C3-convertase (C3bBb) need only one molecule of C3b to form, whereas two or more C3b are required for generation of C5 convertase (C3bBb). It means, when C3b is randomly distributed on the surface of a cell, only C3 convertase activity appears after addition of Factors B and D.

The Bb subunit demonstrates catalytic activity and cleaves new C3 into C3a and C3b. C3bBb recruits additional available C3b to form the C5 convertase, C3bBbC3b, which initiates terminal pathway activation. The C3 convertase undergoes dissociation through spontaneous decay and also secondary to complement regulators such as factor H and CR1.

The binding and cleavage of an additional C3 molecule to C3 convertase forms the APC C5 convertase (C3bBbC3b), that cleaves C5 to C5a and C5b. C5b initiates the terminal complement pathway that forms the MAC, as described above.

Factor B (MW: 93 kDa) is a complement protein unique to the alternative pathway (AP). In the presence of C3b, FB binds to C3b to form the pre-convertase (C3bB). Factor D cleaves factor B releasing Ba (MW: 33 kDa) to generate the active proteolytic enzyme Bb (MW: 66kDa).

2023年4月27日 · We present the cryo-electron microscopy (EM) structures of native C3 and C3b in complex with ISG65 which reveal a set of modes of complement interaction.

Within this C3bBbC3b complex, the first C3b binds covalently to the target surface, and Bb, which bears a catalytic site, binds noncovalently to the first C3b. In the present investigation, we studied the nature of the convertase that is assembled on E surfaces and obtained evidence that the second C3b binds directly to the alpha'-chain of the ...

C3 convertase can be used to refer to the form produced in the alternative pathway (C3bBb) or the classical and lectin pathways (C4bC2b, formerly C4b2a). Once formed, both C3 convertases will catalyze the proteolytic cleavage of C3 into C3a and C3b (hence the name "C3-convertase").

2009年1月20日 · Using single-particle electron microscopy we have determined the 3-dimensional structures of the C3bB and the C3bBb complexes at ≈27Å resolution. The C3bB structure shows that fB undergoes a dramatic conformational change upon binding to C3b.

1988年12月1日 · C5 convertase of the alternative C pathway is a complex enzyme consisting of three C fragments--one molecule of a major fragment of factor B (Bb) and two molecules of a major fragment of C3 (C3b). Within this C3bBbC3b complex, the first C3b binds covalently to the target surface, and Bb, which bears …

2024年12月20日 · C3b在表面的聚集允许形成旁路途径的C5转化酶，C3bBbC3b，并且可以裂解C5。 随后TCC（膜攻击复合物）的组装与经典途径相同。