Figure 4. Isoniazid binding to Mt-trHbN(II).

(A) Difference static and kinetic absorbance spectrum of Mt-trHbN(II) minus Mt-trHbN(II)-isoniazid (dotted line and squares, respectively). (B) Ligand-binding isotherm for isoniazid binding to Mt-trHbN(II). The analysis of data according to Equation 12 allowed the determination of D = (1.2±0.2)×10⁻³ M. (C) Dependence of the pseudo-first-order rate-constant d ^obs for isoniazid binding to Mt-trHbN(II) on the drug concentration. The analysis of data according to Equation 14 allowed the determination of d _on  = (1.3±0.4)×10³ M⁻¹ s⁻¹ and d _off  = 1.5±0.4 s⁻¹. The protein concentration was 1.5×10⁻⁶ M. The isoniazid concentration was 1.0×10⁻² M (panel A). Where not shown, the standard deviation is smaller than the symbol. All data were obtained at pH 7.0 and 20.0°C. For details, see text.