NMR constraints on the location of the retinal chromophore in rhodopsin and bathorhodopsin.

Rhodopsin is the photoreceptor in vertebrate rod cells responsible for vision at low light intensities. The photoreactive chromophore in rhodopsin is 11-cis-retinal bound to the protein via a protonated Schiff base with Glu113 as the counterion. We have used the observed 13C NMR chemical shifts of the conjugated retinal carbons in combination with semiempirical molecular orbital calculations to establish the major charge interactions in the retinal binding site of rhodopsin and its primary photoproduct, bathorhodopsin. In rhodopsin, the NMR data constrain one of the carboxylate oxygens (O1) of Glu113 to be approximately 3 A from the C12 position of the retinal with the second oxygen oriented away from the conjugated retinal chain. The O1-C12-H angle is constrained by taking into account the 500 nm absorption maximum of the protein-bound retinal as well as the chemical shift data. The bathorhodopsin retinal binding site structure is generated from the rhodopsin model by isomerization of the C11 = C12 bond and incorporation of C-C single bond twists from C8 to C15. The resulting structure yields a moderate fit to both the chemical shift data and the 543 nm absorption maximum of bathorhodopsin. In both the rhodopsin and bathorhodopsin models, we have included a structural water molecule hydrogen bonded with the Schiff base to account for the high C = N stretching vibrations previously observed.(ABSTRACT TRUNCATED AT 250 WORDS)