Figure 5. (A) Response curves for increasing 12p1 concentrations binding to immobilized HIV-1_YU2 gp120. (B) 12p1 inhibition of gp120-sCD4 interaction. (C) 12p1 inhibition of gp120-17b interaction. (D) Analysis of sCD4 saturation of HIV-1_YU2 gp120 in the presence versus absence of 12p1. Response at equilibrium (R_eq) values were obtained from fitting sensorgrams charting the response of immobilized HIV-1_YU2 gp120 to increasing concentrations of sCD4 (0–3.5 μm) in the presence or absence of 100 μm 12p1. The resultant R_eq responses were then plotted versus sCD4 concentration and fitted to a steady-state 1:1 binding model to obtain the equilibrium binding constants K_A and K_D. Under the conditions used, the presence of 12p1 had no discernible effect on the kinetics of interaction between HIV-1_YU2 gp120 and sCD4 (K_D values, 721 nm and 429 nm, in the presence or absence of peptide, respectively) but had a marked effect on the total amount of sCD4 bound by HIV-1_YU2 gp120. This indicates a noncompetitive mode of action for the 12p1 inhibitor. (Adapted from Biorn et al. 2004.)