Figure 1.

H-bonding interactions of the conserved arginine–tyrosine grouping in activated rhodopsin (Meta II), without (left) and with (right) the peptide backbone of the bound C-terminal portion of transducin (GαCT, with a backbone shown in green in the right panel). Coordinates were obtained from data published with 3.0 Å (3XPO) and 2.85 Å (3PQR) resolutions, respectively.⁴ In Meta II without GαCT (at left), one of terminal nitrogens of Arg135 from TM3 is only 2.7 Å from the Tyr223 oxygen atom. The other two nitrogens are not within the H-bond distance to donor or acceptor group detected in the crystal structure. Upon binding of Meta II with GαCT (right panel), the latter protein’s Val347 backbone C=O group serves as an H-bond acceptor for the other terminal nitrogen of Arg135, as well as for the intrachain nitrogen, with N–O distances of 3.0 and 3.1 Å, respectively. (Only the former distance is indicated by a dashed line in this figure.) Furthermore, the C-terminal carboxylate of GαCT moves in to become the closest anion, ∼9 Å away from Arg135. Formation of these ionic and H-bonding interactions between GαCT and Arg135 apparently weakens the H-bonding interaction of the arginine with tyr223, as evidenced by an increased N–O distance (3.3 Å in the right panel).