Electrostatic effects in electron transfer reactions of [2Fe‐2S] ferredoxins with inorganic reagents

Vidakovic, Momcilo; Germanas, Juris P.

doi:10.1002/pro.5560050905

Cited by 7 publications

(4 citation statements)

References 38 publications

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“…The oxidation of the dithionite-reduced [2Fe-2S] ferredoxins by nitroimidazoles was monitored under pseudo-first-order conditions as described by us for the reaction of the ferredoxins with inorganic reagents (24). The reaction traces were accurately modeled by single-exponential fits.…”

Section: Resultsmentioning

confidence: 99%

Reactivity of Reduced [2Fe-2S] Ferredoxins Parallels Host Susceptibility to Nitroimidazoles

Vidakovic¹,

Crossnoe

Neidre

et al. 2003

Antimicrob Agents Chemother

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The kinetics of the electron transfer reaction between reduced [2Fe-2S] ferredoxins and select nitroimidazole antimicrobial agents is reported. The ferredoxins from the protozoan Trichomonas vaginalis and the cyanobacterium Anabaena sp. strain 7120 were studied because they are the proximal electron donors to nitroimidazoles in these two organisms with significantly different nitroimidazole susceptibilities. The rates of electron transfer from Anabaena ferredoxin to all nitroimidazoles were 1 to 2 orders of magnitude lower than for T. vaginalis ferredoxin. Quantitative structure-activity analysis of the kinetic data showed that the size of the alkyl substituent on the N-1 position of the imidazole ring strongly influenced the magnitude of the electron transfer rate constant. This implies that the distance between the iron-sulfur cluster and the nitro group of the imidazole is the critical variable in determining the rate of electron transfer. A correlation between the magnitude of the one-electron transfer rate constant with the susceptibility of the host organism to the cytotoxic effects of nitroimidazoles was also discovered. These results demonstrate that reductive activation is the most crucial step in determining the toxicity of nitroimidazoles. (2)] is a member of the nitroheterocycle class of antimicrobial compounds that also includes the nitroimidazole tinidazole and the nitrofuran nitrofurantoin. Metronidazole is a front-line antibiotic in use against a wide range of infectious species. For example, metronidazole is highly effective in treating trichomoniasis and is equally useful against other protozoal infections, including giardiasis and amebiasis (6). Further, metronidazole is the primary drug used to treat postoperative and other infections with anaerobic bacteria, including members of the genera Bacteroides, Fusobacterium, and Clostridium. In addition metronidazole is a component of the therapeutic regimen to eradicate Helicobacter pylori, which is responsible for the majority of cases of peptic ulcer disease and is a risk factor for gastric cancer (4).The importance of metronidazole in treating various infectious diseases and its unusually broad spectrum of activity have led to investigation of its mechanism of action. Most studies have focused on the activity of nitroimidazoles against the protozoan Trichomonas vaginalis, the causative organism of trichomoniasis in humans. Researchers initially believed that the drug inhibited hydrogenase, an enzyme involved in anaerobic carbohydrate metabolism (6). Further studies showed, however, that the ultimate target of the drug was DNA, and that nitroimidazoles not only inhibited DNA synthesis, but also degraded existing DNA (16). Labeling experiments implicated a radical intermediate as the agent responsible for DNA degradation. Within anaerobic cells nitroimidazoles were found to be reduced to radical anion species with the ability to initiate DNA degradation through abstraction of atoms of the nucleic acid backbone (5). The redox potential of nitroimid...

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Section: Resultsmentioning

confidence: 99%

Reactivity of Reduced [2Fe-2S] Ferredoxins Parallels Host Susceptibility to Nitroimidazoles

Vidakovic¹,

Crossnoe

Neidre

et al. 2003

Antimicrob Agents Chemother

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“…The previously published structure of Pdx ( 5), which is also a hybrid structure combining experimental data with modeling of the metal cluster environs, has been used successfully as a guide for mutagenesis studies in Pdx and Adx (31)(32)(33)(34)(35) as well as for a variety of homology modeling and theoretical studies (4,(36)(37)(38)(39)(40). It was noted by the authors of the Adx crystallographic study (11) that in a bestfit superposition of the Pdx o structure with the Adx structure, the crystallographically determined positions of the atoms in the Fe 2 S 2 cluster in Adx superimpose almost precisely on the corresponding atoms as modeled in the Pdx o structure (5).…”

Section: Validity Of the Modelingmentioning

confidence: 99%

A Refined Model for the Solution Structure of Oxidized Putidaredoxin^,

et al. 1999

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A refined model for the solution structure of oxidized putidaredoxin (Pdxo), a Cys4Fe2S2 ferredoxin, has been determined. A previous structure (Pochapsky et al. (1994) Biochemistry 33, 6424-6432; PDB entry ) was calculated using the results of homonuclear two-dimensional NMR experiments. New data has made it possible to calculate a refinement of the original Pdxo solution structure. First, essentially complete assignments for diamagnetic 15N and 13C resonances of Pdxo have been made using multidimensional NMR methods, and 15N- and 13C-resolved NOESY experiments have permitted the identification of many new NOE restraints for structural calculations. Stereospecific assignments for leucine and valine CH3 resonances were made using biosynthetically directed fractional 13C labeling, improving the precision of NOE restraints involving these residues. Backbone dihedral angle restraints have been obtained using a combination of two-dimensional J-modulated 15N,1H HSQC and 3D (HN)CO(CO)NH experiments. Second, the solution structure of a diamagnetic form of Pdx, that of the C85S variant of gallium putidaredoxin, in which a nonligand Cys is replaced by Ser, has been determined (Pochapsky et al. (1998) J. Biomol. NMR 12, 407-415), providing information concerning structural features not observable in the native ferredoxin due to paramagnetism. Third, a crystal structure of a closely related ferredoxin, bovine adrenodoxin, has been published (Müller et al. (1998) Structure 6, 269-280). This structure has been used to model the metal binding site structure in Pdx. A family of fourteen structures is presented that exhibits an rmsd of 0.51 A for backbone heavy atoms and 0.83 A for all heavy atoms. Exclusion of the modeled metal binding loop region reduces overall the rmsd to 0.30 A for backbone atoms and 0.71 A for all heavy atoms.

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“…However, it is also plausible that the mutation for the charge neutralization often affects other interactions between P450cam and Pd such as van der Waals contacts and hydrophobic interactions, which complicates the interpretation of the mutational effects and computer simulation. To simplify the ET reaction system and enhance the effects of electrostatic interaction on the ET reaction, many studies have utilized small inorganic reagents as models for its physiological partner proteins ( − ). By using small and isostructural inorganic compounds as redox partners, the steric and hydrophobic effects can be regulated.…”

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confidence: 99%

“…By using small and isostructural inorganic compounds as redox partners, the steric and hydrophobic effects can be regulated. Such approaches have been applied to the ET reaction of blue copper protein ( , ), cytochrome ( , ), and iron−sulfur proteins ( , ), by which some interaction sites for the ET reactions have been identified and the ET processes have been characterized.…”

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confidence: 99%

Electron Transfer Reactions in Zn-Substituted Cytochrome P450cam

2000

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We have investigated photoinduced electron transfer (ET) reactions between zinc-substituted cytochrome P450cam (ZnP450) and several inorganic reagents by using the laser flash photolysis method, to reveal roles of the electrostatic interactions in the regulation of the ET reactions. The laser pulse irradiation to ZnP450 yielded a strong reductant, the triplet excited state of ZnP450, (3)ZnP450, which was able to transfer one electron to anionic redox partners, OsCl(6)(2-) and Fe(CN)(6)(3-), with formation of the porphyrin pi-cation radical, ZnP450(+). In contrast, the ET reactions from (3)ZnP450 to cationic redox partners, such as Ru(NH(3))(6)(3+) and Co(phen)(3)(3+), were not observed even in the presence of 100-fold excess of the oxidant. One of the possible interpretations for the preferential ET to the anionic redox partner is that the cationic patch on the P450cam surface, a putative interaction site for the anionic reagents, is located near the heme (less than 10 A from the heme edge), while the anionic surface is far from the heme moiety (more than 16 A from the heme edge), which would yield 8000-fold faster ET rates through the cationic patch. The ET rate through the anionic patch to the cationic partner would be substantially slower than that of the phosphorescence process in (3)ZnP450, resulting in no ET reactions to the cationic reagents. These results demonstrate that the asymmetrical charge distribution on the protein surface is critical for the ET reaction in P450cam.

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Electrostatic effects in electron transfer reactions of [2Fe‐2S] ferredoxins with inorganic reagents

Cited by 7 publications

References 38 publications

Reactivity of Reduced [2Fe-2S] Ferredoxins Parallels Host Susceptibility to Nitroimidazoles

Reactivity of Reduced [2Fe-2S] Ferredoxins Parallels Host Susceptibility to Nitroimidazoles

A Refined Model for the Solution Structure of Oxidized Putidaredoxin^,

Electron Transfer Reactions in Zn-Substituted Cytochrome P450cam

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Electrostatic effects in electron transfer reactions of [2Fe‐2S] ferredoxins with inorganic reagents

Cited by 7 publications

References 38 publications

Reactivity of Reduced [2Fe-2S] Ferredoxins Parallels Host Susceptibility to Nitroimidazoles

Reactivity of Reduced [2Fe-2S] Ferredoxins Parallels Host Susceptibility to Nitroimidazoles

A Refined Model for the Solution Structure of Oxidized Putidaredoxin,

Electron Transfer Reactions in Zn-Substituted Cytochrome P450cam

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Product

Resources

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A Refined Model for the Solution Structure of Oxidized Putidaredoxin^,