This complex might interact with another C3b and thus form the alternative pathway C5 convertase.

These together are called the C5 convertase.

The complement component C5 can be also activated by fluid phase C5 convertase.

It is a membrane protein and regulates also C5 convertase of the classical and alternative pathway.

C5 convertase is an enzyme belonging to a family of serine proteases that play key role in the innate immunity.

The modified C5 convertase, C4b2aoxy3b, contains C2a that is derived from C2 oxidized by iodine.

Target of function The target of C5 convertase is complement protein C5.

The membrane attack complex is initiated when the complement protein C5 convertase cleaves C5 into C5a and C5b.

The formation of the alternative pathway C5 convertase (C3bBbC3b) starts by spontaneous cleavage of C3 protein exposing previously hidden thioester bond.

CFHR1 blocks C5 convertase activity and interferes with C5b surface deposition and membrane attack complex (MAC) formation.

C3b can then act as an opsonin or bind to activated bimolecular complex C4b2a to form a trimolecular complex, C5 convertase, which is a specific enzyme for C5.

Once the alternative C3 convertase enzyme is formed on a pathogen or cell surface, it may bind covalently another C3b, to form C3bBbC3bP, the C5 convertase.

Two fluid phase C5 convertases have been described: the classical pathway enzyme, C4b2aoxy3b and the cobra venom factor-dependent C5 convertase, CVFBb.

Eculizumab inhibits the cleavage of C5 to C5a (a potent anaphylatoxin with prothrombotic and proinflammatory properties) and C5b by the C5 convertase, which prevents the generation of the terminal complement complex C5b-9 (which also has prothrombotic and proinflammatory effects).

The classical pathway C5 convertase is composed of the larger fragments of complement proteins, C4b, C2a produced by cleavage mediated by C1 complex, and C3b produced by cleavage mediated by the classical pathway C3 convertase (C4bC2a).