C4b binding protein (C4BP) regulates complement activation of the classical and lectin pathways. It binds to C4b and accelerates the dissociation of C2a from the C3/C5 convertase C4b,C2a. This inactivates the central enzyme of both pathways. It also functions as a cofactor allowing factor I to cleave and permanently inactivate C4b. C4BP has a flower shaped structure with each alpha chain radiating out from a central core. The arms are linked by disulfide bonds in the center and the ends of each arm bind C4b. Studies have shown that on surfaces densely coated with C4b, C4BP can bind up to four C4b molecules simultaneously (Rawal, N. et al. (2009)).
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