Transmembrane channel selectivity

The following conditions must be fulfilled in the ion transport through an ion-selective transmembrane channel ... 

Channels Transmembrane pores selectivity for solutes depends on biophysical properties that are actively regulated... 

A synthetic ion-selective (ion-exchange) membrane is a dense, nonporous, mechanically stable polymer film about 0.01 —0.04 cm thick. By nonporosity we mean the absence of pores (possibly very tortuous transmembrane channels) with a typical radius above 5 — KM (10-8 cm). Structurally the membrane material is a cross-linked polyelectrolyte. This latter is a polymer containing chemical groups that while in contact with an aqueous solvent are capable of dissociation into charges which remain fixed to the polymer core and counterions which are free to move in the solution. 

The chemistry of transport systems has three main goals to design transport effectors, to devise transport processes, and to investigate their applications in chemistry and in biology. Selective membrane permeability may be induced either by carrier molecules or by transmembrane channels (Fig. 10). 

Ions and small molecules may be transported across cell membranes or lipid bilayers by artificial methods that employ either a carrier or channel mechanism. The former mechanism is worthy of brief investigation as it has several ramifications in the design of selectivity filters in artificial transmembrane channels. To date there are few examples where transmembrane studies have been carried out on artificial transporters. The channel mechanism is much more amenable to analysis by traditional biological techniques, such as planar bilayer and patch clamp methods, so perhaps it is not surprising that more work has been done to model transmembrane channels. 

Successful synthetic transmembrane channels must have three characteristics if they are to replicate the behaviour of natural systems. They must span the cell membrane, implying a single molecule or stable self-assembled complex over 4 nm in length. Ideally they should also be able to discriminate in favour of one chemical species, if they are to mimic the highly selective channels, and transport that species at rates in the region of 104 to 108 ions per second to match the efficacy of natural channels. 

In this review, we first discuss the individual properties of polyPs and PHBs related to their function in ion transport, and then consider how these two structurally dissimilar polymers may act in synergy to form ion-selective transmembrane channels. Finally, we examine protein ion transporters that contain PHBs and polyPs, and consider how these three macromolecules may come together in supramolecular channels to refine ion recognition and regulate ion transport. 

Figure 16.5 Illustration of the structure and mechanism of the K+ channel selectivity filter with views from above (left side) and as transmembrane protein structure with a gate, central cavity, and a selectivity filter (right side). To pass through the filter, the K+ needs to be dehydrated. The K+ selectivity filter holds and stabilizes four dehydrated K+ ions by using carbonyl oxygen atoms (red) that resemble the K+ hydration shell. The smaller Na+ ion cannot stabilize in this structure that fits uniquely to the size of the K+ ions only. Based on MacKinnon s X-ray crystallographic studies and proposed mechanism.

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