Abstract

The dynamics of retinal 2H-labeled at the C5-, C9-, and C13-methyl groups have been studied by solid-state deuterium NMR relaxation in the dark, meta I, and meta II states of the G protein-coupled receptor rhodopsin. Relaxation rates and quadrupolar splittings were interpreted in terms of axial rotation and off-axial motion of the methyl groups and revealed interactions between the retinal cofactor and the rhodopsin binding pocket. Surprisingly, in the dark state the crucial C9-methyl group is the most mobile despite its role in stabilizing the polyene chain. The C5-methyl group is slowest which is most likely due to interactions with Glu122 on helix 3. Dynamics of the ligand change significantly after light absorption. However, most of the changes occur between the dark and meta I states, and can be attributed to variations in intra-retinal interactions due to isomerization. Only small changes are observed upon transition from the meta I to meta II state where activation takes place. Overall, the dynamics of the C9- and C13-methyl groups in the meta I and meta II states indicate the absence of significant steric clashes of these groups with the surrounding amino acids. Even more surprising, there is little change in mobility of the β-ionone ring upon light activation. An activation mechanism based on the relaxation data is suggested which assumes that retinal is maintained in a similar environment, and does not experience significant reorientation or displacement upon transition from the pre-activated meta I to the active meta II state.[1] G. F. J. Salgado et al. (2004) Biochemistry 43, 12819.[2] G. F. J. Salgado et al. (2006) JACS 128, 11067.[3] A. V. Struts et al. (2007) J. Mol. Biol. 372, 50.

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