Conformation analysis of glu181 and ser186 in the metarhodopsin I state

Photoactivation of rhodopsin yields a photointermediate, metarhodopsin I, during the formation of the fully activated photointermediate, metarhodopsin II. It is proposed that Glu181 and Ser186, in the second extracellular loop, play important roles in the stabilization of the protonated Schiff base of metarhodopsin I. Glu181 and Ser186 form a network of hydrogen bonds mediated by a water molecule in the dark-state crystal structure of rhodopsin. On the other hand, the counter-ion of the protonated Schiff base, Glu113, is not involved in the hydrogen-bond network, as it is located further than hydrogen-bond distance from Ser186. Herein, the conformations and proton arrangements of the protonated form of Glu181 and Ser186 in the hydrogen-bond network have been investigated by molecular-dynamics calculations of the rhodopsin crystal structure as well as in the structural model of metarhodopsin I. In the metarhodopsin I model, Ser186 mediated the hydrogen-bond network between Glu113 and Glu181, changing the protein's conformation and creating a space by the outward motion of transmembrane segment 3, while the hydroxyl group of Glu181 was favored in the hydrogen-bond network. The hydrogen bond between Glu113 and Ser186 is thought to reduce the basicity of the carboxylate of Glu113, maintaining the protonated state of the Schiff base in the metarhodopsin I state. In the Glu181Gln mutant, the hydroxyl group of Ser186 favored the water molecule as a proton donor in the metarhodopsin I state, since the carbonyl group of the Gln residue was favored in the hydrogen-bond network. These results indicate that the Gln181 residue interferes with the hydrogen-bond between Glu113 and Ser186 in the metarhodopsin I state, facilitating the neutralization of the protonated Schiff base.