There are two forms of C3-convertase ().

The first is an enzyme composed of the C4b-C2b complex, which forms during the Classical pathway of the complement system. It is formed when C1s cleaves off C2a from a membrane-bound C4b-C2 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 in 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 itself become a C3-convertase.

Complement system | EC 3.4.21