Table 1.
Na+ Occupancy and Mg2+ Association Rate Constant
| Value | ||||
|---|---|---|---|---|
| Parameter | Symbol | Units | ||
| No. of external sites | n | 2.0 ± 0.2 | – | |
| Dissociation constant for extracellular Na+ at the external sites (no Mg2+ in the channel) | KNaex | mM | 41.8 ± 12.9 | 113 ± 34.9 |
| 15.2 ± 5.1 | ||||
| Dissociation constant for intracellular Na+ at the external sites (no Mg2+ in the channel, no membrane potential) | K0 Nain | mM | 5.1 ± 2.1 | 2.0 ± 0.8 |
| 7.2 ± 5.5 | ||||
| Fractional electrical distance for Na+ from intracellular compartment to the external sites | α | 0.83 ± 0.07 | 0.92 ± 0.08 | |
| Extracellular Mg2+ association rate constant (no competing ions, no membrane potential) | κ0 +Mg (×108) | M−1s−1 | 4.4 ± 0.9 | 7.8 ± 2.4 |
| Fractional electrical distance for Mg2+ from extracellular compartment to the peak of the entry barrier | δ | 0.27 ± 0.01 | 0.24 ± 0.01 | |
| Goodness of fit | MSC | 5.50 | 5.90 | |
KNaex and K0 Nain are apparent (nonequilibrium) dissociation constants (koff/kon) for Na+ arising from the extracellular and intracellular compartments, respectively, with no Mg2+ in the pore. MSC is the Model Selection Criterion, with the higher number indicating the better fit (after accounting for the different in the number of free parameters; see materials and methods).