Gramicidin transmembrane channel

Urry, D. W. On the Molecular Structure and Ion Transport Mechanism of the Gramicidin Transmembrane Channel. In Membranes and Transport, Vol. 2, (ed. Martonosi, A.), p. 285, Plenum Publishing Corporation, New York 1982... 

Fig. 5. Space filling model of Gramicidin A transmembrane channel.

Fig. 12. Stereoperspectives giving channel view from solution of one-half of a Gramicidin A transmembrane channel.

Fig. 25. Top view of a space-filling model of the proposed Gramicidin A dimer structure, showing the transmembrane channel. [Photograph courtesy of D. A. Haydon]...

The bonding of K+ and Na+ to A-methylacetamide is of interest64 in studies of the interaction of these ions with peptides and proteins, and particularly studies of the ion transport through transmembrane channels such as the gramicidin channel. Roux and Karplus35 have used the complexation of the given alkali ion with two N-methylacetamide molecules and two water molecules as a model for interactions occurring in transmembrane channels. 

The peptide subunit was easily incorporated into lipid bilayers of liposome, as confirmed by absorption and fluorescence spectroscopy. Formation of H-bonded transmembrane channel structure was confirmed by FT IR measurement, which suggests the formation of a tight H-bond network in phosphatidylcholine liposomes. Liposomes were first prepared to make the inside pH 6.5 and the outside pH 5.5. Then the addition of the peptide to such liposomal suspensions caused a rapid collapse of the pH gradient. The proton transport activity was comparable to that of antibiotics gramicidin A and amphotericin B. 

The expected channel shown in Figure 12 is of a bimolecular structure. The rigid channel mouth may prohibit the consecutive long alkyl chains from assembling themselves and to prevent lipid molecules from invading the area. The space thus provided may accommodate water molecules to make the domain sufficiently hydrophilic to pass ions. Such a domain would recognize its counterpart located in another lipid layer to make a tail-to-tail dimer of 8, i.e. a symmetric transmembrane channel, as in the case of Gramicidin A dimer. ... 

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]. 

There are many examples of peptides that cannot form transmembrane channels on their own but can do so through aggregation. The gramicidin antibiotics, produced by bacteria as part of their chemical defence system, are only 1.6 nm or so in length [11], Specific placement of side chains, such as four tryptophan residues towards the C-terminus, ensures that the helix penetrates cell membranes to a particular depth but does not pass through the membrane. 

Figure 33. Stereo pair plots of the Gramicidin A transmembrane channel. It is a single-stranded, left-handed j8-helix with approximately six residues per turn. A, Side view. Two molecules are hydrogen-bonded head to head (amino end to amino end) by means of six hydrogen bonds. The intermolecular hydrogen bonds have the pattern of an antiparallel -pleated sheet (see Figure 24A), whereas the intramolecular hydrogen bonding pattern is a parallel -pleated sheet (see Figure 24B). B, Channel view of a monomer. Reproduced, with permission from [96].

Figure 34. A, Circular dichroism spectra of the Gramicidin A transmembrane channel in phospholipid bilayers (curve a) and of hydrogenated Gramicidin A in trifluoroethanol at high concentration ( 100 mg/ml), which is likely a double-stranded j3-helix. See text for discussion. Mean residue ellipticities are given. B, Absorption spectrum of the Gramicidin A transmembrane channel in phospholipid bilayers. The absorption spectrum is dominated by the four tryptophan residues per pentadecapeptide. The extinction coefficient is given on a per residue basis.

D.W. Urry, N. Jing, T.L. TVapane, C-L. Luan, and M. Waller, (1988a) Ion interactions with the gramicidin A transmembrane channel cesium-133 and calcium-43 NMR studies. In W. Agnew, T. Claudio, and F. Sigworth (eds) Mol Biol of Ionic Channels 33, (vol. pp. 51-90) Academic Press, Inc., New York. 

Some linear peptides such as the gramicidins A, B, and C, alamethicin, suzukacillin, and trichotoxin A-40 do not act as carriers but they form transmembrane channels across which alkali metal ions can migrate. Just as the carrier cavities, these channels display a hydrophilic interior and a lipophilic exterior, but in contrast to the former they exhibit poor ion selectivity. Since no complete X-ray studies of any of these channel forming agents are available only few facts are known about their conformations. Therefore, they will not be treated in this review. 

During the past decade, numerous studies have been undertaken to develop synthetic ionophores that might permit cations or molecules to pass through a lipid bilayer [1]. Naturally-occurring gramicidin A is known to form transmembrane channels [2] and efforts to prepare a cation-conducting channel have been reported as well [3]. In our work, we have studied the selectivity of numerous crown ethers, lariat ethers [4, 5], and multi-armed versions of the latter [6]. Much has been learned about flexible ionophores and we have now attempted to utilize the concepts of flexibility and self-assembly to permit construction of a cation- or molecule-conducting channel. 

FIGU RE 19.6 Fluctuation of ion transport through a lipid bilayer containing a small amount of gramicidin A. The distinct steps in the conductivity reflect the opening and closure of individual transmembrane channels. (Adapted from Haydon, D.A. and Hladky, B.S., Q. Rev. Biophys., 5, 187, 1972.)... 

---