Role of aromatic side chains in the binding of volatile general anesthetics to a four-alpha-helix bundle.

Currently, the mechanism by which anesthesia occurs is thought to involve the direct binding of inhaled anesthetics to ligand-gated ion channels. This hypothesis is being studied using four-alpha-helix bundles as model systems for the transmembrane domains of the natural "receptor" proteins. This study concerns the role in anesthetic binding played by aromatic side chains in the binding cavity of a four-alpha-helix bundle designed to assume a Rop-like fold. Specifically, the effect of the substitution W15Y on bundle structure, stability, and anesthetic binding energetics was investigated. No appreciable effect of substituting W for Y on the secondary structure or the thermodynamic stability of the four-alpha-helix bundle was identified. However, the substitution W15Y resulted in about 6- and 3-fold decreases in halothane and chloroform binding affinities, respectively. This effect may reflect weaker dipole-aromatic quadrupole interactions between the aromatic side chain and the anesthetic in the tyrosine-containing species, which possesses the smaller aromatic ring system. For these anesthetic binding proteins, this class of interaction occurs when the permanent nonspherical distribution of electrons in the aromatic ring systems interact with the weakly acidic CH group of the anesthetics.