V1-ATPase (V1), the catalytic domain of an ion-pumping V-ATPase, is a molecular motor that converts ATP hydrolysis–derived chemical energy into rotation. Here, using a gold nanoparticle probe, we directly observed rotation of V1 from the pathogen Enterococcus hirae (EhV1). We found that 120° steps in each ATP hydrolysis event are divided into 40 and 80° substeps. In the main pause before the 40° substep and at low ATP concentration ([ATP]), the time constant was inversely proportional to [ATP], indicating that ATP binds during the main pause with a rate constant of 1.0 × 107
m−1 s−1. At high [ATP], we observed two [ATP]-independent time constants (0.5 and 0.7 ms). One of two time constants was prolonged (144 ms) in a rotation driven by slowly hydrolyzable ATPγS, indicating that ATP is cleaved during the main pause. In another subpause before the 80° substep, we noted an [ATP]-independent time constant (2.5 ms). Furthermore, in an ATP-driven rotation of an arginine-finger mutant in the presence of ADP, −80 and −40° backward steps were observed. The time constants of the pauses before −80° backward and +40° recovery steps were inversely proportional to [ADP] and [ATP], respectively, indicating that ADP- and ATP-binding events trigger these steps. Assuming that backward steps are reverse reactions, we conclude that 40 and 80° substeps are triggered by ATP binding and ADP release, respectively, and that the remaining time constant in the main pause represents phosphate release. We propose a chemo-mechanical coupling scheme of EhV1, including substeps largely different from those of F1-ATPases.
Several cermet anode materials were investigated for direct internal reforming operation of solid oxide fuel cells. The effects of steam and discharge conditions were examined for internal reforming operation with propane as a hydrocarbon fuel. Even at high steam-to-carbon ͑S/C͒ ratio, power generation characteristics with Ni-yttria-stabilized zirconia ͑YSZ͒ were deteriorated during internal reforming operation with propane at 1000°C, possibly due to carbon formation at low current densities, whereas stable generation behavior was observed for the cells with Ni-scandia-stabilized zirconia ͑ScSZ͒ or Ni-samaria-doped ceria ͑SDC͒ anode by feeding propane at low S/C = 0.8. The carbon deposition rate was measured for several cermet anode materials by feeding dry or humidified methane by the gravimetric technique. The carbon deposition rate over Ni-ScSZ was lower than Ni-YSZ, leading to better generation performance with propane at low S/C. Ni-SDC showed the highest carbon deposition rate among the cermets investigated, even for humidified methane. This implies that carbon formed over Ni-SDC could be effectively removed upon power generation.
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