Water filled channels

Figure 12.11 Schematic diagram of the ion pore of the K+ channel. From the cytosolic side the pore begins as a water-filled channel that opens up into a water-filled cavity near the middle of the membrane. A narrow passage, the selectivity filter, links this cavity to the external solution. Three potassium ions (purple spheres) bind in the pore. The pore helices (red) are oriented such that their carboxyl end (with a negative dipole moment) is oriented towards the center of the cavity to provide a compensating dipole charge to the K ions. (Adapted from D.A. Doyle et al.. Science 280 69-77, 1998.)...

The structural solution for the vast majority of OM proteins is provided in the form of the (3-strand, a secondary fold, which allows portions of the polypeptide chain to organise as a (3-barrel. In this cylindrical structure, hydro-phobic residues point outwards and hydrophilic residues are located inside, which can allow the formation of a water-filled channel [30 33]. 

For the record, according to the classification of Singer (1990), type I proteins are called type la, and type III proteins are called type lb. In Singer s definition, type III proteins represent multispanning proteins and type IV represents a water-filled channel (defined for porins). 

The earliest fully atomistic molecular dynamic (MD) studies of a simplified Nation model using polyelectrolyte analogs showed the formation of a percolating structure of water-filled channels, which is consistent with the basic ideas of the cluster-network model of Hsu and Gierke. The first MD... 

The acetylcholine receptor has four subunits (a, /3, y, and 5, with Mr 52, 56, 63, and 66 x 103, respectively). The complete receptor includes two copies of the a subunit, and one of each of the others. The overall structure, as visualized by electron microscopy, resembles a cylindrical bundle of five, approximately parallel rods, with a water-filled channel along the axis of the cylinder (fig. SI. 12). This assembly projects about 70 A into the synaptic cleft on one side of the membrane and about 40 A into the intracellular... 

While gramicidin and other channel formers can show high transport rates, they do not show the high selectivity that characterizes natural channels. There is much interest at present in a class of proteins called porins, which form natural pores in the outer membranes of Gram-negative bacteria. Several different porin proteins have been isolated from Escherichia coli. These form water-filled channels of various sizes in membranes. Thus the proteins OmpC and OmpF seem to be cation-specific channels while other proteins give larger diameter channels that seem to be specific for anions.34,35... 

Mechanosensitive channels respond to changes in membrane tension. A prokaryotic large-conductance mechanosensitive channel, MscL, opens in response to osmotic stress to form a water filled channel between 3 and 4 nm across [18]. The change in pressure on the bilayer imparts a small movement in a transmembrane helix that is then followed by a dramatic rearrangement of the transmembrane domain to a fully open state. 

Figure 3.3.5 (A) Chemical structure of sulfonated perfluorinated polyethylene (Nafion ). (B) Schematic illustration of the microscopic structure of hydrated Nafion membrane perfluorinated polyethylene backbone chains form spherical hydrophobic clusters. Sulfonic end groups interface with water-filled channels and mediate the migration and diffusion of protons. The channels are filled with water and hydronium ions. Figure adapted from [4].

If we assume that the solute penetrates only through the water-filled channels in the membrane (with volume fraction c/jw), then the solute-water frictional coefficient is close to that of free diffusion D°... 

Figure 3.46. "Inside Out" Amino Acid Distribution in Poiin. The outside of porin (which contacts hydrophobic groups in membranes) is covered largely with hydrophobic residues, whereas the center includes a water-filled channel lined with charged and polar amino acids.

The effect of polymer morphology on membrane structure and conductance has been shown recently. In Ref. 25 hydrogen-based graft-copolymer membranes were compared in terms of morphology and performance to random copolymer membranes with the same ion content. For the hydrated grafted membranes TEM micrographs revealed a picture of a continuous phase-separated network of water-filled channels with diameters of 5 nm. In contrast to that, the random copolymer membranes exhibit a reduced tendency toward microphase separation water is... 

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