Zur Identifizierung und Redoxfunktion der Pigmente von Pseudomonas aureofaciens und P. iodina

Manipulation of microbial extracellular electron transfer by changing molecular structure of phenazine-type redox mediators

Phenazines, as a type of electron shuttle, are involved in various biological processes to facilitate microbial energy metabolism and electron transfer. They constitute a large group of nitrogen-containing heterocyclic compounds, which can be produced by a diverse range of bacteria or by artificial synthesis. They vary significantly in their properties, depending mainly on the nature and position of substitutent group. Thus, it is of great interest to find out the most favorable substituent type and molecular structure of phenazines for electron transfer routes. Here, the impacts of the substituent group on the reduction potentials of phenazine-type redox mediators in aqueous solution were investigated by quantum chemical calculations, and the calculation results were further validated with experimental data. The results show that the reaction free energy was substantially affected by the location of substituent groups on the phenazine molecule and the protonated water clusters. For the main proton addition process, the phenazines substituted with electron-donating groups and those with electron-withdrawing groups interacted with different protonated water clusters, attributed to the proximity effect of water molecules on proton transfer. Thus, high energy conversion efficiency could be achieved by controlling electron flow route with appropriate substituted phenazines to reduce the biological energy acquisition. This study provides useful information for designing efficient redox mediators to promote electron transfer between microbes and terminal acceptors, which are essential to bioenergy recovery from wastes and environmental bioremediation.

Redox reactions of phenazine antibiotics with ferric (hydr)oxides and molecular oxygen

Phenazines are small redox-active molecules produced by a variety of bacteria. Beyond merely serving as antibiotics, recent studies suggest that phenazines play important physiological roles, including one in iron acquisition. Here we characterize the ability of four electrochemically reduced natural phenazines--pyocyanin (PYO), phenazine-1-carboxylate (PCA), phenazine-1-carboxamide, and 1-hydroxyphenazine (1-OHPHZ)--to reductively dissolve ferrihydrite and hematite in the pH range 5-8. Generally, the reaction rate is higher for a phenazine with a lower reduction potential, with the reaction between PYO and ferrihydrite at pH 5 being an exception; the rate decreases as the pH increases; the rate is higher for poorly crystalline ferrihydrite than for highly crystalline hematite. Ferric (hydr)oxide reduction by reduced phenazines can potentially be inhibited by oxygen, where O2 competes with Fe(III) as the final oxidant The reactivity of reduced phenazines with 02 decreases in the order: PYO > 1-OHPHZ > PCA. Strikingly, reduced PYO,which isthe least reactive phenazine with ferrihydrite and hematite at pH 7, is the most reactive phenazine with O2. These results imply that different phenazines may perform different functions in environments with gradients of iron and O2.

The unique biochemistry of methanogenesis

Methanogenic archaea have an unusual type of metabolism because they use H2 + CO2, formate, methylated C1 compounds, or acetate as energy and carbon sources for growth. The methanogens produce methane as the major end product of their metabolism in a unique energy-generating process. The organisms received much attention because they catalyze the terminal step in the anaerobic breakdown of organic matter under sulfate-limiting conditions and are essential for both the recycling of carbon compounds and the maintenance of the global carbon flux on Earth. Furthermore, methane is an important greenhouse gas that directly contributes to climate changes and global warming. Hence, the understanding of the biochemical processes leading to methane formation are of major interest. This review focuses on the metabolic pathways of methanogenesis that are rather unique and involve a number of unusual enzymes and coenzymes. It will be shown how the previously mentioned substrates are converted to CH4 via the CO2-reducing, methylotrophic, or aceticlastic pathway. All catabolic processes finally lead to the formation of a mixed disulfide from coenzyme M and coenzyme B that functions as an electron acceptor of certain anaerobic respiratory chains. Molecular hydrogen, reduced coenzyme F420, or reduced ferredoxin are used as electron donors. The redox reactions as catalyzed by the membrane-bound electron transport chains are coupled to proton translocation across the cytoplasmic membrane. The resulting electrochemical proton gradient is the driving force for ATP synthesis as catalyzed by an A1A0-type ATP synthase. Other energy-transducing enzymes involved in methanogenesis are the membrane-integral methyltransferase and the formylmethanofuran dehydrogenase complex. The former enzyme is a unique, reversible sodium ion pump that couples methyl-group transfer with the transport of Na+ across the membrane. The formylmethanofuran dehydrogenase is a reversible ion pump that catalyzes formylation and deformylation of methanofuran. Furthermore, the review addresses questions related to the biochemical and genetic characteristics of the energy-transducing enzymes and to the mechanisms of ion translocation.

Synthesis, electrochemistry, and molluscicidal activity of nitroaromatic compounds: effects of substituents and the role of redox potential

Abstract-Molluscicidal bioassays and electrochemical studies (measurement of first wave reduction potential, Epcl) were performed on several synthetic nitroaromatics, in relation to possible correlation between biological activity, redox potential and structural effects. Five of them presented a significant molluscicidal activity on Biomphalaria glabrata (LD50 < 20 ppm). The Epc1 values ranged from -0.532 to -0.857 V versus Ag/AgCl (0.1 M) (-0.260 to -0.585 V versus NHE), all of them, in the favorable range for reduction in vivo. Data comparison between Epc1 and molluscicidal activity indicates that the presence of the electroactive nitro group is important for the biological activity. Correlation with redox potential, however, was not evident. Structural effects seem to be the most important parameter. Higher activity is noticeable for phenols, including the para-nitro azo or hydrazo-containing compounds. No activity was observed for compounds having the benzylic substituent in meta position to the nitro group. These results suggest that activity undoubtedly involves more than reduction characteristics and that the possible formation of electrophilic species, after nitro reduction, can play an important role in molluscicidal activity against B. glabrata.

The F420H2 dehydrogenase from Methanosarcina mazei is a Redox-driven proton pump closely related to NADH dehydrogenases

The F(420)H(2) dehydrogenase is part of the energy conserving electron transport system of the methanogenic archaeon Methanosarcina mazei Gö1. Here it is shown that cofactor F(420)H(2)-dependent reduction of 2-hydroxyphenazine as catalyzed by the membrane-bound enzyme is coupled to proton translocation across the cytoplasmic membrane, exhibiting a stoichiometry of 0.9 H(+) translocated per two electrons transferred. The electrochemical proton gradient thereby generated was shown to drive ATP synthesis from ADP + P(i). The gene cluster encoding the F(420)H(2) dehydrogenase of M. mazei Gö1 comprises 12 genes that are referred to as fpoA, B, C, D, H, I, J, K, L, M, N, and O. Analysis of the deduced amino acid sequences revealed that the enzyme is closely related to proton translocating NADH dehydrogenases of respiratory chains from bacteria (NDH-1) and eukarya (complex I). Like the NADH-dependent enzymes, the F(420)H(2) dehydrogenase is composed of three subcomplexes. The gene products FpoA, H, J, K, L, M, and N are highly hydrophobic and are homologous to subunits that form the membrane integral module of NDH-1. FpoB, C, D, and I have their counterparts in the amphipathic membrane-associated module of NDH-1. Homologues to the hydrophilic NADH-oxidizing input module are not present in M. mazei Gö1. Instead, the gene product FpoF may be responsible for F(420)H(2) oxidation and may function as the electron input part. Thus, the F(420)H(2) dehydrogenase from M. mazei Gö1 resembles eukaryotic and bacterial proton translocating NADH dehydrogenases in many ways. The enzyme from the methanogenic archaeon functions as a NDH-1/complex I homologue and is equipped with an alternative electron input unit for the oxidation of reduced cofactor F(420) and a modified output module adopted to the reduction of methanophenazine.

Isolation and characterization of methanophenazine and function of phenazines in membrane-bound electron transport of Methanosarcina mazei Gö1

A hydrophobic, redox-active component with a molecular mass of 538 Da was isolated from lyophilized membranes of Methanosarcina mazei Gö1 by extraction with isooctane. After purification on a high-performance liquid chromatography column, the chemical structure was analyzed by mass spectroscopy and nuclear magnetic resonance studies. The component was called methanophenazine and represents a 2-hydroxyphenazine derivative which is connected via an ether bridge to a polyisoprenoid side chain. Since methanophenazine was almost insoluble in aqueous buffers, water-soluble phenazine derivatives were tested for their ability to interact with membrane-bound enzymes involved in electron transport and energy conservation. The purified F42OH2 dehydrogenase from M. mazei Gö1 showed highest activity with 2-hydroxyphenazine and 2-bromophenazine as electron acceptors when F420H2 was added. Phenazine-1-carboxylic acid and phenazine proved to be less effective. The Km values for 2-hydroxyphenazine and phenazine were 35 and 250 microM, respectively. 2-Hydroxyphenazine was also reduced by molecular hydrogen catalyzed by an F420-nonreactive hydrogenase which is present in washed membrane preparations. Furthermore, the membrane-bound heterodisulfide reductase was able to use reduced 2-hydroxyphenazine as an electron donor for the reduction of CoB-S-S-CoM. Considering all these results, it is reasonable to assume that methanophenazine plays an important role in vivo in membrane-bound electron transport of M. mazei Gö1.

Cyclic voltammetry of phenazines and quinoxalines including mono- and di-N-oxides. Relation to structure and antimicrobial activity

Cyclic voltammetry data were obtained for eight phenazines and phenazine-N-oxides, and eleven quinoxalines and quinoxaline-N-oxides: 1,6-phenazine-diol-5,10-dioxide (iodinin), iodinin copper complex, 6-methoxy-1-phenazinol-5,10-dioxide 1,6-dimethoxyphenazine-5-oxide, 1,6-phenazinediol, 1,6-dimethoxyphenazine, quinoxaline-1,4-dioxide, 2-methylquinoxaline-1,4-dioxide, 2,3-diphenylquinoxaline-1,4-dioxide, 2-carboxyquinoxaline-1,4-dioxide, 5-hydroxyquinoxaline-1,4-dioxide, 5-hydroxy-8-methoxyquinoxaline-1,4-dioxide, 2-methylquinoxaline, 2,3-diphenylquinoxaline, 5-hydroxyquinoxaline, 5-hydroxy-8-methoxyquinoxaline and 2-(2-quinoxalinylmethylene)hydrazine carboxylic acid methyl ester-1,4-dioxide (Carbadox). The di-N-oxides exhibit the most positive E1/2 values within each class. Reversible first wave reductions were observed for iodinin, iodinin copper complex, 1,6-dimethoxyphenazine-5-oxide, 1,6-dimethoxyphenazine, quinoxaline-1,4-dioxide, 2-methylquinoxaline-1,4-oxide and 2,3-diphenylquinoxaline-1,4-dioxide. The results are correlated with structure. Some relationships exist between reduction potential and reported antimicrobial activity. A possible mechanism of drug action is addressed.

Role of pyocyanin in the acquisition of iron from transferrin

Pseudomonas aeruginosa produces a blue pigment called pyocyanin. In the presence of oxidizable substrates, bacteria reduce this pigment to a colorless product, leukopyocyanin. Pyocyanin can also be nonenzymatically reduced by NADH. Leukopyocyanin formed by cell- or NADH-mediated reduction nonenzymatically reduces oxygen or Fe(III). Pyocyanin-dependent iron reduction by whole bacterial cells was measured by the formation of the ferrous-ferrozine complex. In addition, leukopyocyanin reduced chelated Fe(III) including ferric iron in complex with transferrin, the serum iron-binding protein. High-pressure liquid chromatography was used to display the reductive removal of iron from transferrin and the accumulation of iron in the ferrous-ferrozine complex. Pyocyanin stimulated the accumulation of 55Fe from [55Fe]transferrin when it was added to bacteria incubated under low-oxygen conditions. Although bacteria grown in the presence of 100 microM FeCl3 reduced pyocyanin just as rapidly as iron-limited bacteria, these cells did not accumulate iron in the presence or absence of pyocyanin. Therefore, P. aeruginosa participates indiscriminantly in the reduction of pyocyanin, but soluble or available iron generated by the pyocyanin is taken up specifically by iron-limited bacteria.

Charge transfer in the mechanism of drug action involving quinoxaline di-N-oxides

Cyclic voltammetry data were obtained for various 2,3-disubstituted quinoxaline di-N-oxides: dimethyl, bishydroxymethyl, bisacetoxymethyl, bis-N-anilinomethyl, and dicarboxaldehyde hydrate. The dimethyl derivative exhibited the most negative E1/2 value, and along with the diol, showed reversible reduction for the first wave. Rationalizations of the E1/2 values are provided. Reasonable correlations exist for the electrochemical data and drug activity. The results support the diiminium theory of drug action.