Effect of polyP₁₃₀ and heparin on the observed rates of association between C1s and C1-INH. The effect of polyP₁₃₀ (A) or heparin (C) on the observed rate of association between recombinant WT CCP12SP C1s and C1-INH was determined by adding C1s (2 nM) to C1-INH (20 nM), Z-Lys-SBzl (0.2 mM), and DTDP (0.6 mM) in the presence of increasing concentrations of polyP₁₃₀ (0-2000 μM) (A) or heparin (0-100 μg mL⁻¹) (C). The pseudo-first-order rate constant (k_obs) is plotted as a function of polyP₁₃₀ or heparin concentration. The right-hand axis shows the ratio of the k_obs at a particular polyP₁₃₀ or heparin concentration to that in the absence of a cofactor. The effect of polyP₁₃₀ (B) or heparin (D) on the observed rate of association between recombinant exosite (A1) mutant CCP12SP C1s and C1-INH was determined by adding 1.5 nM C1s A1 mutant to 30 nM C1-INH, 0.2 nM Z-Lys-SBzl, and 0.9 mM DTDP in the presence of increasing concentrations of polyP₁₃₀ (0-2000 μM) (B) or heparin (0-500 μg mL⁻¹) (D). The pseudo-first-order rate constant (k_obs) is plotted as a function of polyP₁₃₀ or heparin concentration. The right-hand axis shows the ratio of the k_obs at a particular polyP₁₃₀ or heparin concentration to that in the absence of a cofactor. Results are representative of 3 independent experiments.