coupled to form a diamagnetic S = 0 state. In the one-electron and two-electron reduced states, the reducing electrons were found to localize, consecutively, onto two Fe sites that are rich in oxygen/nitrogen ligands. Based on the X-ray structure and the Mossbauer parameters, attempts could be made to identify the reduced Fe sites. For the two-electron reduced cluster, EPR and Mossbauer data indicate that the cluster is paramagnetic with a nonzero interger spin. For the one-electron reduced cluster, the data suggest a half-integer spin of 9/2 Characteristic fine and hyperfine parameters for all four Fe sites were obtained. Structural implications and the nature of the spin-coupling interactions are discussed.

The complete amino acid sequence of the unusual diheme split-Soret cytochrome c from the sulphate-reducing Desulfovibrio desulfuricans strain ATCC 27774 has been determined using classical chemical sequencing techniques and mass spectrometry. The 247-residue sequence shows almost no similarity with any other known diheme cytochrome c, but the heme-binding site of the protein is similar to that of the cytochromes c3 from the sulphate reducers. The cytochrome-c-like domain of the protein covers only the C-terminal part of the molecule, and there is evidence for at least one more domain containing four cysteine residues, which might bind another cofactor, possibly a non-heme iron-containing cluster. This domain is similar to a sequence fragment of the genome of Archaeoglobus fulgidus, which confirms the high conservation of the genes involved in sulfate reduction.

Some new properties promoted by the formation of a supercomplex between iron hexacyanometallates and the polyazamacrocycle [32]aneN(8) (1,5,9,13,17,21,25,29-octaazacyclodotrane) are described. In the presence of the polyazamacrocycle, thermal and photoinduced electron transfer from iodide to Fe(CN)(6)(3-) were observed in moderately acidic media. The thermal reaction is slow (k(obs) = 8.9 x 10(-4) s(-1), at 25 degrees C) and proceeds to an equilibrium (K = 7 M-2, at 25 degrees C). The reaction is almost isoergonic, with favorable enthalpy and unfavorable entropy changes (Delta G degrees = -4.8 kJ mol(-1), Delta H degrees = -160 kJ mol(-1), Delta S degrees = -0.54 kJ mol(-1) K-1). A photoinduced electron-transfer process, leading to additional iodide oxidation, was observed upon flash irradiation of equilibrated solutions. Following the photoinduced process, the system reverts to the thermal equilibrium in the dark. The promoting role of the macrocycle is thermodynamic for the thermal process (anodic shift in the Fe-II/III potential upon supercomplex formation) and kinetic for the photoinduced process [formation of ion-paired species between hexacyanoferrate(III) and iodide upon supercomplex formation]. The thermal reaction is reversible in basic media (where the macrocycle deprotonates and supercomplex formation is prevented), providing an example of on/off switching by pH changes of an electron-transfer reaction.

A complete study of the structural pH dependent transformations of the synthetic flavylium salt 4',7-dihydroxyflavylium chloride (DHF), occurring in aqueous solutions, including the basic region, is described. The kinetic study of the transformations occurring in acidic media (quinoidal base (A) reversible arrow flavylium cation (AH(+)) reversible arrow hemiacetal (B) reversible arrow cis-chalone (C-cis) reversible arrow trans-chalcone (C-trans)) allowed to conclude that the cis-trans isomerization is faster than the tautomerization and the hydration processes, which is unique in the anthocyanins family. Results obtained with the parent compound 4'7-dimethoxyglavylium chloride (DMF) with relevance to this study are also presented. In equilibrated basic solution the existence of acid-base equilibria involving the trans-Chalcone (C-trans) and its conjugated bases, (C-trans(-) and C-trans(2)), was detected. Freshly prepared solutions at pH >7 show also the presence of a transient species identified as the ionized quinoidal base (A(-)), which is almost completely converted into C-trans(2-) with a pH dependent rate constant.

Conversion of Fe ions in solution to the solid phase in ferritin concentrates iron required for cell function. The rate of the Fe phase transition in ferritin is tissue specific and reflects the differential expression of two classes of ferritin subunits (H and L). Early stages of mineralization were probed by rapid freeze-quench Mossbauer, at strong fields (up to 8 T), and EPR spectroscopy in an H-type subunit, recombinant frog ferritin; small numbers of Fe (36 moles/mol of protein) were used to increase Fe3+ in mineral precursor forms, At 25 ms, four Fe3+-oxy species (three Fe dimers and one Fe trimer) were identified, These Fe3+-oxy species were found to form at similar rates and decay subsequently to a distinctive superparamagentic species designated the `'young core.'' The rate of oxidation of Fe2+ (1026 s(-1)) corresponded well to the formation constant for the Fe3+- tyrosinate complex (920 s(-1)) observed previously [Waldo, G. S., \& Theil, E. C. (1993) Biochemistry 32, 13261] and, coupled with EPR data, indicates that several or possibly all of the Fe3+-oxy species involve tyrosine. The results, combined with previous Mossbauer studies of Y30F human H-type ferritin which showed decreases in several Fe3+ intermediates and stabilization of Fe2+ [Bauminger, E. R., et al. (1993) Biochem, J. 296, 709], emphasize the involvement of tyrosyl residues in the mineralization of H-type ferritins. The subsequent decay of these multiple Fe3+-oxy species to the superparamagnetic mineral suggests that Fe3+ species in different environments may be translocated as intact units from the protein shell into the ferritin cavity where the conversion to a solid mineral occurs.

Structural information obtained from the analysis of nickel K-edge X-ray absorption spectroscopic data of [NiFe]hydrogenases from Desulfovibrio gigas, Thiocapsa roseopersicina, Desulfovibrio desulfuricans (ATCC 27774), Escherichia coli (hydrogenase-1), Chromatium vinosum, and Alcaligenes eutrophus H16 (NAD(+)-reducing, soluble hydrogenase), poised in different redox states, is reported. The data allow the active-site structures of enzymes from several species to be compared, and allow the effects of redox poise on the structure of the nickel sites to be examined. In addition, the structure of the nickel site obtained from recent crystallographic studies of the D. gigas enzyme (Volbeda, A.; Charon, M.-H.; Piras, C.; Hatchikian, E. C.; Frey, M.; Fontecilla-Camps, J. C. Nature 1995, 373, 580-587) is compared with the structural features obtained from the analysis of XAS data from the same enzyme. The nickel sites of all but the oxidized (as isolated) sample of A. eutrophus hydrogenase are quite similar. The nickel K-edge energies shift 0.9-1.5 eV to lower energy upon reduction from oxidized (forms A and B) to fully reduced forms. This value is comparable with no more than a one-electron metal-centered oxidation state change. With the exception of T. roseopersicina hydrogenase, most of the edge energy shift (-0.8 eV) occurs upon reduction of the oxidized enzymes to the EPR-silent intermediate redox level (SI). Analysis of the XANES features assigned to 1s-->3d electronic transitions indicates that the shift in energy that occurs for reduction of the enzymes to the SI level may be attributed at least in part to an increase in the coordination number from five to six. The smallest edge energy shift is observed for the T. roseopersicina enzyme, where the XANES data indicate that the nickel center is always six-coordinate. With the exception of the oxidized sample of A. eutrophus hydrogenase, the EXAFS data are dominated by scattering from S-donor ligands at similar to 2.2 Angstrom. The enzyme obtained from T. roseopersicina also shows evidence for the presence of O,N-donor ligands. The data from A. eutrophus hydrogenase are unique in that they indicate that a significant structural change occurs upon reduction of the enzyme. EXAFS data obtained from the oxidized (as isolated) A. eutrophus enzyme indicate that the EXAFS is dominated by scattering from 3-4 N,O-donor atoms at 2.06(2) Angstrom, with contributions from 2-3 S-donor ligands at 2.35(2) Angstrom. This changes upon reduction to a more typical nickel site composed of similar to 4 S-donor ligands at a Ni-S distance of 2.19(2) Angstrom. Evidence for the presence of atoms in the 2.4-2.9 Angstrom distance range is found in most samples, particularly the reduced enzymes (SI, form C, and R). The analysis of these data is complicated by the fact that it is difficult to distinguish between S and Fe scattering atoms at this distance, and by the potential presence of both S and another metal atom at similar distances. The results of EXAFS analysis are shown to be in general agreement with the published crystal structure of the D. gigas enzyme.

The diheme cytochrome c peroxidase from Paracoccus denitrificans was modified with the histidine-specific reagent diethyl pyrocarbonate. At low excess of reagent, 1 mol of histidine was modified in the oxidized enzyme, and modification was associated with loss of the ability to form the active state. With time, the modification reversed, and the ability to form the active state was recovered. The agreement between the spectrophotometric measurement of histidine modification and radioactive incorporation using a radiolabeled reagent indicated little modification of other amino acids. However, the reversal of histidine modification observed spectrophotometrically was not matched by loss of radioactivity, and we propose a slow transfer of the ethoxyformyl group to an unidentified amino acid. The presence of CN- bound to the active peroxidatic site of the enzyme led to complete protection of the essential histidine from modification. Limited subtilisin treatment of the native enzyme followed by tryptic digest of the C-terminal fragment (residues 251-338) showed that radioactivity was located in a peptide containing a single histidine at position 275. We propose that this conserved residue, in a highly conserved region, is central to the function of the active mixed-valence state.