Gramicidin cation transport

Valinomycin is an ionophore but with different properties from gramicidin A (a) it specifically transports K+, and no other ion, when inserted into membranes or vesicles (b) it can transport K+ only above the phase transition temperature of the membrane, whereas cation transport by gramicidin A is insensitive to temperature. 

A group of antibiotics (e.g., valinomycin, nigericin, and gramicidin A) transport cations across the cell membrane. Such agents, known as ionophores, are widely used to probe membrane structure and function. Ionophores uncouple oxidative phosphorylation. Valinomycin, a cyclic peptide (Figure 14-17), forms a lipid-soluble complex with K+ that readily passes through the inner membrane, whereas K+ by itself does not. In the valinomycin-K complex, hydrophobic groups, present on the outside, facilitate transport of the complex in the lipid environment ... 

The gramicidin channel, formed by dimerization of two molecules in a loose helical conformation (Fig. 3), has been studied extensively as a model system. The helical dimer spans the membrane to produce a channel through the hollow center of the helix with anionic sites or regions to facilitate cation transport. The channel transmits only univalent cations. Divalent calcium ion blocks ion flow through the channel. The channel exhibits the anomalous mole fraction effect. Both univalent thallous ion and sodium ion are very permeable when gramicidin dimers... 

X-ray diffraction studies on gramicidin commenced as early as 1949 218-219> and this early work pointed to a helical structure 220). Recent work by Koeppe et al. 221) on gramicidin A crystallised from methanol (/%) and ethanol (.P212121) has shown that the helical channel has a diameter of about 5 A and a length of about 32 A in both cases. The inclusion complexes of gramicidin A with CsSCN and KSCN (P212121) have channels that are wider (6-8 A) and shorter (26 A) than the uncomplexed dimer 221 222). Furthermore there are two cation binding sites per channel situated either 2.5 A from either end of the channel or 2.5 A on each side of its centre 222) Unfortunately these data do not permit a choice to be made from the helical models (i)—(iv) and it is not certain if the helical canals studied are the same as those involved in membrane ion transport. 

Transmembrane channels represent a special type of multi-unit effector allowing the passage of ions or molecules through membranes by a flow or site-to-site hopping mechanism. They are the main effectors of biological ion transport. Natural and synthetic peptide channels (gramicidin A, alamethicin) allowing the transfer of cations have been studied [6.66-6.68]. 

The linear peptide gramicidin A (15 residues) dimerizes in biological and synthetic membranes in a head-to-head manner30 to give channels of about 5 A in diameter and 32 A in length. These channels are specific for alkali metal cations, and show high transport rates, e.g. 107 Na+ ions s l, a value close to the ion fluxes found for the Na+ channel of nerve cells. Each structural unit appears to contain two channels, each of which contains two binding sites for cations. 

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... 

Artificial systems permitting the transport of cations have been inserted into natural membranes and into artificial vesicles. These are known as ionophores, and there are two types, exemplified by gramicidin A and valinomycin. 

Using the principle of ion pair formation between ammonium cations and the phosphate anions of lipids, Matile et al.33 prepared 8, an amphiphilic polyamine dendrimer. Rather than acting as a membrane channel, 8 was expected to form reversible membrane defects in the lipid bilayer. The steroid moiety was expected to act as the hydrophobic anchor for bilayer orientation and steric bulk was expected to prevent the polyamine penetrating the bilayer. Proton transport was assessed in unilamellar vesicles using the pH-fluorescence technique in which the external pH was increased to 7.8 relative to the internal pH at 7.4. The results demonstrated that 8 was almost as active as gramicidin, and maximal flux was achieved in ca. 20 s. 

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