Figure 1. Isoniazid binding to Mt-trHbN(III).

(A) Difference static and kinetic absorbance spectrum of Mt-trHbN(III) minus Mt-trHbN(III)-isoniazid (dotted line and squares, respectively). (B) Ligand-binding isotherm for isoniazid binding to Mt-trHbN(III). The analysis of data according to Equation 1 allowed the determination of K = (1.1±0.1)×10⁻⁴ M. (C) Dependence of the pseudo-first-order rate-constant k ^obs for isoniazid binding to Mt-trHbN(III) on the drug concentration. The analysis of data according to Equation 3 allowed the determination of k _on = (5.3±0.6)×10³ M⁻¹ s⁻¹ and k _off  = (4.6±0.5)×10⁻¹ s⁻¹. The protein concentration was 4.0×10⁻⁶ M (panels A and B) and 2.0×10⁻⁶ M (panel C). The isoniazid concentration was 4.0×10⁻³ 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.