Figure 10.

Structural changes in H3 and H6 upon rhodopsin activation. Packing interactions are shown between H3 and H6 in the crystal structure of rhodopsin (PDB access code = 1GZM) (A), Meta I (B) and in the crystal structure of Meta II (PDB access code = 3PXO) (C). The Meta I structure is based on MD simulations guided by NMR constraints. NMR measurements between Arg135^3.50 and Met257^6.40, in combination with azido labeling studies²³ and EPR³ measurements of Meta I and Meta II, are consistent with rotation of H6. Trp265^6.48 in the conserved CWxP motif on H6 is locked in place in dark rhodopsin by the 11-cis retinal chromophore. Retinal isomerization releases the packing constraints on the Trp265^6.48 indole ring. Gly121^3.36 on H3 is strictly conserved in the visual receptors.¹ The small side chain facilitates packing of the Trp265^6.48 side chain in dark rhodopsin, but does not hinder H6 rotation. Leu128^3.43 and Met257^6.40 are closely packed in dark rhodopsin. Leu128^3.43 is highly conserved (78%) in GPCRs and is part of a tightly packed transmembrane core.¹ The rotation of H6 is facilitated by the small side chain at position 132^3.48. This position is highly conserved in the GPCRs as either an alanine (36%) or a serine (50%).⁶³ The Arg135^3.50 side chain interacts with Glu247^6.30 in the dark and is prevented from moving toward the Met257^6.40 by Val254^6.37. Val254^6.37 is conserved (63%) as a β-branched amino acid across the Class A GPCRs. Rotation of H6 breaks the Arg135^3.50- Glu247^6.30 interaction and removes the steric interaction with Val254^6.37. The side chain of Glu247^6.30 is in an intermediate position between Arg135^3.50 and Lys231^5.66; a Glu247^6.30 - Lys231^5.66 salt-bridge forms in Meta II.