Hydrogen bonding changes of internal water molecules in rhodopsin during metarhodopsin I and metarhodopsin II formation.
Rhodopsin is a 7-helix, integral membrane protein found in the rod outer segments, which serves as the light receptor in vision. Light absorption by the retinylidene chromophore of rhodopsin triggers an 11-cis-->all-trans isomerization, followed by a series of protein conformational changes, which culminate in the binding and activation of the G-protein transducin by the metarhodopsin II (Meta II) intermediate. Fourier transform IR difference spectroscopy has been used to investigate the structural changes that water, as well as other OH- and NH-containing groups, undergo during the formation of the metarhodopsin I (Meta I) and Meta II intermediates. Bands associated with the OH stretch modes of water are identified by characteristic downshifts upon substitution of H2(18)O for H2O. Compared with earlier work, several negative bands associated with water molecules in unphotolysed rhodopsin were detected, which shift to lower frequencies upon formation of the Meta I and Meta II intermediates. These data indicate that at least one water molecule undergoes an increase in hydrogen bonding upon formation of the Meta I intermediate, while at least one other increases its hydrogen bonding during Meta II formation. Amino acid residue Asp-83, which undergoes a change in its hydrogen bonding during Meta II formation, does not appear to interact with any of the structurally active water molecules. Several NH and/or OH groups, which are inaccessible to hydrogen/deuterium exchange, also undergo alterations during Meta I and Meta II formation.