Table 1.
Activation rate and equilibrium constants for V433 and A433 AChR
| αV433 | αV433 (equal binding) | αA433 (equal binding) | αV433/G153S | αA433/G153S | ||
|---|---|---|---|---|---|---|
| Rate constant | k+(k+1) | 97 ± 3 | 158 ± 2 | 179 ± 3 | 138 ± 15 | 193 ± 14 |
| k−(k−1) | 18 026 ± 585 | 25 267 ± 372 | 28 698 ± 684 | 770 ± 147 | 1620 ± 207 | |
| k+2 | 291 ± 16 | — | — | 770 ± 121 | 860 ± 127 | |
| k−2 | 43 010 ± 1835 | — | — | 9100 ± 647 | 5840 ± 271 | |
| β | 48 950 ± 13 490* | 48 950 ± 13 490* | 58 700 ± 17 420* | 43 800 ± 3632 | 36 500 ± 1081 | |
| α | 2612 ± 24 | 2735 ± 20 | 2850 ± 43 | 775 ± 71 | 998 ± 28 | |
| k+g | 32 ± 3 | 36 ± 3 | 98 ± 17 | — | — | |
| k−g | 362 ± 26 | 377 ± 27 | 564 ± 48 | — | — | |
| k+b | — | — | — | — | — | |
| k−b | — | — | — | — | — | |
| LL | 364 719 | 364 667 | — | — | — | |
| Equilibrium constant | Ka (Kd) (μM) | 186 ± 12 | 160 ± 4 | 160 ± 7 | 5.6 ± 1.8 | 8.4 ± 1.8 |
| Kb (μm) | 148 ± 37 | — | — | 11.8 ± 3.2 | 6.8 ± 1.5 | |
| Θ | 19 ± 5 | 18 ± 5 | 18 ± 9 | 57 | 37 | |
| Kg | 0.09 ± 0.01 | 0.10 ± 0.01 | 0.17 ± 0.01 | — | — | |
| Intervals | 59 402 | 59 402 | 22 801 | 1392 | 7900 |
Rate constants (± standard deviation) were determined by kinetic modelling of single-channel interval dwell times, as described in Methods. The rate constants are defined in Model 1. The equilibrium constants were calculated from ratios of the rate constants. Ka is the dissociation equilibrium constant of the slow transmitter binding site, Kb is the dissociation equilibrium constant of the fast transmitter binding site, Θ is the gating equilibrium constant, and Kg is equilibrium constant for occupying a ∼1 ms closed state (state C6 of Model 1). LL is the log likelihood. All rate constants are in units of s−1, except for k+1, and k+2,which are μm−1 s−1. The αA433 and αV433 values were obtained by fitting Model 1 simultaneously to currents elicited at 20, 50 and 100 μm ACh. For modelling, the opening rate constant was fixed at 50 000 s−1.
Values derived from fit of β' curve (Figs 2C and 3C) to eqn (1). For αG153S, the ACh concentration was 1 μm, and the inclusion of state C6 did not improve the LL and was not part of the model. The number of clusters used in the fit was 118 (αV433), 54 (αA433), 14 (αV433/G153S) and 45 (αA433/G153S).