There are two forms of C3-convertase (EC 3.4.21.43).The first is an enzyme composed of the C4b-C2a complex, which forms during the Classical pathway of the complement system. It is formed when C1s cleaves off C2b from a membrane-bound C4b-C2a 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.

Soluble C3-convertase, also known as C4b2a, catalyzes the proteolytic cleavage of C3 into C3a and C3b as part of the classical complement system as well as the mannan-binding lectin pathway.