Our long-term goal is to improve the design and administration of volatile anesthetics by learning the molecular mechanisms of anesthesia. Our short-term goal is to understand how volatile anesthetic potency is altered by site-directed mutations in the transmembrane domains of ligand-gated ion channels. Our hypothesis is that cavities within transmembrane domains provide a common motif for volatile anesthetic binding sites within the superfamily of GABA, glycine, nicotinic acetyicholine, and 5-NT receptors. We suggest that specific amino acid residues define the dimensions and polarity of these binding sites and thereby determine the relative efficacy of volatile anesthetics. This hypothesis will be tested in two Specific Aims:
Aim 1. We will test the hypothesis that variations in the dimensions of cavities within transmembrane subunits determine the relative potency of anesthetics within the superfamily of GABA, glycine, and nicotinic acetyicholine receptors. Mutation of two critical amino acid residues in transmembrane segments of the glycine alpha 1 receptor (S267 and A288) modulates the potentiation of agonists by volatile anesthetics. The volume of these residues is the best predictor of anesthetic potency. We will build molecular models of the transmembrane domains of these subunits and predict additional residues that may define the dimensions of these putative cavities.
Aim 2. We will test the hypothesis that variations in the polarity of cavities within transmembrane subunits determine the relative potency of volatile anesthetics. Although the volume of amino acid side-chains has a dominant effect, the distinct in vivo and in vitro pharmacology of pairs of anesthetic isomers demonstrate that the polarity and shape of binding sites is important. We will use molecular modeling to rationalize existing data and predict new site-directed mutations for study by our collaborators in an iterative series of experiments. In summary, our initial computational models with two transmembrane alpha helices have been of value in rationalizing and predicting the effect of site-directed mutations. Building a more complete 3-dimensional model of an anesthetic binding site will allow us to define those molecular properties that confer distinct pharmacologies on volatile anesthetics.
