9.A.14.3.11
β1-adrenergic receptor of 477 aas and 7 TMSs.  It can serve as an ATP-independent phospholipid flippase (scramblase) (Goren et al. 2014; Chauhan et al. 2016). Active-state structures of the β₁-adrenoceptor (β₁AR) bound to conformation-specific nanobodies in the presence of agonists of varying efficacy have been solved (Warne et al. 2019). Comparison with inactive-state structures of β₁AR bound to the identical ligands showed a 24 to 42% reduction in the volume of the orthosteric binding site. Potential hydrogen bonds were also shorter, and there was up to a 30% increase in the number of atomic contacts between the receptor and ligand. This explains the increase in agonist affinity of GPCRs in the active state for a wide range of structurally distinct agonists (Warne et al. 2019).