Transmembrane ion transport

Early studies of peptides isolated from microbial sources and possessing antibiotic activity led to the discovery of some a-amino acids not normally found in proteins. The peptides containing these nonproteinogenic, but natural a-amino acids exhibit helical structures which act as channels for transmembrane ion transport 32 Formation of a- or 310-helices in these peptides was ascribed to the unique geometry of these residues and their present use in the induction of these conformations is now widely established. 

Calixarenes, in particular calix[4]arene, have been seen as potential ion selective filters around which ionophore or channel frameworks can be constructed. Calix[4]arenes exist in different conformers, two of which are of interest as platforms for transmembrane ion transport the cone conformer, in which all four... 

PolyP is a participant of transmembrane ion transport processes, both in procaryptes and eukaryotes. It is widely accepted that ion channels are exclusively proteins, but recently the formation of ion-selective, voltage-activated channels by complexes of PolyP and poly-(A )-3-hydroxybutyratcs (PHBs) has been demonstrated (Reusch and Sadoff, 1988 Reusch, 1992, 1999a, 2000). Each of these have unique molecular characteristics that facilitate ion selection, solvation and transport. 

R. N. Reusch (2000). Transmembrane ion transport by polyphosphate/poly-(R)-3-hydroxybutyrate complexes. Biochemistry (Moscow), 65, 280-296. 

The recent discoveries of PHB and polyP in a human calcium pump and bacterial potassium channel suggest that the naked PHB/polyP complexes found in bacteria are progenitors of protein ion transporters. The process by which protein channels and pumps may have evolved from PHB/polyP complexes is unknown however, one may surmise that over time proteins surrounded the complexes to support and regulate their activity. At first, the association may have been nonco-valent, but subsequently PHB may have become tethered to the protein by a covalent bond. By this view, many of the channels and pumps of prokaryotes and eukaryotes may be supramolecular structures in which protein, polyP, and PHB join together for efficient regulation of transmembrane ion transport. 

U. D. Lengweiler, Biopolymers and -oligomers of (J )-3-Hydroxyalkanoic Acids - Contributions of Synthetic Organic Chemists , Ernst Schering Research Foundation, 1995, 28, 7 - 98 D. Seebach, M. G. Fritz, Detection, synthesis, structure, and function of oligo(3-hydroxy-alkanoates) contributions by synthetic organic chemists , Int. J. Biol. Macromol. 1999, 25, 217 - 236 R. N. Reusch, Transmembrane Ion Transport by Polyphosphate/Poly-(J )-3-hydroxybutyrate Complexes , Biochemistry (Moscow) 2000, 65, 280 - 295 S. Das, D. Seebach, R. N. Reusch, Biochemistry 2002,41,5307-5312. 

Theoretical studies with the compounds discussed in this article have provided much important information on the mechanisms of cation complex formation and transmembrane ion transport. The unique molecular properties of these ligands have also led to a number of practical applications in organic synthesis, analytical chemistry, and biochemistry which are briefly represented in this chapter. 

At frequencies below 63 Hz, the double-layer capacitance began to dominate the overall impedance of the membrane electrode. The electric potential profile of a bilayer membrane consists of a hydrocarbon core layer and an electrical double layer (49). The dipolar potential, which originates from the lipid bilayer head-group zone and the incorporated protein, partially controls transmembrane ion transport. The model equivalent circuit presented here accounts for the response as a function of frequency of both the hydrocarbon core layer and the double layer at the membrane-water interface. The value of Cdl from the best curve fit for the membrane-coated electrode is lower than that for the bare PtO interface. For the membrane-coated electrode, the model gives a polarization resistance, of 80 kfl compared with 5 kfl for the bare PtO electrode. Formation of the lipid membrane creates a dipolar potential at the interface that results in higher Rdl. The incorporated rhodopsin may also extend the double layer, which makes the layer more diffuse and, therefore, decreases C. ... 

These peptides induce transmembrane ion transport at rates comparable to those of natural ion-channel proteins, such as gramicidin A, and show considerable promise as antibiotics. 

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