Table 3.
Na+ Equilibrium Binding and Mg2+ Dissociation and Permeation
| Parameter | Symbol | Units | Value | |||
|---|---|---|---|---|---|---|
| No. of external sites | n | 1 (fixed) | 2 (fixed) | 1.3 ± 0.5 | ||
| Equilibrium dissociation for extracellular Na+ at the external site (Mg 2+ present in the channel) | Jd,Naex | mM | 88.5 ± 5.6 | 251 ± 12 | 129 ± 86.3 | |
| Mg 2+ dissociation rate constant (no competing ions, no membrane potential) | κ0 −Mg | s−1 | 9,641 ± 277 | 8,944 ± 205 | 9,318 ± 86.3 | |
| Mg 2+ permeation rate constant (no competing ions, no membrane potential) | κ0 pMg | s−1 | 623 ± 4.8 | 624 ± 5.0 | 624 ± 5.0 | |
| Goodness of fit | MSC | 4.97 | 4.95 | 4.89 | ||
The results were obtained by fitting to assuming one or two (equal) external sites. The electrical distances (see Fig. 6) ɛ (0.36; Mg2+ binding site to the extracellular compartment) and λ (0.03; Mg2+ binding site to the intracellular compartment) were fixed to their values obtained by fitting with (Table ).