The double ??5.6 helix of gramicidin a predominates in unsaturated lipid membranes

Abstract

The structure of the channel-forming polypeptide gramicidin A (GA) incorporated into phosphatidyl-choline (PC) liposomes has been studied as a function of the degree of unsaturation of the acyl chains of PC. The initial conformational state of GA in reconstituted bilayers is determined by the solvent in which the peptide and the lipid are initially co-dissolved, whereas the equilibrium conformational state (after heat incubation) is affected by the lipid structure rather than by the nature of the solvent. The conformational equilibrium of GA has been studied in liposomes prepared from PC having a variable number of double bonds in the fatty acid moiety, by circular dichroism and Fourier transform infrared. Liposomes were prepared from trifluoroethanol or ethanol solutions and incubated at 68 degrees C. GA was shown to retain the conformation of the right-handed pi-->6.3 pi<--6.3 helix in PC with saturated acyl chains and with one double bond, whereas in dilinoleoyl-PC, having two double bond in each chain, the thermodynamically preferred structures are left-handed antiparallel and parallel double pi pi 5.6 helices. Natural soybean PC also favours left-handed pi pi 5.6 helical structures of GA (approximately 75%). This finding is discussed in terms of the role of PC unsaturation in the dynamic properties of the lipid matrix. Differences between observed FTIR spectra of the increases decreases pi pi 5.6 helix in solution (and to a larger extent in the membrane) and the calculated IR spectra can be interpreted as resulting from deviation of the real structure from the theoretically derived ideal helix. The data obtained provide grounds for better understanding of a GA channel functioning in lipids of variable degrees of unsaturation.

Common structural features in gramicidin and other ion channels

“…Despite recent claims to the contrary, (13) it is very clear that the principal conducting form of gramicidin is the HD. (15,16) Only under limited conditions (17,18,19) do DHs form conducting structures. Since gramicidin does not have any polar or hydrophilic amino acids, it forms ion-conducting pathways from the carbonyl oxygens of its polypeptide backbones.…”

Vibrational circular dichroism of gramicidin D in organic solvents

“…8 It has been shown, however, that the conformation of gramicidin is environment sensitive and adopts different conformations with the change of membrane composition. For example, in detergent micelles, which mimic phospholipid bilayer of biomembranes, a head-to-head dimer formed from right-handed single strand ␤ 6.3 helices was proposed, 10,22,23 whereas in lipids composed of unsaturated fatty acids, gramicidin adopts interwound double-helical structure, 8,24 and in artificial lipid membrane it is observed that conformation of gramicidin is determined by its conformational behavior in the organic solvent where it was initially dissolved. 25 A limited number of vibrational absorption studies were undertaken on gramicidin, 9,24,26 -28 while most literature studies on gramicidin were carried out with electronic circular dichroism (ECD), NMR, and X-ray diffraction methods.…”

The effect of temperature and lipid on the conformational transition of gramicidin A in lipid vesicles

“…18 The rate of transition from the double-stranded helix to the single-stranded helical dimer depends on the lipid head group, acyl chain length, and unsaturated content. [19][20][21][22][23] Two models of this conformational change have been proposed. In the ''zipper'' mechanism, the intertwined chains unscrew in opposite directions by progressively shifting the antiparallel chains past each other.…”

“…[16][17][18] On the other hand, when apolar solvents are used, the double-helical conformation initially predominates. 15,19 This has been referred to as the ''solvent history'' of gramicidin A. In a phospholipid bilayer, this gramicidin double-stranded helix is a nonminimum energy conformation, [15][16][17][18][19] but conversion to the single-stranded helical dimer takes several hours if the polypeptide was initially dissolved in double-stranded helix-stabilizing solvents.…”

“…15,19 This has been referred to as the ''solvent history'' of gramicidin A. In a phospholipid bilayer, this gramicidin double-stranded helix is a nonminimum energy conformation, [15][16][17][18][19] but conversion to the single-stranded helical dimer takes several hours if the polypeptide was initially dissolved in double-stranded helix-stabilizing solvents. 18 The rate of transition from the double-stranded helix to the single-stranded helical dimer depends on the lipid head group, acyl chain length, and unsaturated content.…”