The ECL2

In the rhodopsin X-ray structures, the ECL2 region is very long and contains p- sheets that help enclose the active site, but in the high- resolution X- ray structure of the p2AR, the ECL2 has an unexpected a-helix (see Fig. 16.1a,b) with two cysteine bridges, one of which is found within the ECL2 and the other is linked to transmembrane helix 3. The a-helix and rigidity afforded by two cysteine bridges hold ECL2 away from the binding cavity of the transmembrane region, thus providing more accessible ligand entry. In the high-resolution X-ray structure of the p2AR, Phe193 of the extended loop component of the ECL2 comes into contact with the bound ligand, carazolol.

Contrary to the extended p- s heet conformation in rhodopsin or the a -helical structure of the adrenergic receptors, the ECL2 of the A2a adenosine receptor X-ray structure adopts a random coil conformation. However, in analogy to the p2AR X-ray structure, the easily accessible binding cavity of the A2a adenosine receptor X-ray structure is held open by three nearby disul-fide bridges involving ECL1 and ECL2.

Near the extracellular region, the opsin X-ray structures possess an opening between TM5 and TM6, and another opening between TM1 and TM7, both of which arise from extracellular-side helical motions that alter the ligand binding cavity. The openings are necessary because ECL2 blocks the binding-site in opsin and rhodopsin. The authors suggest [31] that the two openings may allow the entry of 11-ds-retinal and the exit of all-trans-retinal, respectively. Due to differences in the ECL2 architecture, the modes of ligand entry and exit are expected to be different in the ligand-mediated GPCR X-ray structures. The two opsin X- ray structures advance the understanding of GPCR activation and may provide insights for developing homology models for agonist binding.

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