Clq Component of Complement Binds to Fibrinogen and Fibrin

The interaction of complement component Clq with Fibrinogen and fibrin was studied by using a solid-phase direct binding assay. Scatchard analysis of radioiodinated fibrinogen binding to Clq indicated at least two high-affinity binding constants (K_d) calculated as 8.5 and 120 nM. In contrast, binding of radioiodinated fibrin to Clq showed only a single class of binding sites with a calculated K_d of 600 nM. Fibrinogen-Clq binding was shown to decrease as a function of increasing salt concentrations, indicating either the presence of charged amino acids in the binding sites or an ionic strength induced conformational dependency of the binding. In direct binding studies using isolated fragments of Clq, both the collagen-like domain of Clq and the globular domains of Clq were shown to bind fibrinogen, indicating at least one binding site for fibrinogen is located in each of the major domains of Clq. Addition of the thrombin-generated peptides of fibrinogen, fibrinopeptides A and B, enhanced Clq-fibrinogen binding, again indicating a complex binding interaction. These results indicate that Clq and fibrinogen are capable of high-affinity interactions that may serve to sequester these complexes in areas of tumors, immune complex deposition, or wounds.