Antiamoebin I in Methanol Solution: Rapid Exchange between Right‐Handed and Left‐Handed 3₁₀‐Helical Conformations

Abstract

Antiamoebin I (Aam-I) is a membrane-active peptaibol antibiotic isolated from fungal species belonging to the genera Cephalosporium, Emericellopsis, Gliocladium, and Stilbella. Antiamoebin I has the amino acid sequence: Ac-Phe(1)-Aib-Aib-Aib-Iva-Gly-Leu-Aib(8)-Aib-Hyp-Gln-Iva-Hyp-Aib-Pro-Phl(16). By using the uniformly (13)C,(15)N-labeled sample of Aam-I, the set of conformationally dependent J couplings and (3h)J(NC) couplings through H-bonds were measured. Analysis of these data along with the data on magnetic nonequivalence of the (13)C(beta) nuclei (Deltadelta((13)C(beta))) in Aib and Iva residues allowed us to draw the univocal conclusion that the N-terminal part (Phe(1)-Gly(6)) of Aam-I in MeOH solution is in fast exchange between the right-handed and left-handed 3(10)-helical conformations, with an approximately equal population of both states. An additional conformational exchange process was found at the Aib(8) residue. The (15)N-NMR-relaxation and CD-spectroscopy measurements confirmed these findings. Molecular modeling and Monte Carlo simulations revealed that both exchange processes are correlated and coupled with significant hinge-bending motions around the Aib(8) residue. Our results explain relatively low activity of Aam-I with respect to other 15-amino acid residue peptaibols (for example, zervamicin) in functional and biological tests. The high dynamic 'propensity' possibly prevents both initial binding of the antiamoebin to the membrane and subsequent formation of stable ionic channels according to the barrel-stave mechanism.

The crystal structure of samarosporin I at atomic resolution

“…In methanol solution, NMR data indicated a left‐handed helix for residues 2–7 followed by a right‐handed C‐terminal helix . A recent combination of NMR data with CD spectroscopy shows that antiamoebin changes the N‐terminal conformation of residues 1–7 between left‐handed and right‐handed 3 10 ‐helices rapidly with an equal population of both states . This highly dynamic process might explain the relative inefficiency of antiamoebin as an antibiotic, even as methanol resembles neither the aqueous nor the membrane phase nor the interface between both.…”

Synthesis, preferred conformation, protease stability, and membrane activity of heptaibin, a medium‐length peptaibiotic

“…Peptaibiotics are tipically divided in three subclasses, depending upon the length of their primary structure: (i) short‐sequence (less than 11 residues), such as trichogin GA IV2, 3, characterized by a long fatty acyl moiety at the N‐terminus; (ii) long‐sequence (e.g. alamethicin)4, with 17–21 residues; and (iii) medium‐length (14–16 residues), such as among others antiamoebin5, efrapeptin6, and tylopeptin7. Despite the interesting antibiotic and antifungal properties exhibited by these last membrane‐active peptides, their exact mechanism of action is still unknown.…”

Engineering a β‐Helical d,l‐Peptide for Folding in Polar Media

“…[42,[69][70][71] A temperature coefficient more positive that À4.5 ppb K À1 is typically considered to be good evidence that a backbone NH group participates in intramolecular hydrogen bonding. [67,71] We note, however, that these coefficients can be strongly influenced by changes in secondary structure upon heating; therefore, these coefficients must be interpreted with caution when studying small peptides having thermally labile conformations.…”

“…[42,[69][70][71] A temperature coefficient more positive that À4.5 ppb K À1 is typically considered to be good evidence that a backbone NH group participates in intramolecular hydrogen bonding. [67,71] We note, however, that these coefficients can be strongly influenced by changes in secondary structure upon heating; therefore, these coefficients must be interpreted with caution when studying small peptides having thermally labile conformations. [67] Nevertheless, the variable-temperature CD spectra of 1 m and 1 w had shown that these structures do not change drastically with temperature; this observation assured us that amide temperature coefficients would provide a dependable gauge of intramolecular hydrogen bonding in 1 m and 1 w. For 1 m, the coefficients for 18 of the 20 NH-containing residues are !…”

“…Backbone NH temperature coefficients for 1 m (blue) and 1 w (red). The dashed line at À4.5 ppb K À1 represents the proposed level [67,71] that divides H-bonded and free amide H-atoms. For analysis, we also calculated an energy-minimized average of this family (Figure 7 b) and found it to have type IV and type I b-turns for l-Pro(10)-Gly(11) and l-Pro(21)-Gly(22), respectively.…”