Thus, the alternative C3 convertase (C3bBb) is formed and is able to cleave C3 via its dimeric Bb subunit. Also Mg2+ ions are necessary for forming a functional C3 convertase. The Factor B (a zymogen) is cleaved by a plasma serine protease Factor D releasing a small fragment called Ba and generating a larger fragment called Bb that remains attached to C3b. After cleavage and binding to cell surface, the C3b fragment is ready to bind a plasma protein called Factor B. C3b, the larger fragment, becomes covalently attached to the microbial surface or to the antibody molecules through the thioester domain at the site of complement activation. C3 convertase formation can occur in three different pathways: the classical, lectin, and alternative pathways.Ĭleavage of complement C3 by a free floating convertase, thrombin, plasmin or even a bacterial enzyme leads to formation of C3a and C3b fragments.
