Table 4.

Equilibrium constants (nM) of reactions in Fig. 1C

A. Kdvalues of peptides for channel alone versus channel saturated with peptide
	Channel Alone	Channel Saturated With Alkaloid
Peptide	None Kd (kBR/kRB)	KIIIA Kd (kCA/kAC)	KIIIA[K7A] Kd (kCA/kAC)
Alkaloid
TTX	38 ± 6	730 ± 800	45 ± 22
STX	8 ± 2	—	6 ± 3
GTX2/3	5 ± 1	—	7 ± 4

B. Kdvalues of peptides for channel alone versus channel saturated with alkaloid
	Channel Alone	Channel Saturated With Alkaloid
Alkaloid	None Kd (kAR/kRA)	TTX Kd (kCB/kBC)	STX Kd (kCB/kBC)	GTX2/3 Kd (kCB/kBC)
Peptide
KIIIA	5 ± 5	105 ± 25	4 (1–12) × 103†	290 ± 30
KIIIA[K7A]	115 ± 40	138 ± 30	87 ± 18	156 ± 62

Equilibrium K_d values, mean ± SD, were calculated from k_off/k_on ratios (notations of corresponding rate constants in Fig. 1C are indicated in parentheses); values for these rate constants were obtained from Tables 1–3, except for those of k_CB and k_CA, which were obtained by numerical simulation, as described in methods. Values of k_CB, in min⁻¹, were (for indicated peptide–alkaloid combinations): 0.021 (KIIIA–TTX); 0.35 (KIIIA–STX); 0.029 (KIIIA–GTX2/3); 0.022 (KIIIA[K7A]–TTX); 0.013 (KIIIA[K7A]–STX); and 0.014 (KIIIA[K7A]–GTX2/3); SDs of k_off values for corresponding ternary complexes in Table 2 served as proxies for SDs for k_CB values and these were used in calculating SDs for corresponding k_CA and K_d values. Mean values of k_CA ± SD, in min⁻¹, were (for indicated peptide–alkaloid combinations): 0.0022 ± 0.0024 (KIIIA–TTX); 0.025 ± 0.012 (KIIIA[K7A]–TTX); 0.0038 ± 0.0018 (KIIIA[K7A]–STX); and 0.004 ± 0.0021 (KIIIA[K7A]–GTX2/3).

^†Values in parentheses indicate range of uncertainty of predicted K_d, from limits of uncertainty for corresponding k_BC value in Table 3.