The Structure, Cation Binding, Transport, and Conductance of Gly₁₅-Gramicidin A Incorporated into SDS Micelles and PC/PG Vesicles^,

Abstract

To further investigate the effect of single amino acid substitution on the structure and function of the gramicidin channel, an analogue of gramicidin A (GA) has been synthesized in which Trp(15) is replaced by Gly in the critical aqueous interface and cation binding region. The structure of Gly(15)-GA incorporated into SDS micelles has been determined using a combination of 2D-NMR spectroscopy and molecular modeling. Like the parent GA, Gly(15)-GA forms a dimeric channel composed of two single-stranded, right-handed beta(6.3)-helices joined by hydrogen bonds between their N-termini. The replacement of Trp(15) by Gly does not have a significant effect on backbone structure or side chain conformations with the exception of Trp(11) in which the indole ring is rotated away from the channel axis. Measurement of the equilibrium binding constants and Delta G for the binding of monovalent cations to GA and Gly(15)-GA channels incorporated into PC vesicles using (205)Tl NMR spectroscopy shows that monovalent cations bind much more weakly to the Gly(15)-GA channel entrance than to GA channels. Utilizing the magnetization inversion transfer NMR technique, the transport of Na(+) ions through GA and Gly(15)-GA channels incorporated into PC/PG vesicles has been investigated. The Gly(15) substitution produces an increase in the activation enthalpy of transport and thus a significant decrease in the transport rate of the Na(+) ion is observed. The single-channel appearances show that the conducting channels have a single, well-defined structure. Consistent with the NMR results, the single-channel conductances are reduced by 30% and the lifetimes by 70%. It is concluded that the decrease in cation binding, transport, and conductance in Gly(15)-GA results from the removal of the Trp(15) dipole and, to a lesser extent, the change in orientation of Trp(11).

The Preference of Tryptophan for Membrane Interfaces

“…The lifetime also depends on hydrophobic matching between the channel exterior and the host lipid bilayer (25). Replacing amino acids other than those at the subunit interface, such as Trp 3 Phe (27) or Trp 3 Gly substitutions (63,64), or changing the D-Leu "spacer" residues between the Trps (65) also alters the single channel lifetime (66), presumably by altering the orientations and/or lipid interactions of the anchoring Trp side chains.…”

Right-Handed Helical Foldamers Consisting of De Novo d-AApeptides

“…12 Certain peptides consisting of both D-amino acids and L-amino acids were studied for their helicity; 13 however, it is very challenging to predict the folding propensity of the mixed peptides due to the intrinsic opposite folding propensity of D- and L-amino acids. 4b,14 For instance, Gramicidin A containing alternating L- and D-amino acid is capable of forming a right-handed helix, 13d,15 while another oligo-L-Val-D-Val peptide was found to form a left-handed helix. 13c It is therefore attractive to explore new helical foldameric scaffolds of this type to better understand the helical propensity of unnatural oligomers bearing D-form synthetic residues, so as to design new molecular entities that could modulate specific biological process, in the long-term journey of biopolymer-mimicking foldamer research.…”

Effect of micellar charge on the conformation and dynamics of melittin

“…The studies on micellar organization and dynamics assume special significance in light of the fact that the general principles underlying the formation of micelles are common to other related assemblies such as reverse micelles, bilayers, liposomes, and biological membranes (Tanford 1978;Israelachvili et al 1980). Micelles have also been applied as membrane mimetics to characterize membrane proteins and peptides (Franklin et al 1994;Lenz et al 1995;Sham et al 2003).…”