Table 1.

Site-specific and stepwise binding parameters for the interactions of glycocholic acid with human ileal bile acid binding protein at 20°C

Parameter type	Dissociation constants*, M	Binding free energies†, kJ/mol
Site-specific binding
Cooperativity	8.6 (± 0.3) × 10−4	−17.2  ± 0.1
Site 1, intrinsic	1.5 (± 0.1) × 10−3	−15.8  ± 0.2
Site 2, intrinsic	2.1 (± 0.4) × 10−3	−15.0  ± 0.5
Stepwise binding‡
Step 1	1.8 (± 0.3) × 10−3	−15.4  ± 0.4
Step 2	1.5 (± 0.9) × 10−6	−32.6  ± 2.2
Hill coefficient§	1.94

^*The inverse of the association constants defined in Figs. 1 and 2. 

^† ΔG° = RTlnK_d or RTln(1/c₁₂), where the standard state is defined as 1 M. 

^‡ The observed stepwise dissociation constants are related to the site-specific dissociation constants as follows: (K)⁻¹ = (2K_d1)⁻¹ + (2K_d2)⁻¹ and K*K = K_d1*K_d2/c₁₂. 

^§ The Hill coefficient, a commonly used measure of macroscopic cooperativity, varies from limiting values of 0 to 2 for extremely negatively and positively cooperative systems, respectively (3). It assumes a value of 1 for a noncooperative system. The Hill coefficient at half saturation is related to the stepwise binding parameters as follows: n_H = 2/[1 + (K/K)^1/2].