Complement C3b

Complement component 3 (C3) is a key member in the complement system, owning to the manifold biological activities of its activation fragments, C3b, which anchors the generation of convertases affecting C3 and C5 activation. C3b is produced by proteolysis of its anaphylatoxin domain of C3, which activates a stable thioester bond, causing the covalent attachment of C3b to cell-surface or protein-surface hydroxyl groups by transesterification. The cleavage and activation of C3 expose binding sites for a variety of proteins, including factors B, H, and I, complement receptor 1 (CR1, CD35), decay-accelerating factor (DAF, CD55), properdin, membrane cofactor protein (MCP, CD46) and so on. C3b binds to these proteins in different configurations and forms complexes causing the activation, amplification, and regulation of the complement response.

Importantly, C3b plays an important role in all three pathways of complement activation. In the classical (CP) and lectin (LP) pathways, C3b gives one of two noncatalytic subunits of the C5 convertase, and in the alternative pathway (AP), it provides the non-catalytic subunits of both C3 convertase and C5 convertases.

Fig. 1 C3b involved in the alternative pathway.¹

Reference

1. From Wikipedia: https://commons.wikimedia.org/wiki/File:Complement_pathway.svg