C3 convertase (EC 3.4.21.43, C42 , C4bC2a, C3bBb, complement C.hivin.4.hivin2, complement C3 convertase) belongs to family of serine proteases and is necessary in innate immunity as a part of complement system which eventuate in opsonisation of particles, release of inflammatory peptides, C5 convertase formation and cell lysis.

C3 convertase can be used to refer to the form produced in the alternative pathway (C3bBb) or the classical and lectin pathways (C4bC2a). Once formed, both C3 convertases will catalyze the proteolytic cleavage of C3 into C3a and C3b (hence the name "C3-convertase").

The smaller fragment called C3a serves to increase vascular permeability and promote extravasation of phagocytes, while the larger C3b fragment can be used as an opsonin or bind to either type of C3 convertase to form the trimolecular C5 convertase to activate C5 for the membrane attack complex.

C3 convertase formation can occur in three different pathways: the classical, lectin, and alternative pathways.

Cleavage of complement C3 by a free floating convertase, thrombin, plasmin or even a bacterial enzyme leads to formation of C3a and C3b fragments. 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. After cleavage and binding to cell surface, the C3b fragment is ready to bind a plasma protein called Factor B. 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. Also Mg2+ ions are necessary for forming a functional C3 convertase. Thus, the alternative C3 convertase (C3bBb) is formed and is able to cleave C3 via its dimeric Bb subunit.

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