Abstract
An alternative pathway C3 convertase is formed by the equilibrium association of Factor B with cobra venom factor (CVF) followed by the activation step catalyzed by Factor D. However, the association of Factor B with CVF has only occasionally been demonstrated and has not been quantitatively analyzed. Here we show that in the absence of metals the two proteins have significant affinity for each other and reversibly associate in a one-to-one stoichiometry with a dissociation constant of 11.6 microM. Upon the addition of metal ions, the complex is stabilized only 2- to 30-fold in the order Ni2+(Kd = 6.6 microM) less than Mg2+(Kd = 1.1 microM) less than Mn2+(Kd = 0.4 microM). These results suggest that metal ions may be less important in stabilizing the CVF.B complex and more important in promoting the subsequent equilibrium association of CVF.B with Factor D. The stability of the CVF.B complex is variously dependent on temperature in the range studied (14-21 degrees C) depending on the metal ion that is present. The complex formation was demonstrated in the analytical ultracentrifuge at sedimentation equilibrium employing a combination of single- and multiple-independent variable nonlinear least squares analytical techniques. Two different numerical approaches gave very similar results.
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