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Hydrogen production and deuterium-proton exchange reactions catalyzed by Desulfovibrio nickel(II)-substituted rubredoxins, Saint-Martin, P., Lespinat P. A., Fauque G., Berlier Y., Legall J., Moura I., Teixeira M., Xavier A. V., and Moura J. J. , Proc Natl Acad Sci U S A, Dec, Volume 85, Number 24, p.9378-80, (1988) AbstractWebsite

The nickel tetrahedral sulfur-coordinated core formed upon metal replacement of the native iron in Desulfovibrio sp. rubredoxins is shown to mimic the reactivity pattern of nickel-containing hydrogenases with respect to hydrogen production, deuterium-proton exchange, and inhibition by carbon monoxide.

Cytochrome b5 reductase is the component from neuronal synaptic plasma membrane vesicles that generates superoxide anion upon stimulation by cytochrome c, Samhan-Arias, A. K., Fortalezas S., Cordas C., Moura I., Moura J. J. G., and Gutierrez-Merino C. , Redox Biol, Volume 15, p.109-114, (2018)
Peroxidase-like activity of cytochrome b5 is triggered upon hemichrome formation in alkaline pH, Samhan-Arias, A., Maia L. B., Cordas C. M., Moura I., Gutierrez-Merino C., and Moura J. J. G. , BBA - Proteins and Proteomics, Volume 1866, p.373-378, (2018)
Ligand accessibility to heme cytochrome b5 coordinating sphere and enzymatic activity enhancement upon tyrosine ionization, Samhan-Arias, A. K., Cordas C. M., Carepo M. S., Maia L. B., Gutierrez-Merino C., Moura I., and Moura J. J. G. , J Biol Inorg Chem, Volume 24, p.317-330, (2019)
Reduction of ascorbate free radical by the plasma membrane of synaptic terminals from rat brain, Samhan-Arias, A. K., Duarte R. O., Martin-Romero F. J., Moura J. J., and Gutierrez-Merino C. , Arch Biochem Biophys, Jan 15, Volume 469, Number 2, p.243-54, (2008) AbstractWebsite

Synaptic plasma membranes (SPMV) decrease the steady state ascorbate free radical (AFR) concentration of 1mM ascorbate in phosphate/EDTA buffer (pH 7), due to AFR recycling by redox coupling between ascorbate and the ubiquinone content of these membranes. In the presence of NADH, but not NADPH, SPMV catalyse a rapid recycling of AFR which further lower the AFR concentration below 0.05 microM. These results correlate with the nearly 10-fold higher NADH oxidase over NADPH oxidase activity of SPMV. SPMV has NADH-dependent coenzyme Q reductase activity. In the presence of ascorbate the stimulation of the NADH oxidase activity of SPMV by coenzyme Q(1) and cytochrome c can be accounted for by the increase of the AFR concentration generated by the redox pairs ascorbate/coenzyme Q(1) and ascorbate/cytochrome c. The NADH:AFR reductase activity makes a major contribution to the NADH oxidase activity of SPMV and decreases the steady-state AFR concentration well below the micromolar concentration range.

Topography of human cytochrome b5/cytochrome b5 reductase interacting domain and redox alterations upon complex formation, Samhan-Arias, A. K., Almeida R. M., Ramos S., Cordas C. M., Moura I., Gutierrez-Merino C., and Moura J. J. G. , Biochim Biophys Acta, Volume 1859, p.78-87, (2018)
Evidence for antisymmetric exchange in cuboidal 3Fe-4S (+) clusters, Sanakis, Y., Macedo A. L., Moura I., Moura J. J. G., Papaefthymiou V., and Munck E. , Journal of the American Chemical Society, Dec 6, Volume 122, Number 48, p.11855-11863, (2000) AbstractWebsite

Iron-sulfur clusters with [3Fe-4S] cores are widely distributed in biological systems. In the oxidized state, designated [3Fe-4S](+), these electron-transfer agents have an electronic ground state with S = 1/2, and; they exhibit EPR signals centered at g = 2.01. It has been established by Mossbauer spectroscopy that the three iron sites of the cluster are high-spin Fe3+; and the general properties of the S = 1/2 ground state have been described with the exchange Hamiltonian H-exch = J(12)S(1).S-2 + J(23)S(2).S-3 + J(13)S(1).S-3 Some [3Fe-4S](+) clusters (type 1) have their g-values confined to the range between g = 2.03 and 2.00 while others (type 2) exhibit a continuous distribution of g-values down to g approximate to 1.85. Despite considerable efforts in various laboratories no model has emerged that explains the g-values of type 2 clusters. The 4.2 K spectra of all [3Fe-4S](+) clusters have broad features,which have been simulated in the past by using Fe-57 magnetic hyperfine tensors with anisotropies that are unusually large for high-spin feme sites. It is proposed here that antisymmetric exchange, H-AS = d.(S-1 x S-2 + S-2 x S-3 + S-3 x S-1), is the cause of the g-value shifts in type 2 clusters. We have been able to fit the EPR and Mossbauer spectra of the 3Fe clusters of beef heart aconitase and Desulfovibrio gigas ferredoxin II by using antisymmetric exchange in combination with distributed exchange coupling constants J(12), J(13), and J(23) (J-strain). While antisymmetric exchange is negligible for aconitase (which has a type 1 cluster), fits of the ferredoxin II spectra require \d\ approximate to 0.4 cm(-1). Our studies show that the data of both proteins can lie fit using the same isotropic Fe-57 magnetic hyperfine coupling constant for th three cluster sites, namely a -18.0 MHz for aconitase and a = -18.5 MHz for the D. gigas ferredoxin. The effects of antisymmetric exchange and J-strain on the Mossbauer and EPR spectra are discussed.

Electrochemical studies on small electron transfer proteins using membrane electrodes, dos Santos, M. M. C., de Sousa P. M. P., Goncalves M. L. S., Krippahl L., Moura J. J. G., Lojou E., and Bianco P. , Journal of Electroanalytical Chemistry, Jan 16, Volume 541, p.153-162, (2003) AbstractWebsite

Membrane electrodes (ME) were constructed using gold, glassy carbon and pyrolytic graphite supports and a dialysis membrane, and used to study the electrochemical behavior of small size electron transfer proteins: monohemic cytochrome c(522) from Pseudomonas nautica and cytochrome c(533) as well as rubredoxin from Desulfovibrio vulgaris. Different electrochemical techniques were used including cyclic voltammetry (CV), square wave voltammetry (SW) and differential pulse voltammetry (DP). A direct electrochemical response was obtained in all cases except with rubredoxin where a facilitator was added to the protein solution entrapped between the membrane and the electrode surface. Formal potentials and heterogeneous charge transfer rate constants were determined from the voltammetric data. The influence of the ionic strength and the pH of the medium on the electrochemical response at the ME were analyzed. The benefits from the use of the ME in protein electrochemistry and its role in modulating the redox behavior are analyzed. A critical comparison is presented with data obtained at non-MEs. Finally, the interactions that must be established between the proteins and the electrode surfaces are discussed, thereby modeling molecular interactions that occur in biological systems. (C) 2002 Elsevier Science B.V. All rights reserved.

Synthesis of WO3 nanoparticles for biosensing applications, Santos, L., Silveira C. M., Elangovan E., Neto J. P., Nunes D., Pereira L., Martins R., Viegas J., Moura J. J. G., Todorovic S., Almeida M. G., and Fortunato E. M. , Sensors and Actuators B: Chemical, Volume 223, p.186-194, (2016)
Ultrasonic multiprobe as a new tool to overcome the bottleneck of throughput in workflows for protein identification relaying on ultrasonic energy, Santos, H. M., Carreira R., Diniz M. S., Rivas M. G., Lodeiro C., Moura J. J., and Capelo J. L. , Talanta, Apr 15, Volume 81, Number 1-2, p.55-62, (2010) AbstractWebsite

We studied in this work the performance of the new ultrasonic multiprobe in terms of throughput, handling and robustness. The study was conducted using the multiprobe to speed two different proteomics workflows. The "classic" method relaying on overnight protein digestion (12h), was used as the standard procedure. This work clearly shows the importance of testing variables such as ultrasonic amplitude and ultrasonic time when adapting an ultrasonic-based treatment to a new ultrasonic device. The results here presented also shown and confirm the advantage of speed up sample treatment workflows with the aid of ultrasonic energy in combination with a 96-well plate. The methods compared were similar in terms of robustness, but the desalting free method was the fastest, requiring only 2 min/sample for completion. In addition it was also the simplest in terms of handling, since no desalting step was needed. The following standard proteins were successfully identified using the methods studied: bovine serum albumin, alpha-lactalbumin, ovalbumin, carbonic anhydrase, fructose-bisphosphate aldolase A, catalase, chymotrypsinogen A. As case study, the identification of the protein Split-Soret cytochrome c from D. desulfuricans ATCC 27774 was carried out.

Electron transfer mechanism studies of cytochrome c3: pH dependence of the redox equilibria, Santos, H., Moura J. J. G., Xavier A. V., and Legall J. , Inorganica Chimica Acta, Volume 79, p.167-169, (1983) AbstractWebsite
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Redox chemistry of low-pH forms of tetrahemic cytochrome c3, Santos, M., Dos Santos M. M., Goncalves M. L., Costa C., Romao J. C., and Moura J. J. , J Inorg Biochem, Dec, Volume 100, Number 12, p.2009-16, (2006) AbstractWebsite

Desulfovibrio vulgaris Hildenborough cytochrome c(3) contains four hemes in a low-spin state with bis-histidinyl coordination. High-spin forms of cytochrome c(3) can be generated by protonation of the axial ligands in order to probe spin equilibrium (low-spin/high-spin). The spin alterations occurring at acid pH, the associated changes in redox potentials, as well as the reactivity towards external ligands were followed by the conjunction of square wave voltammetry and UV-visible, CD, NMR and EPR spectroscopies. These processes may be used for modelling the action of enzymes that use spin equilibrium to promote enzyme activity and reactivity towards small molecules.

An improved clean sonoreactor-based method for protein identification by mass spectrometry-based techniques, Santos, H. M., Mota Cristiano, Lodeiro C., Moura Isabel, Isaac Issa, and Capelo J. L. , Talanta, Dec 15, Volume 77, Number 2, p.870-875, (2008) AbstractWebsite

A new clean fast (8 min) method for in-solution protein digestion Without detergent or urea for protein identification by peptide mass fingerprint and mass spectrometry-based techniques is Proposed. The new method avoids the use of time consuming desalting procedures entailing the following four steps done under the effect of an ultrasonic field provided by a sonoreactor: denaturation (1 min) in a mixed Solution of water:acetonitrile 1/1 (v/v): protein reduction (1 min); protein alkylation (1 min); and protein digestion (5 min). Five Proteins with masses comprised between 14.4 kDa and 97 kDa and the protein splitsoret cytochrome c from D. desulfuricans ATCC27774, Were Successfully identified with this procedure. No differences were found in the sequence coverage or in the number of peptides matched when the new clean method was compared to another one using urea. Twofold better signal-to-noise ratios were obtained in the MALDI spectra from protein samples prepared with the new method when comparing it with a method using urea. The new digestion method avoids the need to remove salt content and increases throughput (six samples at once) while reducing sample loss and contamination from sample handling. (C) 2008 Elsevier B.V. All rights reserved.

Improving sample treatment for in-solution protein identification by peptide mass fingerprint using matrix-assisted laser desorption/ionization time-of-flight mass Spectrometry, Santos, H. M., Rial-Otero R., Fernandes L., Vale G., Rivas M. G., Moura I., and Capelo J. L. , Journal of Proteome Research, Sep, Volume 6, Number 9, p.3393-3399, (2007) AbstractWebsite

Three ultrasonic energy sources were studied to speed up the sample treatment for in-solution protein identification by peptide mass fingerprint using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry. Protein reduction, alkylation, and enzymatic digestion steps were done in 15 min. Nine proteins, including zinc resistance-associated protein precursor from Desulfovibrio desulfuricans strain G20 and split-soret cytochrome c from D. desulfuricans ATCC27774 were successfully identified with the new protocol.

Rubredoxin mutant A51C unfolding dynamics: A Forster Resonance Energy Transfer study, Santos, Andrea, Duarte Americo G., Fedorov Alexander, Martinho Jose M. G., and Moura Isabel , Biophysical Chemistry, May, Volume 148, Number 1-3, p.131-137, (2010) AbstractWebsite

The unfolding dynamics of the rubredoxin mutant A51C (RdA51C) from Desulfovibrio vulgaris (DvRd) was studied on the temperature range from 25 degrees C to 90 degrees C and by incubation at 90 degrees C. By Forster Resonance Energy Transfer (FRET) the donor (D; Trp37) to acceptor (A; 1,5-IAEDANS) distance distribution was probed at several temperatures between 25 degrees C and 90 degrees C, and incubation times at 90 degrees C. From 25 degrees C to 50 degrees C the half-width distributions values (hw) are small and the presence of a discrete D-A distance was considered. At temperatures higher than 60 degrees C broader hw values were observed reflecting the existence of a distance distribution. The protein denaturation was only achieved by heating the solution for 2 h at 90 degrees C, as probed by the increase of the D-A mean distance. From Trp fluorescence it was shown that its vicinity was maintained until similar to 70 degrees C, being the protein hydrodynamic radius invariant until 50 degrees C. However, at similar to 70 degrees C a change in the partial unfolding kinetics indicates the disruption of specific H-bonds occurring in the hydrophobic core. The red shift of 13 nm, observed on the Trp37 emission, confirms the exposition of Trp to solvent after protein incubation at 90 degrees C for 2.5 h. (C) 2010 Elsevier B.V. All rights reserved.

NMR studies of electron transfer mechanisms in a protein with interacting redox centres: Desulfovibrio gigas cytochrome c3, Santos, H., Moura J. J., Moura I., Legall J., and Xavier A. V. , Eur J Biochem, Jun 1, Volume 141, Number 2, p.283-96, (1984) AbstractWebsite

The proton NMR spectra of the tetrahaem cytochrome c3 from Desulfovibrio gigas were examined while varying the pH and the redox potential. The analysis of the NMR reoxidation pattern was based on a model for the electron distribution between the four haems that takes into account haem-haem redox interactions. The intramolecular electron exchange is fast on the NMR time scale (larger than 10(5) s-1). The NMR data concerning the pH dependence of the chemical shift of haem methyl resonances in different oxidation steps and resonance intensities are not compatible with a non-interacting model and can be explained assuming a redox interaction between the haems. A complete analysis at pH* = 7.2 and 9.6, shows that the haem-haem interacting potentials cover a range from -50 mV to +60 mV. The midpoint redox potentials of some of the haems, as well as some of their interacting potentials, are pH-dependent. The physiological relevance of the modulation of the haem midpoint redox potentials by both the pH and the redox potential of the solution is discussed.

The first crystal structure of class III superoxide reductase from Treponema pallidum, Santos-Silva, T., Trincao J., Carvalho A. L., Bonifacio C., Auchere F., Raleiras P., Moura I., Moura J. J., and Romao M. J. , J Biol Inorg Chem, Jul, Volume 11, Number 5, p.548-58, (2006) AbstractWebsite

Superoxide reductase (SOR) is a metalloprotein containing a non-heme iron centre, responsible for the scavenging of superoxide radicals in the cell. The crystal structure of Treponema pallidum (Tp) SOR was determined using soft X-rays and synchrotron radiation. Crystals of the oxidized form were obtained using poly(ethylene glycol) and MgCl2 and diffracted beyond 1.55 A resolution. The overall architecture is very similar to that of other known SORs but TpSOR contains an N-terminal domain in which the desulforedoxin-type Fe centre, found in other SORs, is absent. This domain conserves the beta-barrel topology with an overall arrangement very similar to that of other SOR proteins where the centre is present. The absence of the iron ion and its ligands, however, causes a decrease in the cohesion of the domain and some disorder is observed, particularly in the region where the metal would be harboured. The C-terminal domain exhibits the characteristic immunoglobulin-like fold and harbours the Fe(His)4(Cys) active site. The five ligands of the iron centre are well conserved despite some disorder observed for one of the four molecules in the asymmetric unit. The participation of a glutamate as the sixth ligand of some of the iron centres in Pyrococcus furiosus SOR was not observed in TpSOR. A possible explanation is that either X-ray photoreduction occurred or there was a mixture of redox states at the start of data collection. In agreement with earlier proposals, details in the TpSOR structure also suggest that Lys49 might be involved in attraction of superoxide to the active site.

Crystallization and preliminary X-ray diffraction analysis of the 16-haem cytochrome of Desulfovibrio gigas, Santos-Silva, T., Diasa J. M., Bourenkov G., Bartunik H., Moura I., and Romao M. J. , Acta Crystallographica Section D-Biological Crystallography, May, Volume 60, p.968-970, (2004) AbstractWebsite

High-molecular-weight cytochromes (Hmcs) belong to a large family of multihaem cytochromes in sulfate-reducing bacteria. HmcA is the first cytochrome reported to have 16 c-type haems arranged in its polypeptide chain. The function of this cytochrome is still unknown, although it is clear that it belongs to a membrane-bound complex involved in electron transfer from the periplasm to the membrane. HmcA from Desulfovibrio gigas has been purified and successfully crystallized using the hanging-drop vapour-diffusion method. The crystals grew using PEG and zinc acetate as precipitants to maximum dimensions of 0.2 x 0.2 x 0.2 mm in an orthorhombic space group, with unit-cell parameters a = 88.9, b = 90.9, c = 83.7 Angstrom. The crystals diffracted to beyond 2.07 Angstrom and a MAD data set was collected.

Crystal structure of the 16 heme cytochrome from Desulfovibrio gigas: A glycosylated protein in a sulphate-reducing bacterium, Santos-Silva, Teresa, Dias Joao Miguel, Dolla Alain, Durand Marie-Claire, Goncalves Luisa L., Lampreia Jorge, Moura Isabel, and Romao Maria Joao , Journal of Molecular Biology, Jul 20, Volume 370, Number 4, p.659-673, (2007) AbstractWebsite

Sulphate-reducing bacteria have a wide variety of periplasmic cytochromes involved in electron transfer from the periplasm to the cytoplasm. HmcA is a high molecular mass cytochrome of 550 amino acid residues that harbours 16 c-type heme groups. We report the crystal structure of HmcA isolated from the periplasm of Desulfovibrio gigas. Crystals were grown. using polyethylene glycol 8K and zinc acetate, and diffracted beyond 2.1 angstrom resolution. A multiple-wavelength anomalous dispersion experiment at the iron absorption edge enabled us to obtain good-quality phases for structure solution and model building. DgHmcA has a V-shape architecture, already observed in HmcA isolated from Desulfovibrio vulgaris Hildenborough. The presence of an oligosaccharide molecule covalently bound to an Asn residue was observed in the electron density maps of DgHmcA and confirmed by mass spectrometry. Three modified monosaccharides appear at the highly hydrophobic vertex, possibly acting as an anchor of the protein to the cytoplasmic membrane. (c) 2007 Elsevier Ltd. All rights reserved.

Superoxide reductase from the syphilis spirochete Treponema pallidum: crystallization and structure determination using soft X-rays, Santos-Silva, T., Trincao J., Carvalho A. L., Bonifacio C., Auchere F., Moura I., Moura J. J., and Romao M. J. , Acta Crystallogr Sect F Struct Biol Cryst Commun, Nov 1, Volume 61, Number Pt 11, p.967-70, (2005) AbstractWebsite

Superoxide reductase is a 14 kDa metalloprotein containing a catalytic non-haem iron centre [Fe(His)4Cys]. It is involved in defence mechanisms against oxygen toxicity, scavenging superoxide radicals from the cell. The oxidized form of Treponema pallidum superoxide reductase was crystallized in the presence of polyethylene glycol and magnesium chloride. Two crystal forms were obtained depending on the oxidizing agents used after purification: crystals grown in the presence of K3Fe(CN)6 belonged to space group P2(1) (unit-cell parameters a = 60.3, b = 59.9, c = 64.8 A, beta = 106.9 degrees) and diffracted beyond 1.60 A resolution, while crystals grown in the presence of Na2IrCl6 belonged to space group C2 (a = 119.4, b = 60.1, c = 65.6 A, beta = 104.9 degrees) and diffracted beyond 1.55 A. A highly redundant X-ray diffraction data set from the C2 crystal form collected on a copper rotating-anode generator (lambda = 1.542 A) clearly defined the positions of the four Fe atoms present in the asymmetric unit by SAD methods. A MAD experiment at the iron absorption edge confirmed the positions of the previously determined iron sites and provided better phases for model building and refinement. Molecular replacement using the P2(1) data set was successful using a preliminary trace as a search model. A similar arrangement of the four protein molecules could be observed.

Kinetic, structural, and EPR studies reveal that aldehyde oxidoreductase from Desulfovibrio gigas does not need a sulfido ligand for catalysis and give evidence for a direct Mo-C interaction in a biological system, Santos-Silva, T., Ferroni F., Thapper A., Marangon J., Gonzalez P. J., Rizzi A. C., Moura I., Moura J. J., Romao M. J., and Brondino C. D. , J Am Chem Soc, Jun 17, Volume 131, Number 23, p.7990-8, (2009) AbstractWebsite

Aldehyde oxidoreductase from Desulfovibrio gigas (DgAOR) is a member of the xanthine oxidase (XO) family of mononuclear Mo-enzymes that catalyzes the oxidation of aldehydes to carboxylic acids. The molybdenum site in the enzymes of the XO family shows a distorted square pyramidal geometry in which two ligands, a hydroxyl/water molecule (the catalytic labile site) and a sulfido ligand, have been shown to be essential for catalysis. We report here steady-state kinetic studies of DgAOR with the inhibitors cyanide, ethylene glycol, glycerol, and arsenite, together with crystallographic and EPR studies of the enzyme after reaction with the two alcohols. In contrast to what has been observed in other members of the XO family, cyanide, ethylene glycol, and glycerol are reversible inhibitors of DgAOR. Kinetic data with both cyanide and samples prepared from single crystals confirm that DgAOR does not need a sulfido ligand for catalysis and confirm the absence of this ligand in the coordination sphere of the molybdenum atom in the active enzyme. Addition of ethylene glycol and glycerol to dithionite-reduced DgAOR yields rhombic Mo(V) EPR signals, suggesting that the nearly square pyramidal coordination of the active enzyme is distorted upon alcohol inhibition. This is in agreement with the X-ray structure of the ethylene glycol and glycerol-inhibited enzyme, where the catalytically labile OH/OH(2) ligand is lost and both alcohols coordinate the Mo site in a eta(2) fashion. The two adducts present a direct interaction between the molybdenum and one of the carbon atoms of the alcohol moiety, which constitutes the first structural evidence for such a bond in a biological system.

Physico-chemical and Spectroscopic Properties of the Monohemic Cytochrome C552 from Pseudomonas nautica 617, Saraiva, Lígia M., Fauque Guy, Besson Stéphane, and Moura Isabel , European Journal of Biochemistry, Volume 224, Number 3, p.1011-1017, (1994) AbstractWebsite

A c-type monohemic ferricytochrome c552 (11 kDa) was isolated from the soluble extract of a marine denitrifier, Pseudomonas nautica strain 617, grown under anaerobic conditions with nitrate as final electron acceptor. The NH2-terminal sequence and the amino acid composition of the cytochrome were determined. The heme iron of the cytochrome c552 has histidine-methionine as axial ligands, and a pH-dependent mid-point redox potential, equal to 250 mV at pH 7.6. The presence of methionine was demonstrated by visible, EPR and NMR spectroscopies. The assignment of most of the hemic protons was performed applying two-dimensional NOE spectroscopy (NOESY), and the aromatic region was assigned through two-dimensional correlated spectroscopy (COSY) experiments. The EPR spectrum of the oxidised form of the cytochrome c552 is typical of a low-spin ferric heme.

Characterization of the Dihemic Cytochrome C549 from the Marine Denitrifying Bacterium Pseudomonas nautica 617, Saraiva, L. M., Besson S., Fauque G., and Moura I. , Biochemical and Biophysical Research Communications, Volume 199, Number 3, p.1289-1296, (1994) AbstractWebsite
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Purification and Preliminary Characterization of Three C-Type Cytochromes from Pseudomonas Nautica Strain 617, Saraiva, L. M., Besson S., Moura I., and Fauque G. , Biochemical and Biophysical Research Communications, Volume 212, Number 3, p.1088-1097, (1995) AbstractWebsite
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NMR and EPR studies on a monoheme cytochrome c550 isolated from Bacillus halodenitrificans, Saraiva, Lígia M., Denariaz Gerard, Liu Ming- Y., Payne William J., Legall Jean, and Moura Isabel , European Journal of Biochemistry, Volume 204, Number 3, p.1131-1139, (1992) AbstractWebsite

A c-type monoheme ferricytochrome c550 (9.6 kDa) was isolated from cells of Bacillus halodenitrificans sp.nov., grown anaerobically as a denitrifier. The visible absorption spectrum indicates the presence of a band at 695 nm characteristic of heme–methionine coordination. The mid-point redox potential was determined at several pH values by visible spectroscopy. The redox potential at pH 7.6 is 138 mV. When studied by 1H-NMR spectroscopy as a function of pH, the spectrum shows a pH dependence with pKa values of 6.0 and 11.0. According to these pKa values, three forms designated as I, II and III can be attributed to cytochrome c550. The first pKa is probably associated with protonation of the propionate groups. The second pKa value introduces a larger effect in the 1H-NMR spectrum and is probably due to the ionisation of the axial histidine. Studies of temperature variation of the 1H-NMR spectra for both the ferrous and ferri forms of the cytochrome were performed. Heme meso protons, the heme methyl groups, the thioether protons, two protons from a propionate and the methylene protons from the axial methionine were identified in the reduced form. The heme methyl resonances of the ferri form were also assigned. EPR spectroscopy was also used to probe the ferric heme environment. A signal at gmax∼ 3.5 at pH 7.5 was observed indicating an almost axial heme environment. At higher pH values the signal at gmax∼ 3.5 converts mainly to a signal at g∼ 2.96. The pKa associated with this change is around 11.3. The N-terminal sequence of this cytochrome was determined and compared with known amino acid sequences of other cytochromes.