Conduction channel, mouth

An adequate proton supply to an F0 subunit channel in the mitchondrial ATP synthase is less certain. Using the same logic as before, proton supply from the cytosol at pH 7.5 would be only 20 H+ per channel per second. This discrepancy might be overcome by a much wider channel mouth, a slower rate of ATP synthesis per enzyme, or some additional mechanism by which protons are supplied to the mitochondrial ATP synthase. One possibility is that in mitochondria, where ATP synthesis (and therefore proton flux) is driven by a membrane potential, hydrolysis of water at the channel mouth could be a major source for protons. Kasianowicz et al. (46) found it necessary to invoke this possibility to account for the observed rates of protonophore-mediated proton conductance across lipid bilayers. 

It is conceivable, though, that the full conduction cyde in CF comprises at least two extremely fast reactions in series, one unknown "proton supply" reaction regulated by the protein suirounding of the channel mouth and a "membrane crossing" reaction, comprising the highly proton selective filter. 

A premixture of 8 and soybean lecithin gave stable single channel currents with well-defined transitions between open and closed states with the 0.1-Is time scale. The conductance level detected was 6.1 0.5 pS at 0.5 M KCl solution. At various transmembrane voltages with different molar ratios of 8-to-lipid in the range 1/200 - 1/3000, an identical conductance level was always observed. This observation is therefore compatible with the original idea that monomeric 8 itself defines a pore mouth with a specified diameter in the single lipid layer. It gave a cation/anion... 

This hypothesis finds additional confirmation in the features of the a-LTX truncation mutants described above (Section 3.3) (Li et al. 2005). These mutants have 1 to 8 ankyrin repeats removed from their C-termini and form cation channels of dramatically different conductivities. For example, a-LTxAl mutant mediates an enormous conductance, by far exceeding that of wild-type toxin. It is possible that the removal of the last ankyrin repeat lifts an obstruction for cation movements in the extracellular mouth of the channel. In contrast, a-LTxA2 and a-LTxA3 make very inefficient channels, probably due to perturbations of the channel lining. Finally, a-LTxA8 does not induce any cation currents, as it seemingly cannot form tetramers. 

Sample 4 presents a low concentration of organic matter, giving a residual absorbance on the entire spectrum, except for the shortest wavelengths related to the presence of chloride, the mouth of the canal directly opening into the Channel. This direct influence of salt water explains the high value of conductivity. 

SECM associated with scanning ion conductance microscopy (SICM) requires a double tip, on one side of which is a conventional microdisc electrode and on the other side is a narrow pipette filled with electrolyte and an electrode that measures ionic conductance through the mouth of the pipette with respect to another electrode in the bulk solution. When the pipette mouth is within one pipette tip radius away from the sample surface, the conductance varies sufficiently to be used as a control signal to maintain the z-position of the tip during the scans, thereby affording constant-distance SECM operations [133,134]. This methodology is fast and apparently less-challenging to implement than shear force SECM, but it requires the fabrication of double-barrel tips in which one channel is left empty and the other is filled with a conventional microdisc. 

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