C3-convertase

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 or lectin pathways of the complement system. It is formed when C1s cleaves off a small peptide fragment of C2 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 (C4b2a3b or C3bBb3b) is known as "C5-convertase".

C3 convertase is, in classical terms, C4b2a; in the 1990s there was a motion put forward to change the nomenclature to C4b2b, however this was unsuccessful.

External links

• MeSH C3+convertase