Electron transport in biological procesess II: Electrochemical behaviour of Q10 immersed in a phospholipidic matrix added on a pyrolitic graphite electrode

Reconstitution of respiratory complex I on a biomimetic membrane supported on gold electrodes

For the first time, respiratory complex I has been reconstituted on an electrode preserving its structure and activity. Respiratory complex I is a membrane-bound enzyme that has an essential function in cellular energy production. It couples NADH:quinone oxidoreduction to translocation of ions across the cellular (in prokaryotes) or mitochondrial membranes. Therefore, complex I contributes to the establishment and maintenance of the transmembrane difference of electrochemical potential required for adenosine triphosphate synthesis, transport, and motility. Our new strategy has been applied for reconstituting the bacterial complex I from Rhodothermus marinus onto a biomimetic membrane supported on gold electrodes modified with a thiol self-assembled monolayer (SAM). Atomic force microscopy and faradaic impedance measurements give evidence of the biomimetic construction, whereas electrochemical measurements show its functionality. Both electron transfer and proton translocation by respiratory complex I were monitored, simulating in vivo conditions.

Electrochemical strategy to scout 1,4-naphthoquinones effect on voltage gated potassium channels

Naphthoquinone (NQ) was tested on voltage-gated ion channels expressed in Xenopus laevis oocytes. The activity of potassium Shaker channel with Inactivation domain Removed (ShIR) was not affected; in contrast, NQ diminished Kv1.3 currents. A current decrease was barely observed with the oxidant H(2)O(2). These findings suggested that redox properties were involved in the naphthoquinone-Kv1.3 channel interaction. NQ and some derivatives (NQs) were characterized in DMSO and physiological (ND-96) media by cyclic voltammetry. A typical two-stage mono-electronic reduction mechanism was observed in DMSO, while a one-stage bi-electronic reduction process was found in ND-96 medium. NQs with the lowest and the highest redox potential values were tested on both channels. Voltage-clamp recordings showed that inhibition of Kv1.3 was dependent on NQs redox potential. Results demonstrated that structural features (aromaticity and substituents prone to hydrogen bonds formation) of NQs were also important. This effect could be explained by interactions of some channel residues with NQs that contribute to favor their reduction process in the protein surroundings. The electrochemical strategy presented to simulate the cellular environments (aqueous and non-aqueous) that NQs may face, is an important contribution to pre-select (in a fine and simple way) the best redox compounds for electrophysiological testing.

Utilization of electrically reduced neutral red by Actinobacillus succinogenes: physiological function of neutral red in membrane-driven fumarate reduction and energy conservation

Neutral red (NR) functioned as an electronophore or electron channel enabling either cells or membranes purified from Actinobacillus succinogenes to drive electron transfer and proton translocation by coupling fumarate reduction to succinate production. Electrically reduced NR, unlike methyl or benzyl viologen, bound to cell membranes, was not toxic, and chemically reduced NAD. The cell membrane of A. succinogenes contained high levels of benzyl viologen-linked hydrogenase (12.2 U), fumarate reductase (13.1 U), and diaphorase (109.7 U) activities. Fumarate reductase (24.5 U) displayed the highest activity with NR as the electron carrier, whereas hydrogenase (1.1 U) and diaphorase (0.8 U) did not. Proton translocation by whole cells was dependent on either electrically reduced NR or H2 as the electron donor and on the fumarate concentration. During the growth of Actinobacillus on glucose plus electrically reduced NR in an electrochemical bioreactor system versus on glucose alone, electrically reduced NR enhanced glucose consumption, growth, and succinate production by about 20% while it decreased acetate production by about 50%. The rate of fumarate reduction to succinate by purified membranes was twofold higher with electrically reduced NR than with hydrogen as the electron donor. The addition of 2-(n-heptyl)-4-hydroxyquinoline N-oxide to whole cells or purified membranes inhibited succinate production from H2 plus fumarate but not from electrically reduced NR plus fumarate. Thus, NR appears to replace the function of menaquinone in the fumarate reductase complex, and it enables A. succinogenes to utilize electricity as a significant source of metabolic reducing power.

Kinetics of electron and proton transfer to ubiquinone-10 and from ubiquinol-10 in a self-assembled phosphatidylcholine monolayer

Upon incorporating from 0.5 to 2 mol% ubiquinone-10 (UQ) in a self-assembled monolayer of dioleoylphosphatidylcholine (DOPC) supported by mercury, the kinetics of UQ reduction to ubiquinol-10 (UQH2) as well as that of UQH2 oxidation to UQ were investigated in borate buffers over the pH range from 8 to 9.5 by cyclic voltammetry. A general kinetic approach was adopted to interpret the dependence of the applied potential upon the scan rate at constant pH and upon pH at constant scan rate, while keeping the initial reactant concentration and the faradaic charge constant. The oxidation of UQH2 to UQ in DOPC monolayers occurs via the reversible release of one electron with formation of the semiubiquinone radical cation UQH2.+, followed by its rate-determining deprotonation by hydroxyl ions with formation of the UQH. neutral radical; the latter is then instantaneously oxidized to UQ. Analogously, the rate-determining step in UQ reduction to UQH2 consists in the protonation by hydrogen ions of the semiubiquinone radical anion UQ.- resulting from the reversible uptake of one electron by UQ. However, a non-negligible fraction of UQ.- uptakes protons very slowly, and hence, retains its intermediate oxidation state during the recording of the cyclic voltammetric peak for UQ reduction.