Complement factor 3 C3b Cleavage of C3 by C3 convertases Authored: de Bono, B, 2004-08-04 09:17:50 C4b and C2a bind to form the classical pathway C3-convertase (C4b2a complex), C3b and the Bb fragment of Factor B form the alternative pathway C3 convertase. The C3(H2O):Bb C3 convertase is sometimes called the initiating convertase, and the C5 convertases also have C3 convertase activity (Rawal & Pangburn 2001). EC Number: 3.4.21 Edited: Jupe, S, 2010-11-17 Pubmed11367526 Pubmed15199963 Reactome Database ID Release 43166817 Reactome, http://www.reactome.org ReactomeREACT_7990 Reviewed: D'Eustachio, P, 2006-07-03 21:35:13