Agonists

Ring et al. used three distinct agonists binding to the β2-adrenoceptor in their research. They are documented below.

BI167107

This agonist was chosen due to its lack of the catechol head characterising the
other agonists; without this bulky group, this molecule achieves even tighter
binding than HBI, for example, another high-affinity agonist with the catechol
group.

HBI

In order to compare the binding of a high-affinity catechol-lacking
molecule with a high-affinity catechol-containing molecule, Ring et
al. used HBI. This helped them characterise the effect of the catechol
group surface on the binding affinity of the adrenoceptor.

Epinephrine

A classic agonist of GPCRs, epinephrine binds with significantly lower affinity
to the β2-adrenoceptor than both BI167107 and HBI. The chemical structure
is clearly very different than either agonist and yet it achieves specific binding
to the adrenoceptor.

Their experiments led Ring et al. to conclude that the catechol head was the most important part of the protein-agonist interaction while the tail is involved in only minor stabilisation effects.

(Source, Ring et al., 2013 and the PDB, accession codes 4LDE, 4LDL, 4LDO)