Channel selectivity

Fig. 3. (A) Model of the proposed pore forming part of K channel subunits. Segments S5 and S6 are possibly membrane-spanning helices. The helices are connected by a hydrophobic segment H5 which may be tucked into the lipid bilayer [48]. H5 is flanked by two proline residues P. Adjacent to these proline residues are amino acid side chains ( ) important for external TEA binding [45,46]. Approximately halfway between these two proline residues are amino acid side chains ( ) affecting internal TEA binding [46,47] and K channel selectivity [48]. (B) Mutations are indicated which affect in Shaker channels external TEA (TEAe) or internal TEA (TEA,) binding. Concentrations of TEA for half block of the wild-type and mutant K channels are given at the right-hand side of the corresponding sequence. Data have been compiled from [45-47].

The results in Table 3 show that H-mordenite has a high selectivity and activity for dehydration of methanol to dimethylether. At 150°C, 1.66 mol/kg catal/hr or 95% of the methanol had been converted to dimethylether. This rate is consistent with that foimd by Bandiera and Naccache [10] for dehydration of methanol only over H-mordenite, 1.4 mol/kg catal/hr, when extrt lat to 150°C. At the same time, only 0.076 mol/kg catal/hr or 4% of the isobutanol present has been converted. In contrast, over the HZSM-5 zeolite, both methanol and isobutanol are converted. In fact, at 175 X, isobutanol conversion was higher than methanol conversion over HZSM-5. This presents a seemingly paradoxical case of shape selectivity. H-Mordenite, the zeolite with the larger channels, selectively dehydrates the smaller alcohol in the 1/1 methanol/ isobutanol mixture. HZSM-5, with smaller diameter pores, shows no such selectivity. In the absence of methanol, under the same conditions at 15(fC, isobutanol reacted over H-mordenite at the rate of 0.13 mol/kg catal/hr, higher than in the presence of methanol, but still far less than over H M-5 or other catalysts in this study. 

Subtype Other names Threshold (°C) Other activating stimuli Channel selectivity (Po/Pms) Chromasomal location Tissue localization... 

K+ channels selectively transport K+ across membranes, hyperpolarize cells, set membrane potentials and control the duration of action potentials, among a myriad of other functions. They use diverse forms of gating, but they all have very similar ion permeabilities. All K+ channels show a selectivity sequence of K+ Rb+ > Cs+, whereas the transport of the smallest alkali metal ions Na+ and Li+ is very slow—typically the permeability for K+ is at least 104 that of Na+. The determination of the X-ray structure of the K+-ion channel has allowed us to understand how it selectively filters completely dehydrated K+ ions, but not the smaller Na+ ions. Not only does this molecular filter select the ions to be transported, but also the electrostatic repulsion between K+ ions, which pass through this molecular filter in Indian file, provides the force to drive the K+ ions rapidly through the channel at a rate of 107-108 per second. (Reviewed in Doyle et al., 1998 MacKinnon, 2004.)... 

The sait5)ling rate of the digital voltmeter (DVM) was controlled by the microprocessor and channel selection for monitoring was obtained by utilising a pulse output from the DVM. 

The ion channels which permit transport of ions, are constructed from protein polymers in helical conformations. These helices form the wall of cell channels and their precise conformations result in specific geometrical arrangements of a number of potential ligand-metal binding sites. There are striking differences between the channels, selective towards either Na+ or K+ ions. 

When the cardiac cell membrane becomes permeable to a specific ion (ie, when the channels selective for that ion are open), movement of that ion across the cell membrane is determined by Ohm s law current = voltage -f resistance, or current = voltage x conductance. Conductance is determined by the properties of the individual ion channel protein. The voltage term is the difference between the actual membrane potential and the reversal potential for that ion (the membrane potential at which no current would flow even if channels were... 

Gao, Y. D., and Garcia, M. L. 2003. Interaction of agitoxin2, charybdotoxin, and iberiotoxin with potassium channels selectivity between voltage-gated and Maxi-K channels. Proteins 52 146-154. 

From these observations, it became apparent that the frequency dependent capacitance must be due, at least partially, to gating particles, and, in particular, to those of sodium channels. If the capacitance change shown in Figure 6 is indeed due to sodium channels, then the change must be affected by TTX, which is known to block sodium channels selectively. Figure 7 shows the membrane capacitance at various potentials. As can be seen, TTX effectively eliminates the voltage dependence of... 

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