INTRAMEMBRANE SIGNALING MEDIATED BY HYDROGEN-BONDING OF WATER AND CARBOXYL GROUPS IN BACTERIORHODOPSIN AND RHODOPSIN

The light-induced mechanism for proton pumping of bacteriorhodopsin wa s studied by Fourier transform infrared spectroscopy of the discrete s equential intermediate states, L, M, and N, Attention is focused on L in the early microsecond time range, as a transition state in which th e Schiff base forms strong H-bonding with a water molecule coordinated with Asp85, This structure leads to transfer of the Schiff base proto n to Asp85 in the L-to-M process, which then triggers proton release f rom Glu204 to the extracellular surface, H-bonding of Arg82 and water molecules are involved in this process, Chloride can replace Asp85 in the D85T mutant, and this anion will be then transported instead of a proton, In L, structural perturbations are induced also around Asp96, through a string of ii-bonding mediated by internal water molecules an d peptide carbonyls in helices B and C, and Trp182 in helix F, These m ay cause the structural changes that occur later in the M-to-N process , Similar interactions, through internal water molecules and the pepti de bonds in helices B and C, take place in bovine rhodopsin, They tran sduce changes across the membrane from the Schiff base to the cytoplas mic surface, where the activation of the transducin occurs.