Calcium-induced conformational changes of the elicitor and membrane-active fengycin

Fengycin is a natural lipopeptide synthetized by Bacillus subtilis strains. It is characterized by strong antifungal and low hemolytic activities. It seems also play a role in the promoting of elicitor activities of other compounds. The target of the biological activities of fengycin is supposed to be plasma membrane of sensitive cells.

Even though the natural fengycin from has been discovered 25 years ago, nowadays, there is an increase of interest for this compound because of its potent applications.

Until 15 years ago, the primary structure of fengycin was a matter of open debate before the publication of the corrected structure obtained by nuclear magnetic resonance and mass spectroscopy techniques. Although the infrared and ultraviolet absorption spectra of the lipopeptide were measured, no detailed analysis of these data was performed probably because of the unconventional sequence of the lipopeptide making these kinds of analyses complicated.

In this work, our attempt was to analyze the conformational properties of fengycin as well as the calcium-induced changes using two complementary spectroscopic methods, Fourrier transformed infrared spectroscopy (FTIR) and circular dichroism (CD).

In a first step, we have characterized the conformational properties of pure fengycin. The lipopeptide adopts turn conformation in trifluoroethanol, a membrane-mimicking solvent. D-aminoacids seem to be involved in intra molecular hydrogen bonds. In a second step, we have investigated the role played by Ca2+ ions on the possible conformational changes of fengycin. The addition of calcium gives rise to important modifications of the conformation. As fengycin has two glutamate residues, calcium is supposed to bind to their side chains.

In conclusion, we have demonstrated that the conformation of fengycin is closely depending of the environment and the presence of calcium ions play an important role on the conformational changes of the lipopeptide. Moreover, spectra obtained both FTIR and CD methods ascertain the presence of turn conformation.