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The classical and alternative complement pathways.

There are two forms of C3-convertase (EC 3.4.21.43).

  • 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.

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