scholarly journals Kinetic Characterization of OmcA and MtrC, Terminal Reductases Involved in Respiratory Electron Transfer for Dissimilatory Iron Reduction in Shewanella oneidensis MR-1

2009 ◽  
Vol 75 (16) ◽  
pp. 5218-5226 ◽  
Author(s):  
Daniel E. Ross ◽  
Susan L. Brantley ◽  
Ming Tien
Keyword(s):  
Electron Transfer ◽  
Steady State ◽  
Rate Constants ◽  
Iron Reduction ◽  
Shewanella Oneidensis ◽  
Transient State ◽  
Whole Cells ◽  
Soluble Iron ◽  
Reactive Iron ◽  
Dissimilatory Iron Reduction

ABSTRACT We have used scaling kinetics and the concept of kinetic competence to elucidate the role of hemeproteins OmcA and MtrC in iron reduction by Shewanella oneidensis MR-1. Second-order rate constants for OmcA and MtrC were determined by single-turnover experiments. For soluble iron species, a stopped-flow apparatus was used, and for the less reactive iron oxide goethite, a conventional spectrophotometer was used to measure rates. Steady-state experiments were performed to obtain molecular rate constants by quantifying the OmcA and MtrC contents of membrane fractions and whole cells by Western blot analysis. For reduction of soluble iron, rates determined from transient-state experiments were able to account for rates obtained from steady-state experiments. However, this was not true with goethite; rate constants determined from transient-state experiments were 100 to 1,000 times slower than those calculated from steady-state experiments with membrane fractions and whole cells. In contrast, addition of flavins to the goethite experiments resulted in rates that were consistent with both transient- and steady-state experiments. Kinetic simulations of steady-state results with kinetic constants obtained from transient-state experiments supported flavin involvement. Therefore, we show for the first time that OmcA and MtrC are kinetically competent to account for catalysis of soluble iron reduction in whole Shewanella cells but are not responsible for electron transfer via direct contact alone with insoluble iron-containing minerals. This work supports the hypothesis that electron shuttles are important participants in the reduction of solid Fe phases by this organism.

scholarly journals Genomic Plasticity Enables a Secondary Electron Transport Pathway in Shewanella oneidensis

2012 ◽  
Vol 79 (4) ◽  
pp. 1150-1159 ◽  
Author(s):  
M. Schicklberger ◽  
G. Sturm ◽  
J. Gescher
Keyword(s):  
Electron Transfer ◽  
Cell Surface ◽  
Outer Membrane ◽  
Iron Reduction ◽  
Shewanella Oneidensis ◽  
Transport Pathway ◽  
Content Type ◽  
Reducing Conditions ◽  
Dissimilatory Iron Reduction ◽  
Membrane Spanning

ABSTRACTMicrobial dissimilatory iron reduction is an important biogeochemical process. It is physiologically challenging because iron occurs in soils and sediments in the form of insoluble minerals such as hematite or ferrihydrite.Shewanella oneidensisMR-1 evolved an extended respiratory chain to the cell surface to reduce iron minerals. Interestingly, the organism evolved a similar strategy for reduction of dimethyl sulfoxide (DMSO), which is reduced at the cell surface as well. It has already been established that electron transfer through the outer membrane is accomplished via a complex in which β-barrel proteins enable interprotein electron transfer between periplasmic oxidoreductases and cell surface-localized terminal reductases. MtrB is the β-barrel protein that is necessary for dissimilatory iron reduction. It forms a complex together with the periplasmic decahemec-type cytochrome MtrA and the outer membrane decahemec-type cytochrome MtrC. Consequently,mtrBdeletion mutants are unable to reduce ferric iron. The data presented here show that this inability can be overcome by a mobile genomic element with the ability to activate the expression of downstream genes and which is inserted within the SO4362 gene of the SO4362-to-SO4357 gene cluster. This cluster carries genes similar tomtrAandmtrBand encoding a putative cell surface DMSO reductase. Expression of SO4359 and SO4360 alone was sufficient to complement not only anmtrBmutant under ferric citrate-reducing conditions but also a mutant that furthermore lacks any outer membrane cytochromes. Hence, the putative complex formed by the SO4359 and SO4360 gene products is capable not only of membrane-spanning electron transfer but also of reducing extracellular electron acceptors.

scholarly journals Involvement of theShewanella oneidensisDecaheme Cytochrome MtrA in the Periplasmic Stability of the β-Barrel Protein MtrB

2010 ◽  
Vol 77 (4) ◽  
pp. 1520-1523 ◽  
Author(s):  
Marcus Schicklberger ◽  
Clemens Bücking ◽  
Bjoern Schuetz ◽  
Heinrich Heide ◽  
Johannes Gescher
Keyword(s):  
Gene Expression ◽  
Outer Membrane ◽  
Quantitative Pcr ◽  
Protein Complex ◽  
Iron Reduction ◽  
Heterologous Gene Expression ◽  
Shewanella Oneidensis ◽  
Heterologous Gene ◽  
Dissimilatory Iron Reduction ◽  
Membrane Spanning

ABSTRACTTheShewanella oneidensisouter membrane β-barrel protein MtrB is part of a membrane-spanning protein complex (MtrABC) which is necessary for dissimilatory iron reduction. Quantitative PCR, heterologous gene expression, and mutant studies indicated that MtrA is required for periplasmic stability of MtrB. DegP depletion compensated for this MtrA dependence.

scholarly journals Electron-transfer steps involved in the reactivity of Hansenula anomala flavocytochrome b2 as deduced from deuterium isotope effects and simulation studies

1991 ◽  
Vol 274 (1) ◽  
pp. 207-217 ◽  
Author(s):  
C Capeillère-Blandin
Keyword(s):  
Electron Transfer ◽  
Steady State ◽  
Phase I ◽  
Rate Constants ◽  
Intramolecular Electron Transfer ◽  
Simulation Studies ◽  
First Order ◽  
Flavocytochrome B2 ◽  
Hansenula Anomala ◽  
Phase Phase

The L-lactate-flavocytochrome b2-ferricyanide electron-transfer system from the yeast Hansenula anomala was investigated by rapid-reaction techniques. The kinetics of reduction of oxidized flavocytochrome b2 by L-lactate and L-[2H]lactate were biphasic both for flavin and haem prosthetic groups and at all concentrations tested. The first-order rate constants of the rapid and slow phases depended upon substrate concentrations, a saturation behaviour being exhibited. Substitution of the C alpha-H atom by 2H was found to cause appreciable changes in the rate constants for the initial reduction of flavin and haem (phase I), which were respectively about 3-fold and 2-fold less than with L-lactate. In contrast, no significant isotope effect was noted on the apparent reduction rate constants of the slow phase, phase II. Under steady-state conditions an isotope effect of 2.0 was found on the overall electron transfer from L-lactate to ferricyanide. These transient reduction results were discussed in terms of a kinetic model implying intra- and inter-protomer electron exchanges between flavin and haem b2, all of which have been experimentally described. Computer simulations indicate that the reaction scheme provides a reasonable explanation of the fast-reduction phase, phase I (in absence of acceptor). The pseudo-first-order rate constant for oxidation of reduced haem b2 in flavocytochrome b2 increased with increasing ferricyanide concentration in a hyperbolic fashion. The limiting value at infinite ferricyanide concentration, which was attributed to the intramolecular electron-transfer rate from ferroflavocytochrome b2 to the iron of ferricyanide within a complex, was 920 +/- 50 s-1 at pH 7.0 and 5 degrees C. Stopped-flow and rapid-freezing measurements showed haem b2 and flavin to be 90 and 44% oxidized respectively under steady-state conditions in presence of ferricyanide. Simulation studies were carried out to check the participation of the proposed reduction sequence in the overall catalytic reaction together with the role of reduced haem b2 (Hr) and flavin semiquinone (Fsq) as electron donors to ferricyanide. When the rate of the intramolecular electron-transfer exchange between Fsq and ferricyanide was adjusted to 200 s-1, simulated data accounted for molar activities defined under various conditions of L-lactate, [2H]lactate and ferricyanide concentrations. Simulation studies were extended to data obtained using cytochrome c as acceptor and reaction catalysed by Saccharomyces cerevisiae flavocytochrome b2. The differences in reactivity observed for Hr and Fsq with ferricyanide and cytochrome c were discussed in terms of redox potentials, electrostatic interactions, distances and accessibility of the participating groups.

scholarly journals Interpretation of kinetic isotope fractionation between aqueous Fe(II) and ferrihydrite under a high degree of microbial reduction

2020 ◽  
Author(s):  
Lei Jiang ◽  
Chuanjun Wu ◽  
Mingqing Li ◽  
Xuegong Li ◽  
Jiwei Li
Keyword(s):  
Isotope Exchange ◽  
Iron Reduction ◽  
Isotope Fractionation ◽  
Shewanella Oneidensis ◽  
Microbial Reduction ◽  
Fractionation Factor ◽  
Terminal Electron Acceptor ◽  
Iron Mineral ◽  
Dissimilatory Iron Reduction ◽  
Kinetic Fractionation

Abstract. Microbial dissimilatory iron reduction (DIR) often ceases when the degree of iron mineral reduction is low, at which point isotope fractionation occurs between an aqueous Fe(II) solution and a reactive Fe(III) phase on the surface of ferric (oxyhydro) oxides, forming an equilibrium fractionation factor (~ 3 ‰). Recent experimental abiotic studies suggest that Fe(II) adsorption onto the mineral surface may affect the isotope fractionation, which reminds us that the isotope exchange may be greatly inhibited during the DIR process. In this study, ferrihydrite is used as a terminal electron acceptor to conduct Shewanella piezotolerans WP3 and Shewanella oneidensis MR-1 experiments at 0.1 and 15 MPa to ensure a significant variation in the degree of reduction. During the 30-day experiment, the degree of ferrihydrite reduction by S. piezotolerans WP3 is 14 % (at 0.1 MPa) and 8 % (at 15 MPa), whereas the degree of ferrihydrite reduction by S. oneidensis MR-1 is 39 % (at 0.1 MPa) and 36 % (at 15 MPa). Based on the isotope mass balance, the estimated ranges of iron isotope fractionation for S. piezotolerans WP3 and S. oneidensis MR-1 are obtained. The former ranges between −3.58 ‰ and −0.88 ‰ (at 0.1 MPa) and between −2.37 ‰ and −0.66 ‰ (at 15 MPa), and the latter ranges between −0.39 ‰ and 0.10 ‰ (at 0.1 MPa) and between −0.6 ‰ and −0.16 ‰ (at 15 MPa). However, it is difficult to distinguish variations in the same bacteria at 0.1 and 15 MPa due to the large estimation ranges of isotope fractionation. In the S. oneidensis MR-1 experiment, the fractionation factor obtained is significantly different from that obtained in the S. piezotolerans WP3 experiment, indicating that kinetic fractionation occurred. In combination with previous studies, we propose a transient modified Fe(II) adsorption mechanism to explain the isotope fractionation between aqueous Fe(II) and ferrihydrite. When the adsorbed Fe(II) exceeds the surface saturation, the atom (isotope) exchange will be suppressed.

scholarly journals Extracellular Electron Transfer: Respiratory or Nutrient Homeostasis?

2020 ◽  
Vol 202 (7) ◽  
Author(s):  
Lars J. C. Jeuken ◽  
Kiel Hards ◽  
Yoshio Nakatani
Keyword(s):  
Electron Transfer ◽  
Iron Reduction ◽  
Ferric Iron ◽  
Shewanella Oneidensis ◽  
Geobacter Sulfurreducens ◽  
Extracellular Electron Transfer ◽  
Gram Positive ◽  
Content Type ◽  
Gram Positive Bacteria ◽  
Nutrient Homeostasis

ABSTRACT Exoelectrogens are able to transfer electrons extracellularly, enabling them to respire on insoluble terminal electron acceptors. Extensively studied exoelectrogens, such as Geobacter sulfurreducens and Shewanella oneidensis, are Gram negative. More recently, it has been reported that Gram-positive bacteria, such as Listeria monocytogenes and Enterococcus faecalis, also exhibit the ability to transfer electrons extracellularly, although it is still unclear whether this has a function in respiration or in redox control of the environment, for instance, by reducing ferric iron for iron uptake. In this issue of Journal of Bacteriology, Hederstedt and colleagues report on experiments that directly compare extracellular electron transfer (EET) pathways for ferric iron reduction and respiration and find a clear difference (L. Hederstedt, L. Gorton, and G. Pankratova, J Bacteriol 202:e00725-19, 2020, https://doi.org/10.1128/JB.00725-19), providing further insights and new questions into the function and metabolic pathways of EET in Gram-positive bacteria.

Formation and photoinduced properties of zinc porphyrin-SWCNT and zinc phthalocyanine-SWCNT nanohybrids using diameter sorted nanotubes assembled via metal-ligand coordination and π–π stacking

2011 ◽  
Vol 15 (09n10) ◽  
pp. 1033-1043 ◽  
Author(s):  
Sushanta K. Das ◽  
Navaneetha K. Subbaiyan ◽  
Francis D'Souza ◽  
Atula S. D. Sandanayaka ◽  
Takatsugu Wakahara ◽  
...  
Keyword(s):  
Electron Transfer ◽  
Steady State ◽  
Rate Constants ◽  
Charge Separation ◽  
Ion Pairs ◽  
Free Energy Calculations ◽  
Zinc Phthalocyanine ◽  
Ir Absorption ◽  
Zinc Porphyrin ◽  
Π Stacking

Photoinduced electron transfer processes in self-assembled zinc porphyrin ( ZnP ) or zinc phthalocyanine ( ZnPc ) with semiconducting (7,6)- and (6,5)-enriched SWCNTs were investigated. To bind photosensitizers to SWCNTs, first, pyrene covalently functionalized with a phenylimidazole (Im-Pyr) entity was treated with SWCNTs. Exfoliation of SWCNTs occurred due to π–π stacking of pyrene with nanotubes walls leaving the imidazole entity that was subsequently used to coordinate ZnP or ZnPc in o-dichlorobenzene (DCB). The donor-acceptor nanohybrids thus formed were characterized by TEM imaging, steady-state UV-visible-near IR absorption and fluorescence spectra. Free-energy calculations suggested possibility of electron transfer from the photoexcited ZnP or ZnPc to Im-Pyr/SWCNT(n,m) in the nanohybrids. Consequently, steady-state and time-resolved fluorescence studies revealed efficient quenching of the singlet excited state of ZnP or ZnPc with the rate constants of charge separation (k CS ) in the range of (3–6) × 109 s-1. Nanosecond transient absorption technique confirmed the electron transfer products, ZnP·+←Im-Pyr/SWCNT·- and ZnPc·+←Im-Pyr/SWCNT·- (and opposite charged pairs) having characteristic absorptions with the decay rate constants due to charge recombination (k CR ) in the range of (1.4–2.4) × 107 s-1, corresponding to lifetimes of radical ion-pairs in the 70–100 ns range. The SWCNT·- was further utilized to mediate electrons to hexyl-viologen dication (HV2+) resulting in an electron-accumulation process in the presence of sacrificial electron donor, offering additional proof for the occurrence of photoinduced charge-separation and potential utilization of these materials in light energy harvesting applications. Further, photoelectrochemical cells have been constructed on FTO/ SnO2 electrodes to verify their ability to directly convert light into electricity. An IPCE efficiency of up to 7% has been achieved in case of ZnP←Im-Pyr/SWCNT modified electrode.

Sign in / Sign up

Export Citation Format

Share Document