Comparison of agonist binding affinity for single and double mutations of serines 204 and 207 in TM5 of β2AR
| Receptor | Kd | ΔG | n | Mutation Change | Expected Sum from the Single Mutations |
|---|---|---|---|---|---|
| μM ± S.E.M. | kJ · mol−1 ± S.E.M. | kJ · mol−1 ± S.E.M. | kJ · mol−1 ± S.E.M. | ||
| Wild type β2AR | 0.2 (± 0.02) | − 39.8 (± 0.25) | 4 | — | |
| (S204A) β2AR | 5.6 (± 1.4) | − 31.4 (± 0.61) | 4 | +8.3 (± 0.5) | |
| (S207A) β2AR | 3.4 (± 0.4) | − 32.5 (± 0.35) | 4 | +7.2 (± 0.3) | |
| (S204A,S207A) β2AR | 14.1 (± 2.1) | − 28.8 (± 0.34) | 4 | +10.9 (± 0.3) | +15.5 (± 0.7)1-a |
| (S204C) β2AR | 6.3 (± 0.3) | − 30.9 (± 0.17) | 4 | +8.9 (± 0.2) | |
| (S207C) β2AR | 3.9 (± 0.5) | − 32.2 (± 0.35) | 4 | +7.6 (± 0.6) | |
| (S204C,S207C) β2AR | 15.2 (± 0.7) | − 28.6 (± 0.13) | 4 | +11.2 (± 0.2) | +16.4 (± 0.6)1-b |
Binding parameters for isoproterenol in competition for125I-pindolol were estimated using LIGAND (Munson and Rodbard, 1980). In all cases, curves were fitted satisfactorily assuming a single class of binding sites. Mutation change is the net change of agonist binding energy caused by the mutations, calculated as ΔΔG = ΔGwt − ΔGmut. The data are the means of four independent experiments in each of which the binding to the wild-type receptor and all six mutants was measured in parallel. Statistics on free energy data were computed after calculating for each experiment the mutation changes and the expected sum of the single mutations. The significance of the differences between the sum expected from single mutants (col. 5) and the energy measured for double mutants (col. 4) was evaluated using t-statistics (paired two-tailedt-tests).