One form of C3-convertase, also known as C4b2a, is formed by a heterodimer of activated forms of C4 and C2. It catalyzes the proteolytic cleavage of C3 into C3a and C3b, generated during activation through the classical pathway as well as the lectin pathway. C3a is an anaphylotoxin and the precursor of some cytokines such as ASP, and C3b serves as an opsonizing agent. Factor I can cleave C3b into C3c and C3d, the latter of which plays a role in enhancing B cell responses. In the alternative complement pathway, C3 is cleaved by C3bBb, another form of C3-convertase composed of activated forms of C3 (C3b) and factor B (Bb). Once C3 is activated to C3b, it exposes a reactive thioester that allows the peptide to covalently attach to any surface that can provide a nucleophile such as a primary amine or a hydroxyl group. Activated C3 can then interact with factor B. Factor B is then activated by factor D, to form Bb. The resultant complex, C3bBb, is called the alternative pathway (AP) C3 convertase.
C3bBb is deactivated in steps. First, the proteolytic component of the convertase, Bb, is removed by complement regulatory proteins having decay-accelerating factor (DAF) activity. Next, C3b is broken down progressively to first iC3b, then C3c + C3dg, and then finally C3d. Factor I is the protease that performs these cuts but it requires the help of another protein to supply cofactor activity.
^Matsuyama W, Nakagawa M, Takashima H, Muranaga F, Sano Y, Osame M (December 2001). "Molecular analysis of hereditary deficiency of the third component of complement (C3) in two sisters". Intern. Med.40 (12): 1254–8. doi:10.2169/internalmedicine.40.1254. PMID11813855.
^Janssen BJ, Huizinga EG, Raaijmakers HC, Roos A, Daha MR, Nilsson-Ekdahl K, Nilsson B, Gros P (September 2005). "Structures of complement component C3 provide insights into the function and evolution of immunity". Nature437 (7058): 505–11. doi:10.1038/nature04005. PMID16177781.
^Janssen BJ, Christodoulidou A, McCarthy A, Lambris JD, Gros P (November 2006). "Structure of C3b reveals conformational changes that underlie complement activity". Nature444 (7116): 213–6. doi:10.1038/nature05172. PMID17051160.
^Wiesmann C, Katschke KJ, Yin J, Helmy KY, Steffek M, Fairbrother WJ, McCallum SA, Embuscado L, DeForge L, Hass PE, van Lookeren Campagne M (November 2006). "Structure of C3b in complex with CRIg gives insights into regulation of complement activation". Nature444 (7116): 217–20. doi:10.1038/nature05263. PMID17051150.
^Fredslund F, Jenner L, Husted LB, Nyborg J, Andersen GR, Sottrup-Jensen L (August 2006). "The structure of bovine complement component 3 reveals the basis for thioester function". J. Mol. Biol.361 (1): 115–27. doi:10.1016/j.jmb.2006.06.009. PMID16831446.
^Soames CJ, Sim RB (September 1997). "Interactions between human complement components factor H, factor I and C3b". Biochem. J. 326 ( Pt 2): 553–61. PMC1218704. PMID9291131.
^Jokiranta TS, Westin J, Nilsson UR, Nilsson B, Hellwage J, Löfås S, Gordon DL, Ekdahl KN, Meri S (March 2001). "Complement C3b interactions studied with surface plasmon resonance technique". Int. Immunopharmacol.1 (3): 495–506. doi:10.1016/S1567-5769(00)00042-4. PMID11367533.