Interfacial tryptophan residues: a role for the cation-pi effect?

Area of Science:

• Biophysics
• Computational Biology
• Structural Biology

Background:

• Integral membrane proteins often feature aromatic residues, like tryptophan, at the lipid-water interface.
• The precise forces driving this preference, involving steric and electrostatic interactions, remain incompletely understood.
• Tryptophan's structure, with its aromatic pi-electron cloud, suggests specific interactions with lipid headgroups.

Purpose of the Study:

• To investigate the cation-pi interactions between gramicidin (gA) tryptophan residues and lipid headgroups.
• To compare these interactions in two distinct lipid bilayers: POPE and POPC.
• To elucidate the role of these interactions in the preference of integral membrane proteins for specific lipid environments.

Main Methods:

• Molecular dynamics (MD) simulations were employed to model gramicidin (gA) embedded in POPE and POPC lipid bilayers.
• A classical force field model was used for tryptophan, considering implicit polarizability.
• Cation-pi interactions were analyzed based on defined distance and angular criteria.

Main Results:

• Cation-pi interactions were observed between specific gramicidin tryptophan residues (W11, W13, W15) and lipid headgroups, notably in POPE.
• Tryptophan W9 did not exhibit significant cation-pi interactions in either lipid environment.
• These interactions were more prevalent in POPE than in POPC bilayers, while H-bonding capabilities were similar.

Conclusions:

• Cation-pi interactions play a significant role in anchoring interfacial tryptophans to lipid headgroups.
• The higher frequency of cation-pi interactions in POPE suggests a stronger affinity between transmembrane proteins and ethanolamine headgroup lipids.
• These findings contribute to understanding the molecular basis of membrane protein-lipid interactions and lipid selectivity.