Gramicidin A

Fig. 3. Step conductance changes across planar bilayer due to Gramicidin A (traces A and C), D Leu2 Gramicidin A (trace B) and N-acetyl desformyl Gramicidin A (trace D).

C. and D. Monoolein/hexadecane membranes, 100 mV applied potential 1 M RbCl and 23 °C. Note that, during the time period of an average Gramicidin A channel (above), the N-acetyl desformyl Gramicidin A channel turns on and off many times and that the conductance step is smaller. The N-acetyl methyl replacing the formyl proton also crowds and destabilizes the head to head junction and results in less favorable lateral coordination of the cation at the junction. Reproduced with permission from Ref.111... 

Fig. 4. A, Histogram of single channel conductances, y, of Gramicidin A in diphytanoyl L-a-lecithin/n-decane membranes for 1 M KC1, 130 mV potential and 40 °C. Note that single channel conductance is simply the single channel current, i , divided by the applied potential. Reproduced from reference 12) with permission.

B, Histogram of channel lifetimes for Gramicidin A in diphytanoyl L-a-lecithin/n-decane membranes for 1 M KC1, 130 mV applied potential and 15 °C. 

C. Histogram of single channel conductances of Gramicidin A in glyceryl monoolein/hexadecane membranes for 1 M KC1, 103 mV applied potential and 23 °C. Reproduced with permission from Ref. 13>... 

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

A. Side view of channel spanning the lipid layer of a planar lipid bilayer, The structure is comprised of two monomers, each in a left-handed, single stranded p -helical conformation, and joined together at the head or formyl end by means of six, intermolecular hydrogen bonds. The two formyl protons are seen at the center of the structure in this view. Replacement of these protons by methyls destabilizes the conducting dimer as shown with N-acetyl desformyl Gramicidin A (Fig. 3D). 

For a more complete treatment of the derivations and determination of experimental rate constants (to be discussed briefly below) refer to Ref. 46 for Gramicidin A and Ref. 47 for the malonyl dimer of Gramicidin A. (Malonyl Gramicidin A is formed by deformylation of Gramicidin A and then joining to amino ends together using the malonyl moiety, —CO—CH2—CO—, to form the covalent dimer.)... 

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

Fig. 13. Location of calcium ion binding site in the Gramicidin A channel —. For comparison, the sodium ion binding site is included...

A. lCBr titration of Enniatin B in methanol. Since exchange of cation between solution and carrier is relatively rapid only one signal is seen per chemically distinct carbonyl. The titration shows the magnitude of the chemical shift observed. Since Enniatin B may be considered a cyclic analogue of Gramicidin A, these chemical shifts indicate the magnitudes of chemical shifts that can be expected in the Gramicidin A channel (see Fig. 6 and 13) for direct interaction of carbonyl with cation. 

If a small amount of gramicidin A is dissolved in a BLM (this substance is completely insoluble in water) and the conductivity of the membrane is measured by a sensitive, fast instrument, the dependence depicted in Fig. 6.15 is obtained. The conductivity exhibits step-like fluctuations, with a roughly identical height of individual steps. Each step apparently corresponds to one channel in the BLM, open for only a short time interval (the opening and closing mechanism is not known) and permits transport of many ions across the membrane under the influence of the electric field in the case of the experiment shown in Fig. 6.15 it is about 107 Na+ per second at 0.1 V imposed on the BLM. Analysis of the power spectrum of these... 

Fig. 6.15 Fluctuation of the conductivity of BLM in the presence of gramicidin A. (According to D. A. Haydon and B. S. Hladky)...

Many of these materials form voltage-gated transmembrane ion channels, i.e. they exhibit non-linear current voltage curves, unlike the polyene antibiotics and gramicidin A which have linear current voltage curves. Of the peptaibol antibiotics the most thoroughly investigated substance to date is alamethicin 201 203>. 

Fig. 23. Possible helical models for the gramicidin A ion channel. The polypeptides are represented as helical strips, one molecule being stippled for clarity. Numbers refer to the substituted terminal amino acid residues. Model (i), proposed originally by Urry, is the one now generally accepted...

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