Complement C5 Convertase

Complement C5 convertase is a kind of serine protease that cleaves the C5 into fragments C5a, an anaphylatoxin and a chemotactic factor, C5b, the first component of the membrane attack complex (MAC). C5 convertase is made up of three fragments, in the classical or lectin pathways is C4b, C2a, and C3b, while in the alternative pathway is one C3b and two Bb. C5 convertase is generated on activating surfaces after the deposition of C3b molecules on the target surface in the vicinity of C3 convertase complexes.

Normally, C5 convertase cleaves the C5 is the final enzymatic step in the complement activation cascade resulting in the generation of the cytolytic proteolytically activated form of C5b-9 complex. Besides, C3 convertase binding to the additional C3b molecules nearby was thought to be needed for forming C5 convertase activity. In addition, high C5 convertase activity is related to the generation of C3b-C3b or C3b-C4b dimers in which the additional C3b molecule was shown to be covalently attached to a specific site on the first C3b or C4b molecule.

Fig. 1 C5 convertase is involved in classical and alternative pathways.¹

Reference

1. From Wikipedia: By Tossh_eng, CC BY-SA 4.0 https://upload.wikimedia.org/wikipedia/commons/b/b0/Complement-pathways.png