C3-convertase

There are two forms of C3-convertase.


 * The first is an enzyme composed of the C4b-C2a complex, which forms during the classical or lectin pathways of the complement system. It is formed when C1s cleaves off C2a from a membrane-bound C4b-C2b complex.


 * The second consists of the C3bBb complex, which forms during the alternative complement pathway. It is formed when membrane-bound C3b binds to factor B, which is, in turn, cleaved by factor D into Ba and Bb. Bb remains bound to C3b while Ba is released into the surrounding medium.

Once formed, both C3-convertases will catalyze the proteolytic cleavage of C3 into C3a and C3b (hence the name "C3-convertase"). C3b can then act as an opsonizer or bind to activated bimolecular complex C4b2a to form a trimolecular complex, C5-convertase, which is a specific enzyme for C5.

A convertase (of either type) with an additional 3b (C4b2a3b or C3bBb3b) is known as "C5-convertase".

C3 convertase was, in classical terms, C4b2a; however, in keeping with the number and letter scheme, it was changed to C4b2b in the 1990s to reflect the enzymatic activity of the C2b fragment.