Amino acid volume and hydropathy of a transmembrane site determine glycineand anesthetic sensitivity of glycine receptors

Two specific amino acid residues in transmembrane segments (TM) 2 and 3 are critical for the enhancement of glycine receptor (GlyR) function by volati le anesthetics, To determine which physicochemical characteristics of these sites determine their roles in anesthetic actions, an extensive series of single amino acid mutations at amino acid residue 288 (Ala-288) in TM3 of t he alpha 1 GlyR subunit was tested for modulation by volatile anesthetics. The mutations changed the apparent affinities of receptors for glycine; rep lacements with larger volumes and less hydropathy exhibited higher affiniti es for glycine. Potentiation by anesthetics was reduced by specific mutatio ns at Ala-288. The molecular volume of the substituents was negatively corr elated with the extent of potentiation by isoflurane, enflurane, and 1-chlo ro-1,2,2-trifluorocyclobutane, whereas there was no correlation between ane sthetic enhancement and polarity, hydropathy, or hydrophilicity of substitu ents, In contrast to anesthetics, no correlation was found between the effe cts of the nonanesthetics 1,2-dichlorohexafluorocyclobutane or 2,3-dichloro octafluorobutane and any physicochemical property of the substituent. These results suggest that the molecular volume and hydropathy of the amino acid at position 288 in TM3 regulate glycine and anesthetic sensitivity of the GlyR and that this residue might represent one determinant of an anesthetic binding site.