Alice S. Pereira

SORs (superoxide reductases) are enzymes involved in bacterial resistance to reactive oxygen species, catalysing the reduction of superoxide anions to hydrogen peroxide. So far three structural classes have been identified. Class I enzymes have two ironcentre-containing domains. Most studies have focused on the catalytic iron site (centre II), yet the role of centre I is poorly understood. The possible roles of this iron site were approached by an integrated study using both classical and fast kinetic measurements, as well as direct electrochemistry. A new heterometallic form of the protein with a zinc-substituted centre I, maintaining the iron active-site centre II, was obtained, resulting in a stable derivative useful for comparison with the native all-iron from. Second-order rate constants for the electron transfer between reduced rubredoxin and the different SOR forms were determined to be 2.8 x 10(7) M(-1) . s(-1) and 1.3 x 10(6) M(-1) . s(-1) for SOR(Fe(IIII)-Fe(II)) and for SOR(Fe(IIII)-Fe(III)) forms respectively, and 3.2 x 10(6) M(-1) s(-1) for the SOR(Zn(II)-Fe(III)) form. The results obtained seem to indicate that centre I transfers electrons from the putative physiological donor rubredoxin to the catalytic active iron site (intramolecular process). In addition, electrochemical results show that conformational changes are associated with the redox state of centre I, which may enable a faster catalytic response towards superoxide anion. The apparent rate constants calculated for the SOR-mediated electron transfer also support this observation.

The effect of kefir grains on the proteolysis of major milk proteins in milk kefir and in a culture of kefir grains in pasteurized cheese whey was followed by reverse phase-HPLC analysis. The reduction of kappa-, alpha-, and beta-caseins (CN), alpha-lactalbumin (alpha-LA), and beta-lactoglobulin (beta-LG) contents during 48 and 90 h of incubation of pasteurized milk (100 mL) and respective cheese whey with kefir grains (6 and 12 g) at 20 degrees C was monitored. Significant proteolysis of alpha-LA and kappa-, alpha-, and beta-caseins was observed. The effect of kefir amount (6 and 12 g/100 mL) was significant for alpha-LA and alpha- and beta-CN. alpha-Lactalbumin and beta-CN were more easily hydrolyzed than alpha-CN. No significant reduction was observed with respect to beta-LG concentration for 6 and 12 g of kefir in 100 mL of milk over 48 h, indicating that no significant proteolysis was carried out. Similar results were observed when the experiment was conducted over 90 h. Regarding the cheese whey kefir samples, similar behavior was observed for the proteolysis of alpha-LA and beta-LG: alpha-LA was hydrolyzed between 60 and 90% after 12 h (for 6 and 12 g of kefir) and no significant beta-LG proteolysis occurred. The proteolytic activity of lactic acid bacteria and yeasts in kefir community was evaluated. Kefir milk prepared under normal conditions contained peptides from proteolysis of alpha-LA and kappa-, alpha-, and beta-caseins. Hydrolysis is dependent on the kefir: milk ratio and incubation time. beta-Lactoglobulin is not hydrolyzed even when higher hydrolysis time is used. Kefir grains are not appropriate as adjunct cultures to increase beta-LG digestibility in whey-based or whey-containing foods.

Nitric Oxide Reductase (NOR) is an integral membrane protein performing the reduction of NO to N(2)O. NOR is composed of two subunits: the large one (NorB) is a bundle of 12 transmembrane helices (TMH). It contains a b type heme and a binuclear iron site, which is believed to be the catalytic site, comprising a heme b and a non-hemic iron. The small subunit (NorC) harbors a cytochrome c and is attached to the membrane through a unique TMH. With the aim to perform structural and functional studies of NOR, we have immunized dromedaries with NOR and produced several antibody fragments of the heavy chain (VHHs, also known as nanobodies (TM)). These fragments have been used to develop a faster NOR purification procedure, to proceed to crystallization assays and to analyze the electron transfer of electron donors. BIAcore experiments have revealed that up to three VHHs can bind concomitantly to NOR with affinities in the nanomolar range. This is the first example of the use of VHHs with an integral membrane protein. Our results indicate that VHHs are able to recognize with high affinity distinct epitopes on this class of proteins, and can be used as versatile and valuable tool for purification, functional study and crystallization of integral membrane proteins.

The isolation and characterization of a new metalloprotein containing Cu and Fe atoms is reported. The as-isolated Cu-Fe protein shows an UV-visible spectrum with absorption bands at 320 nm, 409 nm and 615 nm. Molecular mass of the native protein along with denaturating electrophoresis and mass spectrometry data show that this protein is a multimer consisting of 14 +/- 1 subunits of 15254.3 +/- 7.6 Da. Mossbauer spectroscopy data of the as-isolated Cu-Fe protein is consistent with the presence of [2Fe-2S](2+) centers. Data interpretation of the dithionite reduced protein suggest that the metallic cluster could be constituted by two ferromagnetically coupled [2Fe-2S](+) spin delocalized pairs. The biochemical properties of the Cu-Fe protein are similar to the recently reported molybdenum resistance associated protein from Desulfovibrio, D. alaskensis. Further-more, a BLAST search from the DNA deduced amino acid sequence shows that the Cu-Fe protein has homology with proteins annotated as zinc resistance associated proteins from Desulfovibrio, D. alaskensis, D. vulgaris Hildenborough, D. piger ATCC 29098. These facts suggest a possible role of the Cu-Fe protein in metal tolerance. (C) 2009 Published by Elsevier Inc.

The characterization of a novel Mo-Fe protein (MorP) associated with a system that responds to Mo in Desulfovibrio alaskensis is reported. Biochemical characterization shows that MorP is a periplasmic homomultimer of high molecular weight (260 +/- 13 kDa) consisting of 16-18 monomers of 15321.1 +/- 0.5 Da. The UV/visible absorption spectrum of the as-isolated protein shows absorption peaks around 280, 320, and 570 nm with extinction coefficients of 18700, 12800, and 5000 M(-1) cm(-1), respectively. Metal content, EXAFS data and DFT calculations support the presence of a Mo-2S-[2Fe-2S]-2S-Mo cluster never reported before. Analysis of the available genomes from Desulfovibrio species shows that the MorP encoding gene is located downstream of a sensor and a regulator gene. This type of gene arrangement, called two component system, is used by the cell to regulate diverse physiological processes in response to changes in environmemtal conditions. Increase of both gene expression and protein production was observed when cells were cultured in the presence of 45 mu M molybdenum. Involvement of this system in Mo tolerance of sulfate reducing bacteria is proposed.

The multicopper enzyme nitrous oxide reductase (N2OR) catalyzes the final step of denitrification, the two-electron reduction of N2O to N-2. This enzyme is a functional homodimer containing two different multicopper sites: CuA and CuZ. CuA is a binuclear copper site that transfers electrons to the tetranuclear copper sulfide CuZ, the catalytic site. In this study, Pseudomonas nautica cytochrome C-552 was identified as the physiological electron donor. The kinetic data show differences when physiological and artificial electron donors are compared [cytochrome vs methylviologen (MV)]. In the presence of cytochrome c(552), the reaction rate is dependent on the ET reaction and independent of the N2O concentration. With MV, electron donation is faster than substrate reduction. From the study of cytochrome c(552) concentration dependence, we estimate the following kinetic parameters: K-mc512 = 50.2 +/- 9.0 mu M and V-maxc551 1.8 +/- 10.6 units/mg. The N2O concentration dependence indicates a K-mN2O of 14.0 +/- 2.9 mu M using MV as the electron donor. The pH effect on the kinetic parameters is different when MV or cytochrome c(552) is used as the electron donor (pK(a) = 6.6 or 8.3, respectively). The kinetic study also revealed the hydrophobic nature of the interaction, and direct electron transfer studies showed that CuA is the center that receives electrons from the physiological electron donor. The formation of the electron transfer complex was observed by H-1 NMR protein-protein titrations and was modeled with a molecular docking program (BiGGER). The proposed docked complexes corroborated the ET studies giving a large number of solutions in which cytochrome c(552) is placed near a hydrophobic patch located around the CuA center.

Adenylate kinase (AK) mediates the reversible transfer of phosphate groups between the adenylate nucleotides and contributes to the maintenance of their constant cellular level, necessary for energy metabolism and nucleic acid synthesis. The AK were purified from crude extracts of two sulfate-reducing bacteria (SRB), Desulfovibrio (D.) gigas NCIB 9332 and Desulfovibrio desulfuricans ATCC 27774, and biochemically and spectroscopically characterised in the native and fully cobalt- or zinc-substituted forms. These are the first reported adenylate kinases that bind either zinc or cobalt and are related to the subgroup of metal-containing AK found, in most cases, in Gram-positive bacteria. The electronic absorption spectrum is consistent with tetrahedral coordinated cobalt, predominantly via sulfur ligands, and is supported by EPR. The involvement of three cysteines in cobalt or zinc coordination was confirmed by chemical methods. Extended X-ray absorption fine structure (EXAFS) indicate that cobalt or zinc are bound by three cysteine residues and one histidine in the metal-binding site of the "LID" domain. The sequence (129)Cys-X(5)-His-X(15)-Cys-X(2)-Cys of the AK from D. gigas is involved in metal coordination and represents a new type of binding motif that differs from other known zinc-binding sites of AK. Cobalt and zinc play a structural role in stabilizing the LID domain. (C) 2008 Elsevier Inc. All rights reserved.

We report the 98% assignment of the apo-form of an orange protein, containing a novel Mo-Cu cluster isolated from Desulfovibrio gigas. This protein presents a region where backbone amide protons exchange fast with bulk solvent becoming undetectable. These residues were assigned using C-13-detection experiments.

An orange-coloured protein (ORP) isolated from Desulfovibrio gigas, a sulphate reducer, has been previously shown by extended X-ray absorption fine structure (EXAFS) to contain a novel mixed-metal sulphide cluster of the type [S2MoS2CuS2MoS2] [J. Am. Chem. Soc. 122 (2000) 8321]. We report here the purification and the biochemical/spectroscopic characterisation of this novel protein. ORP is a soluble monomeric protein (11.8 kDa). The cluster is non-covalently bound to the polypeptide chain. The presence of a MoS42- moiety in the structure of the cofactor contributes with a quite characteristic UV-Vis spectra, exhibiting an orange colour, with intense absorption peaks at 480 and 338 nm. Pure ORP reveals an Abs(480)/Abs(338) ratio of 0.535. The gene sequence coding for ORP as well as the amino acid sequence was determined. The putative biological function of ORP is discussed. (C) 2003 Elsevier Inc. All rights reserved.

Nitrous oxide (N(2)O) is a greenhouse gas, the third most significant contributor to global warming. As a key process for N(2)O elimination from the biosphere, N(2)O reductases catalyze the two-electron reduction of N(2)O to N(2). These 2 x 65 kDa copper enzymes are thought to contain a CuA electron entry site, similar to that of cytochrome c oxidase, and a CuZ catalytic center. The copper anomalous signal was used to solve the crystal structure of N(2)O reductase from Pseudomonas nautica by multiwavelength anomalous dispersion, to a resolution of 2.4 Angstrom. The structure reveals that the CuZ center belongs to a new type of metal cluster, in which four copper ions are liganded by seven histidine residues. N(2)O binds to this center via a single copper ion. The remaining copper ions might act as an electron reservoir, assuring a fast electron transfer and avoiding the formation of dead-end products.

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 CUA and Cut. Cut 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 similar to 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 T, g(z) =0.178, A(z) = 4 mT) and absorption bands at 480, 540, and similar to 800 nm. The difference between the two purified forms of nitrous oxide reductase is interpreted as a difference in the oxidation state of the CuA center. In form A, CUA is predominantly oxidized (S = 1/2, Cu1.5+-Cu1.5+), while in form B it is mostly in the one-electron reduced state (S = 0, Cu1+-Cu1+). In both forms, Cu-Z remains reduced (S = 1/2). Complete crystallographic data at 2.4 Angstrom 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 ai. (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.

An air-stable formate dehydrogenase (FDH), an enzyme that catalyzes the oxidation of formate to carbon dioxide, was purified from the sulfate reducing organism Desulfovibrio gigas (D. gigas) NCIB 9332. D. gigas FDH is a heterodimeric protein [alpha (92 kDa) and beta (29 kDa) subunits] and contains 7 +/- 1 Fe/protein and 0.9 +/- 0.1 W/protein, Selenium was not detected. The UV/visible absorption spectrum of D, gigas FDH is typical of an iron-sulfur protein. Analysis of pterin nucleotides yielded a content of 1.3 +/- 0.1 guanine monophosphate/mol of enzyme, which suggests a tungsten coordination with two molybdopterin guanine dinucleotide cofactors. Both Mossbauer spectroscopy performed on D. gigas FDH grown in a medium enriched with Fe-57 and EPR studies performed in the native and fully reduced state of the protein confirmed the presence of two [4Fe-4S] clusters. Variable-temperature EPR studies showed the presence of two signals compatible with an atom in a d(1) configuration albeit with an unusual relaxation behavior as compared to the one generally observed for W(V) ions.

The soluble methane monooxygenase system from Methylococcus capsulatus (Bath) catalyzes the oxidation of methane to methanol and water utilizing dioxygen at a non-heme, carboxylate-bridged diiron center housed in the hydroxylase (H) component. To probe the nature of the reductive activation of dioxygen in this system, reactions of an analogous molecule, nitric oxide, with the diiron(II) form of the enzyme (H-red) Were investigated by both continuous and discontinuous kinetics methodologies using optical, EPR, and Mossbauer spectroscopy. Reaction of NO with H-red affords a dinitrosyl species, designated H-dinitrosyl, with optical spectra (lambda(max) = 450 and 620 nm) and Mossbauer parameters (delta = 0.72 mm/s, Delta E-Q = 1.55 mm/s) similar to those of synthetic dinitrosyl analogues and of the dinitrosyl adduct of the reduced ribonucleotide reductase R2 (RNR-R2) protein. The H-dinitrosyl species models features of the H-peroxo intermediate formed in the analogous dioxygen reaction. In the presence of protein B, H-dinitrosyl builds up with approximately the same rate constant as H-peroxo (similar to 26 s(-1)) at 4 degrees C. In the absence of protein B, the kinetics of H-dinitrosyl formation were best fit with a biphasic A –> B –> C model, indicating the presence of an intermediate species between H-red and H-dinitrosyl. This result contrasts with the reaction of H-red with dioxygen, in which the H-peroxo intermediate forms in measurable quantities only in the presence of protein B. These findings suggest that protein B may alter the positioning but not the availability of coordination sites on iron for exogenous ligand binding and reactivity.

This communication reports the isolation, purification and characterization of key enzymes involved in dissimilatory sulfate reduction of a sulfate reducing bacterium classified as Desulfovibrio desulfuricans subspecies desulfuricans New Jersey (NCIMB 8313) (Ddd NJ). The chosen strain, originally recovered from a corroding cast iron heat exchanger, was grown in large scale batch cultures. Physico-chemical and spectroscopic studies of the purified enzymes were carried out. These analyses revealed a high degree of similarity between proteins isolated from the DddNJ strain and the homologous proteins obtained from Desulfomicrobium baculatus Norway 4. In view of the results obtained, taxonomic reclassification of Desulfovibrio desulfuricans subspecies desulfuricans New Jersey (NCIMB 8313) into Desulfomicrobium baculatus (New Jersey) is proposed. (C) 1996 Academic Press, Inc.