Tryptophan hydrogen bonding and electric dipole moments: functional roles in the gramicidin channel and implications for membrane proteins.

The known high-resolution structure and dynamics characterization of the lipid bilayer-bound polypeptide gramicidin A provides a unique opportunity to study structure-function and dynamics-function correlations in a model membrane protein. In particular, the indoles have a variety of very important functional roles in this cation channel that will undoubtedly be recognized in membrane proteins. That indoles and phenols are oriented at the hydrophobic-hydrophilic interface of lipid bilayers is already well-recognized in membrane proteins. The most buried indole of the gramicidin channel, Trp9, is shown by 15N solid state NMR to be exposed to the hydrophilic surface through hydrogen exchange. Here the importance of the indole dipole moments is described for cation conductance. Preparation of samples with high concentrations of Na+ is shown by high-resolution orientational constraints derived from 2H NMR to have no structural effect on the indole side chain conformations. These dipoles stabilize cations in the binding sites near the channel entrance and substantially reduce the potential energy barrier at the bilayer center. This latter finding conclusively documents that the rate-limiting step in cation conductance by this channel involves the barrier at the bilayer center. Furthermore, dynamics of the indole rings cause significant fluctuations in the energy of stabilization at the binding site that may result in a rapid mechanism for gating the channel.