Computational approaches to amino acid side-chain conformation using combined NMR theoretical and experimental results: leucine-67 in Desulfovibrio vulgaris flavodoxin.

In this paper, we show that the combination of NMR theoretical and experimental results can help to solve the molecular structure of peptides, here it is used as an example the residue Leucine-67 in Desulfovibrio vulgaris flavodoxin. We apply a computational protocol based on the leucine amino acid dipeptide, which, using calculated and experimental spin-spin coupling constants, allows us to obtain the conformation of the amino acid side chain. Calculated results show that the best agreement is obtained when three conformers around the lateral chain angle $\chi _1$ are considered or when the dynamic effect in the torsional angles is included. The population of each structure is estimated and analyzed according to the correlation between those two approaches. Independently of the approach, the estimated $\chi _1$ angle in solution is close to the staggered value of -60$^\circ $ and deviates significantly from the average x-ray angle of -90$^\circ $.

Structure and function of an unusual flavodoxin from the domain Archaea.

Flavodoxins, electron transfer proteins essential for diverse metabolisms in microbes from the domain Bacteria, are extensively characterized. Remarkably, although genomic annotations of flavodoxins are widespread in microbes from the domain Archaea, none have been isolated and characterized. Herein is described the structural, biochemical, and physiological characterization of an unusual flavodoxin (FldA) from Methanosarcina acetivorans, an acetate-utilizing methane-producing microbe of the domain Archaea In contrast to all flavodoxins, FldA is homodimeric, markedly less acidic, and stabilizes an anionic semiquinone. The crystal structure reveals an flavin mononucleotide (FMN) binding site unique from all other flavodoxins that provides a rationale for stabilization of the anionic semiquinone and a remarkably low reduction potentials for both the oxidized/semiquinone (-301 mV) and semiquinone/hydroquinone couples (-464 mV). FldA is up-regulated in acetate-grown versus methanol-grown cells and shown here to substitute for ferredoxin in mediating the transfer of low potential electrons from the carbonyl of acetate to the membrane-bound electron transport chain that generates ion gradients driving ATP synthesis. FldA offers potential advantages over ferredoxin by (i) sparing iron for abundant iron-sulfur proteins essential for acetotrophic growth and (ii) resilience to oxidative damage.

Expression, purification and characterization of two Clostridium acetobutylicum flavodoxins: potential electron transfer partners for CYP152A2.

Two flavodoxin genes from Clostridium acetobutylicum, CacFld1 (CAC0587) and CacFld2 (CAC3417), were expressed in Escherichia coli and investigated for their ability to support activity of CYP152A2, a fatty acid hydroxylase from C. acetobutylicum. E. coli flavodoxin reductase (FdR) was used as a redox partner, since flavodoxin reductase CacFdR (CAC0196) from C. acetobutylicum could not be purified in a functional form. CacFld1 was shown to accept electrons from FdR and transfer them to CYP152A2. Since H2O2 was generated by uncoupling at different stages of the reconstituted electron transfer chain, catalase was used as H2O2 scavenger in order to exclude peroxygenation by CYP152A2. The reconstituted P450 system with CacFld1 and FdR oxidized myristic acid with a K(M) of 137 µM and a k(cat) of 36 min⁻¹. Furthermore, the hydroxylase activity of CYP152A2 towards myristic acid with CacFld1 was 17-fold higher than without CacFld1. Along with CYP152A2 and a physiological flavodoxin reductase, CacFld1 is therefore likely to be involved in oxygen detoxification in C. acetobutylicum. Flavodoxin CacFld2 did not accept electrons from NADPH-reduced FdR, though it cannot be excluded as a candidate redox partner for CYP152A2 in the presence of an appropriate physiological reductase.

NrdI, a flavodoxin involved in maintenance of the diferric-tyrosyl radical cofactor in Escherichia coli class Ib ribonucleotide reductase.

Ribonucleotide reductase (RNR) catalyzes the conversion of nucleotides to deoxynucleotides and is essential in all organisms. Class I RNRs consist of two homodimeric subunits: alpha2 and beta2. The alpha subunit contains the site of nucleotide reduction, and the beta subunit contains the essential diferric-tyrosyl radical (Y*) cofactor. Escherichia coli contains genes encoding two class I RNRs (Ia and Ib) and a class III RNR, which is active only under anaerobic conditions. Its class Ia RNR, composed of NrdA (alpha) and NrdB (beta), is expressed under normal aerobic growth conditions. The class Ib RNR, composed of NrdE (alpha) and NrdF (beta), is expressed under oxidative stress and iron-limited growth conditions. Our laboratory is interested in pathways of cofactor biosynthesis and maintenance in class I RNRs and modulation of Y* levels as a means of regulating RNR activity. Our recent studies have implicated a [2Fe2S]-ferredoxin, YfaE, in the NrdB diferric-Y* maintenance pathway and possibly in the biosynthetic and regulatory pathways. Here, we report that NrdI is a flavodoxin counterpart to YfaE for the class Ib RNR. It possesses redox properties unprecedented for a flavodoxin (E(ox/sq) = -264 +/- 17 mV and E(sq/hq) = -255 +/- 17 mV) that allow it to mediate a two-electron reduction of the diferric cluster of NrdF via two successive one-electron transfers. Data presented support the presence of a distinct maintenance pathway for NrdEF, orthogonal to that for NrdAB involving YfaE.

Mapping solvation dynamics at the function site of flavodoxin in three redox states.

Flavoproteins are unique redox coenzymes, and the dynamic solvation at their function sites is critical to the understanding of their electron-transfer properties. Here, we report our complete characterization of the function-site solvation of holoflavodoxin in three redox states and of the binding-site solvation of apoflavodoxin. Using intrinsic flavin cofactor and tryptophan residue as the local optical probes with two site-specific mutations, we observed distinct ultrafast solvation dynamics at the function site in the three states and at the related recognition site of the cofactor, ranging from a few to hundreds of picoseconds. The initial ultrafast motion in 1-2.6 ps reflects the local water-network relaxation around the shallow, solvent-exposed function site. The second relaxation in 20-40 ps results from the coupled local water-protein fluctuation. The third dynamics in hundreds of picoseconds is from the intrinsic fluctuation of the loose loops flanking the cofactor at the function site. These solvation dynamics with different amplitudes well correlate with the redox states from the oxidized form, to the more rigid semiquinone and to the much looser hydroquinone. This observation of the redox control of local protein conformation plasticity and water network flexibility is significant, and such an intimate relationship is essential to the biological function of interprotein electron transfer.

Cloning, expression and purification of cindoxin, an unusual Fmn-containing cytochrome p450 redox partner.

Cytochromes P450 (P450s) belong to a superfamily of haemoproteins that catalyse a remarkable variety of oxidative transformations. P450 catalysis generally requires that cognate redox proteins transfer electrons, derived ultimately from NAD(P)H, to the P450 for oxygen activation. P450(cin) (CYP176A1) is a bacterial P450 that is postulated to allow Citrobacter braakii to live on cineole as its sole carbon source by initiating cineole biodegradation. Here we report the cloning, expression, purification and characterisation of one of its postulated redox partners, cindoxin (Cdx), which has strong similarity to the FMN domain of cytochrome P450 reductase. Cindoxin reductase (CdR), which displays strong similarity to NADPH-dependent ferredoxin reductases, was unable to be expressed in a functional form. Mass spectrometric and HPLC analyses confirmed that the flavin cofactor of cindoxin was FMN. Redox potentiometric titrations were performed with cindoxin within the range 6

Noncooperative Formation of the off-pathway molten globule during folding of the alpha-beta parallel protein apoflavodoxin.

During folding of many proteins, molten globules are formed. These partially folded forms of proteins have a substantial amount of secondary structure but lack virtually all tertiary side-chain packing characteristic of native structures. Molten globules are ensembles of interconverting conformers and are prone to aggregation, which can have detrimental effects on organisms. Consequently, molten globules attract considerable attention. The molten globule that is observed during folding of flavodoxin from Azotobacter vinelandii is a kinetically off-pathway species, as it has to unfold before the native state of the protein can be formed. This intermediate contains helices and can be populated at equilibrium using guanidinium hydrochloride as denaturant, allowing the use of NMR spectroscopy to follow molten globule formation at the residue level. Here, we track changes in chemical shifts of backbone amides, as well as disappearance of resonances of unfolded apoflavodoxin, upon decreasing denaturant concentration. Analysis of the data shows that structure formation within virtually all parts of the unfolded protein precedes folding to the molten globule state. This folding transition is noncooperative and involves a series of distinct transitions. Four structured elements in unfolded apoflavodoxin transiently interact and subsequently form the ordered core of the molten globule. Although hydrophobic, tryptophan side chains are not involved in the latter process. This ordered core is gradually extended upon decreasing denaturant concentration, but part of apoflavodoxin's molten globule remains random coil in the denaturant range investigated. The results presented here, together with those reported on the molten globule of alpha-lactalbumin, show that helical molten globules apparently fold in a noncooperative manner.

A Research-inspired biochemistry laboratory module-combining expression, purification, crystallization, structure-solving, and characterization of a flavodoxin-like protein.

Many laboratory courses consist of short and seemingly unconnected individual laboratory exercises. To increase the course consistency, relevance, and student engagement, we have developed a research-inspired and project-based module, "From Gene to Structure and Function". This 2.5-week full-day biochemistry and structural biology module covers protein expression, purification, structure solving, and characterization. The module is centered around the flavodoxin-like protein NrdI, involved in the activation of the bacterial ribonucleotide reductase enzyme system. Through an in-depth focus on one specific protein, the students will learn the basic laboratory skills needed in order to generate a broader knowledge and breadth within the field. With respect to generic skills, the students report their findings as a scientific article, with the aim to learn to present concise research results and write scientific papers. The current research-inspired project has the potential of being further developed into a more discovery-driven project and extended to include other molecular biological techniques or biochemical/biophysical characterizations. In student evaluations, this research-inspired laboratory course has received very high ratings and been highly appreciated, where the students have gained research experience for more independent future work in the laboratory. © 2019 The Authors. Biochemistry and Molecular Biology Education published by Wiley Periodicals, Inc. on behalf of International Union of Biochemistry and Molecular Biology, 47(3):318-332, 2019.

Effect of Hofmeister ions on protein thermal stability: roles of ion hydration and peptide groups?

We have systematically explored the Hofmeister effects of cations and anions (0.3-1.75 M range) for acidic Desulfovibrio desulfuricans apoflavodoxin (net charge -19, pH 7) and basic horse heart cytochrome c (net charge +17, pH 4.5). The Hofmeister effect of the ions on protein thermal stability was assessed by the parameter dT trs/d[ion] (T trs; thermal midpoint). We show that dT trs/d[ion] correlates with ion partition coefficients between surface and bulk water and ion surface tension effects: this suggests direct interactions between ions and proteins. Surprisingly, the stability effects of the different ions on the two model proteins are similar, implying a major role of the peptide backbone, instead of charged groups, in mediation of the interactions. Upon assessing chemical/physical properties of the ions responsible for the Hofmeister effects on protein stability, ion charge density was identified as most important. Taken together, our study suggests key roles for ion hydration and the peptide group in facilitating interactions between Hofmeister ions and proteins.

Crystallization of a flavodoxin involved in nitrogen fixation in Rhodobacter capsulatus.

Flavodoxins are small electron-transfer proteins that contain one molecule of noncovalently bound flavin mononucleotide (FMN). The flavodoxin NifF from the photosynthetic bacterium Rhodobacter capsulatus is reduced by one electron from ferredoxin/flavodoxin:NADP(H) reductase and was postulated to be an electron donor to nitrogenase in vivo. NifF was cloned and overexpressed in Escherichia coli, purified and concentrated for crystallization using the hanging-drop vapour-diffusion method at 291 K. Crystals grew from a mixture of PEG 3350 and PEG 400 at pH 5.5 and belong to the tetragonal space group P4(1)2(1)2, with unit-cell parameters a = b = 66.49, c = 121.32 A. X-ray data sets have been collected to 2.17 A resolution.

Complete activity profile of Clostridium acetobutylicum [FeFe]-hydrogenase and kinetic parameters for endogenous redox partners.

In Clostridium acetobutylicum, [FeFe]-hydrogenase is involved in hydrogen production in vivo by transferring electrons from physiological electron donors, ferredoxin and flavodoxin, to protons. In this report, by modifications of the purification procedure, the specific activity of the enzyme has been improved and its complete catalytic profile in hydrogen evolution, hydrogen uptake, proton/deuterium exchange and para-H2/ortho-H2 conversion has been determined. The major ferredoxin expressed in the solvent-producing C. acetobutylicum cells was purified and identified as encoded by ORF CAC0303. Clostridium acetobutylicum recombinant holoflavodoxin CAC0587 was also purified. The kinetic parameters of C. acetobutylicum [FeFe]-hydrogenase for both physiological partners, ferredoxin CAC0303 and flavodoxin CAC0587, are reported for hydrogen uptake and hydrogen evolution activities.

One-electron photo-oxidation of reduced Desulfovibrio vulgaris flavodoxin on laser excitation at 355 nm.

Electron ejection from the reduced flavin in flavodoxin from Desulfovibrio vulgaris was obtained on exposure of the protein to the third harmonic radiation (354.7 nm) generated from a pulsed Nd/YAG laser. The results indicate that the reaction is due to stepwise two-photon excitation of the reduced flavin via the excited singlet state. The absorption spectrum of the neutral flavosemiquinone radical formed in this process was obtained. This spectrum remains stable over the time of study (0.2 ms) in the pH range studied, except for a slight evolution during the first microseconds, attributed to conformational readjustments of the active site. This two-photon excitation method provides a convenient means of generating the flavosemiquinone for ultrafast kinetic studies.

Purification and properties of a flavodoxin from the heterocystous cyanobacterium Anabaena sphaerica.

A flavodoxin was purified to homogeneity from the nitrogen-fixing heterocystous cyanobacterium Anabaena sphaerica grown under iron-limited conditions. The protein has a molecular mass of 21 kDa, and its spectral properties and amino-acid composition are very close to that of flavodoxins from other cyanobacteria. A. sphaerica flavodoxin supported the activities of A. sphaerica NADP reductase and Clostridium butyricum hydrogenase in reconstituted systems with illuminated plant chloroplasts as reductant. With the use of polyclonal anti-flavodoxin antiserum it was found that nitrogen-fixing cultures of A. sphaerica grown under iron-sufficient conditions contain low but significant amounts of flavodoxin (0.2-0.6 micrograms/mg crude extract protein) which increased dramatically (to 8-15 micrograms/mg crude extract protein) after the iron concentration in the medium was decreased to below 1 microM Fe. The flavodoxin content of both iron-limited and iron-sufficient. A. sphaerica was also shown to depend upon the growth phase of the (batch) cultures with a maximum at early exponential phase, coinciding with maximal in-vivo nitrogenase activity. These results suggest that A. sphaerica flavodoxin not only substitutes for ferredoxin under iron-limiting conditions, but also fulfills some specific role under iron-sufficient conditions.

Flavodoxin and rubredoxin from Desulphovibrio salexigens.

A flavodoxin and a rubredoxin have been isolated from the sulfate-reducing bacterium Desulphovibrio salexigens (strain British Guiana, NICB 8403). Their amino acid composition and spectral characteristics did not differ markedly from the homologous proteins presented in other Desulphovibrio spp. Flavodoxin was shown to be active in the electron transport of the sulfite reductase system.

Ferredoxin and flavodoxin from the cyanobacterium Synechocystis sp PCC 6803.

The unicellular cyanobacterium Synechocystis sp PCC 6803 is capable of synthesizing two different Photosystem-I electron acceptors, ferredoxin and flavodoxin. Under normal growth conditions a [2Fe-2S] ferredoxin was recovered and purified to homogeneity. The complete amino-acid sequence of this protein was established. The isoelectric point (pI = 3.48), midpoint redox potential (Em = -0.412 V) and stability under denaturing conditions were also determined. This ferredoxin exhibits an unusual electrophoretic behavior, resulting in a very low apparent molecular mass between 2 and 3.5 kDa, even in the presence of high concentrations of urea. However, a molecular mass of 10,232 Da (apo-ferredoxin) is calculated from the sequence. Free thiol assays indicate the presence of a disulfide bridge in this protein. A small amount of ferredoxin was also found in another fraction during the purification procedure. The amino-acid sequence and properties of this minor ferredoxin were similar to those of the major ferredoxin. However, its solubility in ammonium sulfate and its reactivity with antibodies directed against spinach ferredoxin were different. Traces of flavodoxin were also recovered from the same fraction. The amount of flavodoxin was dramatically increased under iron-deficient growth conditions. An acidic isoelectric point was measured (pI = 3.76), close to that of ferredoxin. The midpoint redox potentials of flavodoxin are Em1 = -0.433 V and Em2 = -0.238 V at pH 7.8. Sequence comparison based on the 42 N-terminal amino acids indicates that Synechocystis 6803 flavodoxin most likely belongs to the long-chain class, despite an apparent molecular mass of 15 kDa determined by SDS-PAGE.

Purification and some properties of the nitrite reductase from the cyanobacterium Phormidium laminosum.

Assimilatory ferredoxin-nitrite reductase (EC 1.7.7.1, ammonia: ferredoxin oxidoreductase) has been purified 5300-fold with a specific activity of 625 units/mg protein from the filamentous non-heterocystous cyanobacterium Phormidium laminosum. The enzyme was soluble and consisted of a single polypeptidic chain of 54 kDa. It catalyzed the reduction of nitrite to ammonia using ferredoxin or flavodoxin as electron donor. Methyl and benzyl viologens were also effective as electron donors but neither flavins nor NAD(P)H were. The apparent Michaelis constants for nitrite, ferredoxin and methyl viologen were 40, 22 and 215 microM, respectively. Nitrite reductase activity was inhibited effectively by cyanide and thiol reagents. The enzyme exhibited absorption maxima at 281, 391 (Soret), 570 (alpha) and 695 nm, with epsilon 391 of 4.3 x 10(4) M-1 cm-1, and an absorbance ratio A281/A391 of 1.95, suggesting the presence of siroheme as prosthetic group. These results show that this enzyme is similar to those of eukaryotic organisms.

Towards a new therapeutic target: Helicobacter pylori flavodoxin.

Helicobacter pylori flavodoxin is the electronic acceptor of the pyruvate-oxidoreductase complex (POR) that catalyzes pyruvate oxidative decarboxilation. Inactivation of this metabolic route precludes bacterial survival. Because flavodoxin is not present in the human host, substances interfering electronic transport from POR might be well suited for eradication therapies against the bacterium. H. pylori flavodoxin presents a peculiar cofactor (FMN) binding site, compared to other known flavodoxins, where a conserved aromatic residue is replaced by alanine. A cavity thus appears under the cofactor that can be filled with small organic molecules. We have cloned H. pylori fldA gene, expressed the protein in Escherichia coli and characterized the purified flavodoxin. Thermal up-shift assays of flavodoxin with different concentrations of benzylamine, as well as fluorescence titration experiments indicate benzylamine binds in the pocket near the FMN binding site. It seems thus that low affinity inhibitors of H. pylori flavodoxin can be easily found that, after improvement, may give rise to leads.

Cloning and expression of the gene encoding flavodoxin from Desulfovibrio vulgaris (Miyazaki F).

The gene encoding a flavodoxin of Desulfovibrio vulgaris (Miyazaki F) was cloned, and overexpressed in Escherichia coli. A 1.6-kbp DNA fragment, isolated from D. vulgaris (Miyazaki F) by double digestion with SalI and EcoRI, contained the flavodoxin gene and its regulatory region. An expression system for the flavodoxin gene under control of the T7 promoter was constructed in E. coli. The purified protein was soluble and exhibited a characteristic visible absorption spectrum. HPLC analysis of the recombinant flavodoxin revealed the presence of an identical FMN to that found in the native D. vulgaris flavodoxin, and its dissociation constant with FMN was determined to be 0.38 nM. In vitro H2 reduction analysis indicated that the recombinant flavodoxin is active, and its redox potential was determined to be E1 = -434 and E2 = -151 mV using methyl viologen and 2-hydroxy-1,4-naphthoquinone, respectively. Its redox behavior was also examined with the recombinant flavodoxin adsorbed onto a graphite electrode. The mutant, A16E, was also produced, which revealed the feature of a conserved Glu residue at the surface of the molecule.

Relationship of the flavodoxin isoforms from Porphyra umbilicalis.

The macroalga Porphyra umbilicalis contained two flavodoxins in approximately 5:1 ratio and differing in Mr by ca 1000. The N-terminal sequences of the isoforms were identical and there was strong immunochemical identity. Peptide mapping gave similar fragments which differed in Mr by constant amount for the two isoforms. The flavodoxins may therefore differ only at the C-terminus, possibly as a consequence of in vivo processing since inclusion of protease inhibitors during extraction had no effect on the ratio of the isoforms.