Molecular Mechanisms of Ligand Binding and Activation in α1-Adrenergic Receptors

Publisher Summary To rationally design selective drugs, an understanding of subtype differences in the ligand binding pockets would be invaluable. It is hypothesized that only a few nonconserved residues are critical for the ligand-binding specificity of each α 1 AR subtype. Nothing is known about the agonist-dependent molecular mechanisms of α 1 AR stimulation. The activation mechanism for a related G-protein-coupled receptor, rhodopsin, has been studied. The ligand isomerization of retinal when exposed to light breaks a constraining salt-bridge between transmembranes (TMs) 3 and 7, allowing the rhodopsin receptor to adopt an active conformation that can now signal through transducin. A “constraining factor” has also been postulated for the α 1b AR subtype, holding the receptor in a basal configuration until bound by a receptor agonist. However, no molecular evidence for this α 1 BR constraining factor has been presented. In this chapter, the activational mechanism is also explored by testing the hypothesis that α 1 ARs conserved the activational mechanism of rhodopsin in which a saltbridge between TM 3 and TM 7 is disrupted. If the salt-bridge hypothesis is correct, eliminating the negative charge at position 125 should also generate constitutively active α 1b ARs. To investigate this possibility, the D 125 of the WT α 1b AR was changed to an A or a K. The binding properties for these D 125 mutations showed no significant changes in affinity for antagonists when compared with the WT (wild-type) receptor. However, there was a significantly lowered epinephrine affinity (threefold) for these D 125 mutant receptors when compared with the WT α 1b R. This decrease in the epinephrine-binding affinity is likely because of the elimination of the conserved negative charged at position 125, shown in the β AR system to be responsible for docking with the protonated amine of epinephrine.