C3-convertase

The classical and alternative complement pathways.

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.

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 (C4b2b3b or C3bBb3b) is known as "C5-convertase".

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

External links

• MeSH C3+convertase