ISSN:
1432-1017
Keywords:
Gramicidin A
;
Ion channel
;
Membrane proteins
;
FTIR
;
Circular dichroism
Source:
Springer Online Journal Archives 1860-2000
Topics:
Biology
,
Physics
Notes:
Abstract The structure of the channel-forming polypeptide gramicidin A (GA) incorporated into phosphatidylcholine (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°C. GA was shown to retain the conformation of the right-handed π $$\overleftarrow 6$$ .3 π $$\overrightarrow 6$$ .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 ππ5.6 helices. Natural soybean PC also favours left-handed ππ5.6 helical structures of GA (≈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 ↑↓ππ=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.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1007/BF00180262
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