The intrinsic energy of the gating isomerization of a neuromuscular acetylcholine receptor channel

Nicotinic acetylcholine receptor (AChR) channels at neuromuscular synapses rarely open in the absence of agonists, but many different mutations increase the unliganded gating equilibrium constant (E ₀) to generate AChRs that are active constitutively. We measured E ₀ for two different sets of mutant combinations and by extrapolation estimated E ₀ for wild-type AChRs. The estimates were 7.6 and 7.8 × 10 ^-7 in adult-type mouse AChRs (-100 mV at 23 ^oC). The values are in excellent agreement with one obtained previously by using a completely different method (6.5 × 10 ^-7, from monoliganded gating). E ₀ decreases with depolarization to the same extent as does the diliganded gating equilibrium constant, e-fold with ~60 mV. We estimate that at -100 mV the intrinsic energy of the unliganded gating isomerization is +8.4 kcal/mol (35 kJ/mol), and that in the absence of a membrane potential, the intrinsic chemical energy of this global conformational change is +9.4 kcal/mol (39 kJ/mol). Na ⁺ and K ⁺ in the extracellular solution have no measureable effect on E ₀, which suggests that unliganded gating occurs with only water occupying the transmitter binding sites. The results are discussed with regard to the energy changes in receptor activation and the competitive antagonism of ions in agonist binding.