Probing the (cid:1) 2 Adrenoceptor Binding Site with Catechol Reveals Differences in Binding and Activation by Agonists and Partial Agonists*

The (cid:1) 2 adrenergic receptor ( (cid:1) 2 AR) is a prototypical fam- ily A G protein-coupled receptor (GPCR) and an excellent model system for studying the mechanism of GPCR activation. The (cid:1) 2 AR agonist binding site is well characterized, and there is a wealth of structurally related ligands with functionally diverse properties. In the present study, we use catechol (1,2-benzenediol, a structural component of catecholamine agonists) as a molecular probe to identify mechanistic differences between (cid:1) 2 AR activation by cate- cholamine agonists, such as isoproterenol, and by the structurally related non-catechol partial agonist salbutamol. Using biophysical and pharmacologic approaches, we show thatthearomaticringofsalbutamolbindstoadifferentsiteonthe (cid:1) 2 AR than the aromatic ring of catecholamines. This difference is important in receptor activation as it has been hypothesized that the aromatic ring of catecholamines plays a role in triggering receptor activation through interactions with a conserved cluster of aromatic residues in the sixth transmembrane segment by a rotamer toggle switch mechanism. Our experiments indicate that the aromatic ring of salbutamol does not activate this mechanism either directly or indirectly. Moreover, the non-catechol ring of partial agonists does not interact optimally with serine residues solubilized membrane pro- teins chelating Sepharose with nickel by M1- affinity of (cid:3) s were reconstituted buffer 20 m M m M m M

G protein coupled receptors (GPCRs) 1 are remarkably versatile signaling molecules. Many are capable of interacting with more than one G protein, and some have been observed to signal through non-G protein pathways (1,2). The activity of many GPCRs can be regulated by ligands having a spectrum of efficacies ranging from inverse agonists to agonists. Moreover, there is a growing body of evidence that GPCRs are conformationally complex, with different ligands inducing ligand-specific states (3,4). The ␤ 2 adrenoreceptor (␤ 2 AR) is one of the most extensively studied members of the family A GCPRs. Its agonist binding site has been mapped in considerable detail using both site-directed mutagenesis and modified ligands (5-7) (see Fig. 1A).
Much of what is known about the structure and mechanism of activation of GPCRs comes from studies of rhodopsin. However, rhodopsin is limited as an experimental system to investigate the mechanism of activation by diffusible ligands and the structural basis for ligand efficacy. We have used environmentally sensitive fluorophores including fluorescein (3,8) and tetramethylrhodamine (9) to monitor ligand-induced conformational changes in purified ␤ 2 AR. These studies provide evidence that, upon activation, the ␤ 2 AR undergoes structural changes that are similar to those observed upon activation of rhodopsin (8). They also demonstrate that agonists and partial agonists induced distinguishable active states (3) and that the process of activation occurs through at least two kinetically distinguishable steps (9). Based on these results, we proposed a model whereby agonist binding occurs through a series of discrete conformational intermediates as the receptor engages different components of the ligand.
In the present study, we examined differences in the mechanism of activation of the ␤ 2 AR by catecholamine agonists and the non-catechol partial agonist salbutamol. We used catechol (1,2-benzenediol), a fragment of catecholamine agonists, as a molecular probe to characterize differences in binding and activation using biophysical and pharmacologic approaches. Our studies demonstrated that salbutamol binds to and activates the ␤ 2 AR in a manner different from catecholamine agonists. Moreover, they provided further evidence that activation occurs through a series of conformational intermediates having distinct functional properties.
Receptor Purification and Labeling-␤ 2 AR was expressed in Sf9 cells and solubilized using methods described previously (10) with modifica-* This work was supported by grants from the National Institutes of Health (Grant 5 RO1 NS28471) and the Mather's Charitable Foundation (to B. K.). The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
‡ Supported by the Secretaria de Estado de Educacion y Universidades (Spain) and the European Social Fund (European Union).
Purification of Tethered G ␣ -Tethered G␣s (Tet-G ␣s ) is a membranetethered form of G ␣ s that has been shown to couple more efficiently to the ␤ 2 AR than unmodified G ␣ s (11). The construction and characterization of Tet-G ␣s has previously been described (11). Briefly, the membrane tether of Tet-G ␣s consists of the FLAG epitope (Sigma) followed by amino acids 1-64 of the ␤ 2 AR (containing the amino terminus and first transmembrane domain) followed by amino acids 343-412 from the carboxyl terminus of the ␤ 2 AR. This ␤ 2 AR sequence is linked via a His 6 sequence to the amino terminus of G ␣ s. SF9 cells expressing Tet-G ␣s (11) were lysed in buffer containing 20 mM Tris-HCl, pH 7.4, 1 mM EDTA, 3 mM MgCl 2 , 100 mM NaCl, 5 mM NaF, 20 M AlCl 3 , 10 M GDP, 10 mM ␤-mercaptoethanol, and a mixture of protease inhibitors. The lysate was homogenized (with 20 strokes of a Dounce homogenizer) and centrifuged at 18,000 rpm for 20 min at 4°C. The pelleted membranes were solubilized in buffer containing 1% n-dodecyl maltoside, 50 mM Tris-HCl, pH 7.4, 3 mM MgCl 2 , 100 mM NaCl, 5 mM NaF, 20 M AlCl 3 , 10 M GDP, 10 mM ␤-mercaptoethanol, and protease inhibitors for 1 h at 4°C with gentle stirring followed by centrifugation at 18,000 rpm for 20 min. Tet-G ␣s was purified from solubilized membrane proteins by successive chromatography on chelating Sepharose Fast Flow resin (Amersham Biosciences) charged with nickel followed by M1-FLAG affinity resin (Sigma). The peak fractions of pure Tet-G ␣s subunit were pooled and dialyzed against reconstituted buffer containing 20 mM Tris-HCl, pH 7.4, 3 mM MgCl 2 , 100 mM NaCl, 0.05% dodecyl maltoside, 10 M GDP, and 10% glycerol for 3 h. The purified Tet-G ␣s was frozen at Ϫ80°C.
Preparation of Lipids-Phospholipid 18:1 DOPC was purchased from Avanti Polar Lipids Inc., and their purity was ascertained by high performance liquid chromatography. Cholesterol hemisuccinate was purchased from Steraloids Inc. A stock solution of 20 mg/ml DOPC and 10 mg/ml cholesterol hemisuccinate was made in chloroform. 3 mg of DOPC and 0.3 mg of cholesterol hemisuccinate were aliquoted into a glass vial from respective stock solutions. The chloroform was evaporated under a stream of argon and then vacuum-dried for 1 h to remove any residual chloroform. The lipid mixture was then hydrated in Buffer F. Then the lipid mixture was vortexed vigorously and sonicated for over 2 h with 10-min intervals in an ice/water bath. This mixture was stored in Ϫ80°C and used for reconstitution.
Preparation of Reconstituted Receptor-TMR-␤ 2 AR (100 l of 2 M solution) was mixed with 75 l of lipid mixture (3 mg DOPC ϩ 0.3 mg of cholesterol hemisuccinate) and diluted with 25 l of Buffer F. The receptor-lipid mixture was mixed well and allowed to reconstitute on ice for 2 h. The vesicles formed by removing detergent on a 25 ϫ 0.8 cm Sephadex G-50 (fine).
Fluorescence Spectroscopy-Experiments were performed on an SPEX FluoroMax-3 spectrofluorometer with photon counting mode using an excitation and emission band pass of 3.2 nm. For time course experiments, excitation was at 541 nm, and emission was monitored at 571 nm. Unless otherwise indicated, all experiments were performed at 25°C, and the sample underwent constant stirring. Fluorescence intensity was corrected for dilution by ligands in all experiments and normalized to the initial value. All of the compounds tested had an absorbance of less than 0.01 at 541 and 571 nm at the concentrations used, excluding any inner filter effect in the fluorescence experiments.
Ligand Binding Assays-All the binding assays were performed on purified, reconstituted ␤ 2 AR. All of the assays were performed for 1 h at room temperature with shaking at 230 rpm. Competition assays were carried out with 1 nM [ 3 H]DHA and the indicated concentration of competing ligand. Binding data were analyzed by non-linear regression analysis using Prism from GraphPad Software, San Diego, CA.
[ 35 S]GTP␥S Binding-Purified ␤ 2 AR and Tet-G ␣s protein were mixed in a molar ratio of 1:5 and reconstituted as described above. Reconstituted receptor and Tet-G␣ s were suspended in 500 l of cold binding buffer (75 mM Tris-HCl, pH 7.4, 12.5 mM MgCl 2 , and 1 mM EDTA) supplemented with 0.05% (w/v) bovine serum albumin, 0.4 nM [ 35 S]GTP␥S, GDP (1 M) with or without ␤ 2 AR ligands. Incubations were performed for 30 min at 25°C with shaking at 230 rpm. Nonspecific binding was determined in the presence of 100 M GTP␥S and was always less than 0.2% of total binding. Bound [ 35 S]GTP␥S was separated from free [ 35 S]GTP␥S by filtration through glass fiber filters followed by three washes with 3 ml of cold binding buffer. Filter-bound radioactivity was determined by liquid scintillation counting.
Molecular Modeling of the ␤ 2 AR-The residues in the transmembrane (TM) segments of ␤ 2 AR are numbered according to their position in the ␤ 2 AR human sequence and using the Ballesteros general numbering scheme (12). A model of the TM domain and the cytoplasmic helix 8 of ␤ 2 AR was built by homology modeling using the crystal structure of bovine rhodopsin (Protein Data Bank ID code 1L9H (13)) as a template. SCWRL-3.0 (14) was employed to add the side chains to the non-conserved amino acids, whereas the conformation of conserved residues was adjusted to reproduce key local interactions that are likely to be preserved in family A GPCRs. Subsequently, loops were added to the transmembrane bundle, using the Modeler (15) and SYBYL 6.9.2 (Tripos, Inc.) loop-building routines. The volume of the binding pocket was calculated using Grasp (16). Isoproterenol, salbutamol, and catechol were parameterized with the Antechamber program using the general amber force field (17) and HF/6-31G*-derived RESP atomic charges. These ligands were docked in the binding pocket of the ␤ 2 AR according to the experimentally inferred interactions with Asp-113 3.32 , Ser-203 5.42 , Ser-204 5.43 , Ser-207 5.46 , and Asn-293 6.55 (6,18,19). The receptor models with the docked ligands were immersed in a patch of pre-equilibrated palmitoyl oleoyl phosphatidyl-choline lipidic bilayer solvated with water. These systems were energy-minimized and then equilibrated (500 ps) and simulated (500 ps) using molecular dynamics. The simulations were carried out with the Sander module of AMBER 8 (Case et al. (31)) at constant pressure, using the particle mesh Ewald method and the ff2002EP polarizable force field, SHAKE bond constraints in all bonds, a 2-fs integration time step, and constant temperature coupled to a heat bath. The figures were created using MolScript 2.1.2 (20) and Raster3D 2.7 (21).

Non-catechol Partial Agonists Induce a Slow, Monophasic
Conformational Change in TMR-␤ 2 AR-To monitor ligand-induced conformational changes, we labeled purified ␤ 2 AR at Cys-265 ( Fig. 1A) with tetramethylrhodamine maleimide (TMR-␤ 2 AR), as described previously (9). Ligand-induced conformational changes lead to a change in the molecular environment of the covalently bound tetramethylrhodamine that results in a change in emission intensity. We followed fluorescence intensity as a function of time before and after the addition of a saturating concentration of ligand. Fig. 2A shows the biphasic response of TMR-␤ 2 AR to a saturating concentration of norepinephrine. The dotted black lines indicate the magnitudes of the rapid and slow components. We have previously shown that interactions between the catechol ring and the receptor are necessary for the rapid component of the conformational change, whereas interactions between the receptor and the chiral ␤-hydroxyl are required for the slow phase (9). Biphasic conformational changes are also observed upon binding to epinephrine and isoproterenol (9). Fig. 2B shows the fluorescence response of TMR-␤ 2 AR as a function of time following the addition of a saturating concentration of the non-catechol partial agonist salbutamol. This ligand shares similar structural features with catecholamines, and it is likely that, as with catecholamines, the amine of these ligands interacts with Asp-113 3.32 . However, salbutamol differs from catecholamines in the structure of the aromatic component. In the meta-position of the aromatic ring, salbutamol has a hydroxymethyl instead of a hydroxyl group (Fig. 2B). When a saturating concentration of salbutamol was added to TMR-␤ 2 AR, we observed only a slow phase, which is comparable with the slow component of the norepinephrine response ( Fig. 2A).
The results are consistent with our previous studies suggesting that the catechol ring is required for the rapid conformational change observed in TMR-␤ 2 AR (9).
Catechol Can Bind to ␤ 2 AR Occupied by Non-catechol Partial Agonists and Antagonists-Catechol is a structural component of catecholamine agonists but not of the non-catechol partial agonist salbutamol or of antagonists (Fig. 1B). Catechol alone can induce a rapid, monophasic conformational change in TMR-␤ 2 AR (Fig. 2C) (9). The fluorescence response to catechol is saturable with an apparent K D of 160 M (data not shown). The location of the binding site for catechol in the ␤ 2 AR appears to overlap with the binding site for catecholamines such as norepinephrine, epinephrine, and isoproterenol. This can be shown by the inability of catechol to induce a change in fluorescence in TMR-␤ 2 AR occupied by catecholamine agonists (Fig. 3, A and C). In contrast, catechol produces a response in TMR-␤ 2 AR bound to a saturating concentration of salbutamol (Fig. 3B) that is comparable with the response observed in unliganded TMR-␤ 2 AR (Fig. 3C). This indicates that the binding sites for catechol and salbutamol do not overlap.
Antagonists are believed to share a common interaction with agonists and partial agonists at Asp-113 3.32 . Unlike agonists and partial agonists, ␤AR antagonists cause little or no change in the fluorescence intensity of TMR-␤ 2 AR. However, as with partial agonists, catechol induces an increase in fluorescence in ␤ 2 AR bound to the antagonist alprenolol as well as timolol and ICI-118,551 (Fig. 3C). Therefore, the FIG. 1. A, binding site for norepinephrine in the ␤ 2 AR. Amino acids in the ␤ 2 AR involved in ligand binding have been identified using a combination of site-directed mutagenesis and modified ligands (6,7,22). The catecholamine nitrogen interacts with Asp-113 in TM3 (22). Hydroxyls on the catechol ring interact with serines 203 (7), 204, and 207 (19) in TM6. The chiral ␤-hydroxyl interacts with Asn-293 in TM6 (6), and the aromatic ring interacts with Phe-290 in TM6 (22). Also shown is the relative position of Cys-265, the labeling site for tetramethylrhodamine maleimide. B, the structures of agonist and antagonists used or discussed in this report. binding sites for these antagonists do not overlap with the binding site for catechol.
To confirm the results of the biophysical studies, we performed more conventional equilibrium competition binding assays (Fig. 4A). As expected, isoproterenol, norepinephrine, and salbutamol all compete with the antagonist [ 3 H]DHA for binding sites on the ␤ 2 AR. In contrast, catechol does not displace [ 3 H]DHA even at concentrations up to 10 mM. This is consistent with the ability of catechol to induce a response in TMR-␤ 2 AR occupied by the antagonist alprenolol (Fig. 3C). To determine the ability of catechol to compete with agonists and partial agonists, we performed competition binding studies in the absence and the presence of 1 and 10 mM catechol. In the presence of catechol, the apparent EC 50 for isoproterenol is shifted to the right (Fig. 4B), demonstrating that they compete for a common binding site. In contrast, catechol has no effect on the ability of salbutamol to compete for [ 3 H]DHA binding sites (Fig. 4C). Taken together, the results from conformational studies using TMR-␤ 2 AR and conventional ligand binding assays using purified, labeled receptor demonstrated that the aromatic rings of the non-catechol partial agonist salbutamol and the antagonists alprenolol, timolol, and ICI-118,551 do not occupy the same space in the ␤ 2 AR binding pocket as does the catechol ring of catecholamines.
Functional Effects of Catechol-induced Conformational Changes-To determine the functional consequence of ligandinduced conformational changes in the ␤ 2 AR, we reconstituted purified receptor with purified membrane Tet-G ␣s (11) and monitored the effect of ligands on [ 35 S]GTP␥S binding. We have previously shown that Tet-G ␣s couples more efficiently to ␤ 2 AR than does wild type G ␣s when expressed in insect cells (11). Fig.  5A shows the maximum response to a saturating concentration of isoproterenol, dopamine, and catechol. The response to catechol is small but significantly greater than no drug (Fig. 5C). Moreover, the catechol response is not due to a nonspecific effect on Tet-G ␣s as no response was observed in the absence of receptor (data not shown). Fig. 5B shows the effect of 100 M catechol on dose-response curves for salbutamol and isoproterenol. Catechol has no significant effect on isoproterenol. In contrast, catechol reduced the maximal response to salbutamol without significantly altering the EC 50 .
Catechol Activates ␤ 2 AR Occupied by an Inverse Agonist-As shown in Fig. 3C, catechol can induce a conformational change in ␤ 2 AR occupied by a saturating concentration of the inverse agonist ICI-118,551. Fig. 5C shows that this conformational change is associated with an enhanced coupling to Tet-G ␣s . Fig. 6A shows isoproterenol in the ligand binding space defined by TM3, TM5, TM6, and the second extracellular loop (ECL2) in a model of the ␤ 2 AR based on the structure of rhodopsin (see "Experimental Procedures"). The docking of isoproterenol in the ␤ 2 AR is based on interactions with the receptor that have been identified by mutagenesis studies summarized in Fig. 1. Isoproterenol occupies the bottom of the binding pocket interacting with the aromatic residues Phe-289 6.51 and Phe-290 6.52 in TM6 (22,23). It also interacts with Ser-203 5.42 , Ser-204 5.43 , and Ser-207 5.46 in TM5 (7,19). The fact that isoproterenol and catechol compete with each other in binding (Fig. 4B) and conformational assays (Fig. 3C) suggests that catechol occupies the same space in the ␤ 2 AR as that occupied by the aromatic ring of isoproterenol. In contrast, our experimental evidence suggests that the aromatic ring of salbutamol does not occupy the same binding space as catechol. It is therefore unlikely that the binding site for salbutamol is identical to that for isoproterenol. Examination of the ␤ 2 AR binding space identified aromatic amino acids Tyr-174 and Phe-193 in ECL2, as well as Tyr-199 5.38 in TM5, which may interact with salbutamol. Photoaffinity labeling studies suggest a role for Tyr-199 5.38 in antagonist binding (25). Of interest, our results demonstrate that catechol can bind to an antagonist-occupied receptor (Figs. 3C and 4A), suggesting that, like salbutamol, the binding site for the aromatic ring of antagonists does not overlap the binding site for the aromatic ring of catecholamine agonists. In the model shown in Fig. 6B, the aromatic ring of salbutamol is shifted to the top of the binding pocket close to Tyr-174, . In this position, there is room for the simultaneous binding of catechol in the lower part of the binding pocket.

Molecular Modeling of the Salbutamol Binding Site-
We used molecular dynamics simulations to examine the stability of interactions between ligand and receptor when isoproterenol and salbutamol are docked in the lower binding pocket (Fig. 6A). Analysis of multiple trajectories and their energies showed that isoproterenol remains stable in the lower binding pocket (root mean square (isoproterenol) Ͻ 1.5 Å), interacting strongly with residues in the lower region of the binding site, mainly Ser-203 5.42 , Ser-204 5.43 , Ser-207 5.46 , Trp-286 6.48 , and Phe-289 6.51 . On the other hand, salbutamol is less stable in the binding pocket (root mean square (salbutamol) Ϸ 2.5 Å) and adopts different orientations during the trajectories. This ligand is able to interact with the aromatic residues facing the upper region of the binding pocket, mainly with Phe-193 in ECL2. Interestingly, the non-catechol aromatic ring of salbutamol interacted only weakly with the serine residues in TM5, which may explain why salbutamol has a weaker affinity for the ␤ 2 AR than does isoproterenol.

Structurally Similar, but Functionally Diverse, Ligands-
Ligand binding affinity consists of the sum of the energies of interaction between different structural components of the ligand and the amino acids within the binding site of the receptor. The energetic costs associated with ligand-induced conformational changes and ligand desolvation also contribute to binding affinity. Ligands for the ␤ 2 AR share remarkable structural homology (Fig. 1B). Common features include a primary or secondary amine, a chiral ␤-hydroxyl, and an aromatic ring. For agonists and partial agonists, the aromatic ring and the amine are separated by two carbons, whereas for antagonists and inverse agonists, they are separated by three carbons and an oxygen. Interactions between the agonist isoproterenol and the ␤ 2 AR have been mapped in considerable detail by a series of mutagenesis experiments from several laboratories (Fig. 1A) (5)(6)(7). Evidence suggests that the amines of agonists, partial agonists, and antagonists all share an interaction with Asp-113 3.32 (22); therefore, structural differences in the aromatic components of these ligands are the primary determinants of efficacy.
The structural differences between isoproterenol and salbutamol are relatively subtle (Fig. 1B), and one might expect them to occupy a similar space within the ␤ 2 AR binding pocket and activate the ␤ 2 AR by a similar mechanism. We used catechol as a probe to explore the differences between binding and activation by catecholamines and salbutamol. Our results show that the binding site for catechol is the same as the binding site for the catechol ring in catecholamines. Catechol cannot induce a detectable conformational change in TMR-␤ 2 AR occupied by a catecholamine agonist (Fig. 3, A and C), and catechol competes with isoproterenol in binding to the ␤ 2 AR (Fig. 4B). Moreover, we observe that catechol is a weak partial agonist (Fig. 5, A and C). In contrast, binding experiments (Fig. 4C) demonstrate that there is no competition between catechol and salbutamol for binding to the ␤ 2 AR, and fluorescence experiments demonstrate that catechol can bind and induce a conformational change in TMR-␤ 2 AR occupied by salbutamol (Fig. 3,  B and C).
Based on these observations, we conclude that the aromatic ring of salbutamol occupies a binding space in the ␤ 2 AR that does not overlap with the binding space occupied by the aromatic ring of catechol or of catecholamines. Thus, a difference in the meta-position of the aromatic ring (-OH for isoproterenol, -CH 2 OH for salbutamol, Fig. 1B) has a dramatic effect on its location in the binding site and on the mechanism of activation. A catechol moiety optimizes the interactions with the serine residues in TM5 through the formation of a complex network of hydrogen bonds. However, the apparently minor substitution of the meta -OH for a -CH 2 OH group destabilizes this network such that the aromatic ring of salbutamol no longer occupies this space in the ␤ 2 AR binding pocket. Based on the study of the structure of the binding site of ␤ 2 AR, we propose that the aromatic ring of salbutamol may interact with aromatic residues in the second extracellular loop and the carboxyl-terminal end of TM6 (Fig. 6B). In this position, the chiral ␤-hydroxyl would not be expected to interact with Asn-293 6.55 . This is consistent with previous studies showing that the binding affinities of other non-catechol partial agonists are not affected by mutation of Asn-293 6.55 to Ala (6).
Differences in the Mechanism of Activation of Salbutamol and Catecholamines-It has been suggested that interactions between the aromatic ring of catecholamine agonists and Phe-290 6.52 in TM6 play an important role in some of the conformational changes associated with receptor activation by a rotamer toggle switch mechanism (26). Monte Carlo simulations suggest that rotameric positions of Phe-290 6.52 and Trp-286 6.48 are coupled and modulate the bend angle of TM6 around the highly conserved proline kink at Pro-288 6.50 , leading to the movement of the cytoplasmic end of TM6 (26). It is likely that the rapid change in fluorescence observed upon binding of catechol and catecholamine agonists to TMR-␤ 2 AR represents the conformational changes associated with this movement of TM6 relative to TM5.
Based on our experimental results and the model in Fig. 6B, salbutamol does not interact with this aromatic cluster and therefore does not directly activate the receptor by this rotamer toggle switch mechanism. One may hypothesize that the inability of salbutamol to fully activate the receptor may be due to its failure to directly engage the toggle switch. In this case, the combination of catechol and salbutamol might be expected to induce a more active conformation; however, we found that catechol partially inhibits activation by salbutamol (Fig. 5B). These results are most consistent with salbutamol inducing an active conformation distinct from the conformation induced by catecholamine agonists. They suggest that this active conformation does not involve the same movement of TM6 around Pro-288 6.50 that occurs upon activation of the ␤ 2 AR by catecholamines. These studies provide mechanistic insight into previous fluorescence lifetime experiments demonstrating that salbutamol and isoproterenol induce distinct conformational changes in the cytoplasmic end of TM6 of the ␤ 2 AR (3,9,27).
Catechol and Inverse Agonism-The mechanism of inverse agonism is poorly understood. As shown in Fig. 5C, we can detect inverse agonism of ICI-118,551 in a functional assay by its inhibition of basal GTP␥S binding, yet our conformational reporter on Cys-265 is not sensitive to this conformational change. Nevertheless, we can conclude that ICI-118,551 does not restrict movement of TM6 as it is still possible to detect a conformational response (Fig. 3C) and functional response (Fig.  5C) to catechol in the presence of a saturating concentration of this inverse agonist.
Using Ligand Fragments to Dissect the Process of Ligand Binding and Activation-Evidence from several studies suggests that agonists activate GPCRs through a series of conformational intermediates (3,9,(27)(28)(29)(30). These studies form the basis for our current hypothesis regarding the mechanism of activation (9). The unliganded receptor is maintained in a relatively inactive state by a series of intramolecular interactions that stabilize a specific arrangement of the TM segments. Ligands activate the receptor by disrupting these stabilizing interactions and/or by stabilizing new, more active arrangements of the TM segments. The fully active conformation occurs when all of the stabilizing intramolecular interactions have been broken and the new ones have been formed. The unliganded state of the receptor is conformationally dynamic, and at any one moment in time, the amino acids that interact with the ligand are not arranged to form a complete binding site for the ligand (as envisioned in a lock-and-key model of ligand binding). As such, all of the contacts between receptor and agonist do not form at the same time but sequentially through a series of conformational intermediates. As one component of the ligand engages the receptor, some of the stabilizing intramolecular interactions are broken, and there is an increased probability that additional contacts will form as the receptor explores its conformational space. As each contact is formed, the receptor assumes a more active state.
Catechol and dopamine can be considered fragments of catecholamine agonists and can therefore provide some insight into the functional properties of intermediate conformational states. Catechol disrupts and/or stabilizes a specific interaction between TM5 and TM6, and even this relatively small conformational change is associated with an increase in activity toward Gs (Fig. 5C). Catechol has a remarkably high affinity (K D ϭ 160 M, based on a conformational assay) considering its size (110 Da). This is consistent with an agonist fragment, in which a high proportion of the catechol atoms is involved in binding interactions with the receptor. Moreover, the relatively high binding affinity suggests that the energetic cost of the conformational changes required for optimal interactions between the ␤ 2 AR and catechol are small. The observation that sensitive biophysical (Fig. 2C) and functional assays (Fig. 5C) can detect binding and activation by a small agonist fragment such as catechol suggests that fragment-based screening strategies may be applicable to drug discovery for GPCRs (24).
The binding of the catechol ring of dopamine results in the same structural change that occurs upon binding of catechol alone, but the interaction between its amine and Asp-113 3.32 also stabilizes a specific arrangement of TM3 relative to TM5 and TM6. This additional conformational change imparts a much greater activity toward Gs (Fig. 5A). It is interesting to note that binding affinity for dopamine (K i ϭ 350 M) is similar to that for catechol. This is surprising considering that the interaction between the primary amine and Asp-113 3.32 makes the strongest contribution to the binding energy. Part of the binding energy associated with the interaction between dopa-mine and Asp-113 3.32 might be offset by the energetic cost of the conformational change needed for the binding interaction to occur. Thus, in the inactive state, TM5 and TM6 are positioned such that little energy is needed to accommodate the binding of the catechol ring. In contrast, the movement of TM3 relative to TM5 and TM6 required for binding of dopamine may involve the breaking of intramolecular interactions, thereby consuming part of the energy provided by the ionic interaction.
The process of binding and activation by a full agonist such as isoproterenol and epinephrine involves the same interactions that form with catechol and dopamine as well as additional interactions between the receptor and the ␤-OH and the alkyl substituent on the amine. These interactions make significant contributions to binding affinity and to the active state of the receptor.
Conclusion-We have used catechol as a molecular probe to investigate the location of the binding site for the partial agonist salbutamol, and the mechanism by which activation of the receptor by this partial agonist differs from the mechanism of activation by full agonist catecholamines. Despite the structural similarity of salbutamol and catecholamine agonists, the aromatic ring of salbutamol occupies a different space in the ␤ 2 AR and thereby induces a functionally different active state. The results provide further evidence for structural plasticity of GPCRs and the possibility of achieving more than one active state through different interactions between receptors and ligands.