Ionophores channel-forming

In analogy to a series of polypeptide channel forming quasi ionophores, a model of pardaxin tetramer in water and in the membrane is presented ... 

Other ionophore antibiotics such as gramicidin and valinomycin are channel-forming ionophores because they open pores that extend through the membrane. 

Carrier (left) and channel-forming modes of transport ionophors. 

A second group of ionophores are considered to promote the formation of cylindrical channels through the membrane. The cation diffuses through the channel from one membrane surface to the other. The known channel-forming ionophores (the open-chain peptide derivative, gramicidin A, is one example) are non-cyclic species and, as such, lie outside the scope of this discussion. 

Streptomyces spp. produce both peptide- and non-peptide-based ionophores, the latter type comprising the channel-forming amphotericin B and nystatin. Amphotericin B is a polyhydroxylic lactone which can apparently span half the thickness of a lipid bilayer about 10 molecules are thought to assemble to provide a hydrophilic channel of about 0.5 nm diameter, which allows the passage of small molecules and hydrated metal ions. 

A polymeric stack of macrocycles has been synthesized [6.72] and a cyclodextrin-based model of a half-channel has been reported [6.73]. Channel-type conduction of Na+ ions has been reported for a tris-macrocyclic ligand [6.74]. A derivative of the acyclic polyether ionophore monensin forms lithium channels in vesicles [6.75a], which may be sealed by diammonium salts [6.75b]. 

Table 4 lists some antibiotics that function as ionophores, including some that bind Group IIA cations. Ionophores may be classed as channel formers or carriers by several approaches. A carrier ionophore is dependent on membrane fluidity and so cannot function below the transition temperature of the phospholipids in the membrane, as these are now frozen . The channel formers are much less dependent on membrane fluidity. A channel-forming ionophore cannot function if it fails to span the membrane, and so if an ionophore ceases to function in thicker membranes then it is probably a channel former. The kinetics of ion transport may provide a good indication of the type of ionophore. If an ionophore functions at a rate of more than 104 ions s 1 then it must be a channel former, as this level of ion flux cannot be provided by the diffusion of a carrier complex across the membrane. 

Ionophores are classified as either channel or carrier ionophores. Channel ionophores form channels across the membrane through which ions can diffuse down a concentration gradient. The nature of the channel depends on the ionophore, for example, gramicidin A channels are formed by two gramicidin molecules, N-terminus to N-terminus, each molecule forming a left-handed helix (Figure 7.1(a)). Carrier ionophores pick up an ion on one side of the membrane, transport it across, and release it into the fluid on the other side of the membrane. They usually transport specific ions. For example, valinomycin transports K+ but not Na+ Li+ ions (Figure 7.1(b)). 

Figure 7.1 The general mode of action of ionophores in ion transport, (a) A channel formed by two gramicidin A molecules, N-terminus to N-terminus. (b) The sequence of events in the operation of a carrier ionophore such as valinomycin. Valinomycin is a cyclic peptide consisting of three repeating units with the structure shown...

This technique essentially comprises two aqueous compartments containing electrolyte connected by a pinhole across which a bilayer of phospholipid molecules is spread. An electrical potential is applied across the bilayer and in the presence of an ionophore the resultant current flowing between the two chambers is measured as a function of time.12 The method is often used to assess the activity of synthetic peptides, and has been used to measure the channel activity of several non-peptide channels. Of particular significance is its use in allowing the measurement of the single channel properties of isolated channel-forming proteins that have been reintroduced back into planar bilayer membranes.13... 

Structure of valinomycin (a) and its complex with K" " (b). Valinomycin, which consists of three identical fragments of D-valyl-L-lactyl-L-valyl-D-a-hydroxyisovaleric acid (D-Val-L-Lac-L-Val-D-Hyi), is a mobile or channel-forming ionophore and an uncoupler of oxidative phosphorylation. Note the hydrophobic exterior and the hydrophilic interior of the complex. [Structure (b) is reproduced, with permission, from B. C. Pressman Biological application of ionophores. Amu. Rev. Biochem. 45,501 (1976). 1976 by Annual Reviews Inc.]... 

Alamethicin [Trichoderma viride] 1964.33 methanol channel-forming peptide ionophore for monovalent cations H >Cs =Rb =Na =Li ... 

Carrier (left) and channel-forming modes of transport ionophores. (Reprinted with permission of John Wiley Sons, Inc. from Voet, D. and Voet, J. G. Biochemistry, Second Edition. 1995 Voet, D. and Voet, J. G.)... 

Certain microbes synthesize small organic molecules, ionophores, that function as shuttles for the movement of ions across membranes. These ionophores contain hy-drophihc centers that bind specific ions and are surrounded by peripheral hydrophobic regions this arrangement allows the molecules to dissolve effectively in the membrane and diffuse transversely therein. Others, Hke the well-smdied polypeptide gramicidin, form channels. 

A variety of toxins are capable of forming their own channels in cell membranes. For example, okadaic acid has been shown to be an ionophoric polyether (27,29). 

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