The Retinal Energy Landscape as a Function of the Rhodopsin Photocycle

Abstract

Retinal is the covalently bound inverse-agonist of the prototypical G protein-coupled receptor, rhodopsin. It comprises a β-ionone ring and polyene chain covalently bound to Lys296 of rhodopsin by a protonated Schiff base (PSB). During the course of rhodopsin activation, retinal initially undergoes an 11-cis → all-trans isomerization, followed by a deprotonation of the Schiff base. Using quantum chemical calculations at the MP2 level of theory and solid state NMR spectroscopy we demonstrate substantial differences in retinal structure and dynamics between the protonated and deprotonated species. The delocalization of positive charge from the PSB results in perturbations of the entire retinal moiety upon deprotonation. For example, methyl rotation barriers are shifted as much as 200% [1]. Surprisingly, deprotonation of retinal drastically affects the energetics of β-ionone ring rotation, producing an extra minimum in the C5=C6-C7=C8 torsional energy surface. This results in a proton affinity (PA), and hence pKa, that depends on β-ionone ring orientation. Specifically, the PA of retinal is lowered for non-planar conformations of the β-ionone ring, in turn lowering the pKa and facilitating the deprotonation required for formation of the Meta I pre-activated state of rhodopsin. In order to extend these calculations to account for interactions within the rhodopsin binding pocket we have used the QM data, including MP2 level torsion scans of every dihedral angle, to refine new retinal force fields for both protonation. This data opens the door to future molecular dynamics studies of retinal proteins that include the activated Meta II state. [1] B. Mertz et al. (2011) Biophys. J. 101: L17.