Table 2.

Binding energies and dissociation constants (K_d) of tetracaine (Ttc) bound to closed and open muscle-type nicotinic acetylcholine receptors (nAChRs).

Receptor state	Binding site location	Mean binding energy (kcal/mol)	(n)	Kd (M)
Resting	1	4.79 ± 0.54	133	3.08 × 10−4
	2	3.67 ± 0.67*	12	2.05 × 10−3
	3	4.75 ± 0.19	66	3.31 × 10−4
Open	1	4.53 ± 0.66	107	4.74 × 10−4
	2	3.48 ± 0.36*	33	2.79 × 10−3
	3	3.94 ± 0.51#	58	1.28 × 10−3

The main Ttc binding sites are denoted as (1) at the extracellular domain (ECD); (2) at the low-affinity site within the pore; and (3) at the high-affinity site within the pore. See right-hand figure for their location within the nAChR. Different binding energies of the same locations were averaged to get a single energy value, which is indicated together with its standard deviation. The K_ds were calculated using Equation 4 (see section Materials and Methods). Asterisks indicate significant differences between Ttc binding energies of site 2 and those of either sites 1 or 3, in both the open and closed conformations (p < 0.05, t-test); the pound sign denotes significant differences between sites 1 and 3 in the open state; n is the number of docking solutions that were averaged.