Complement Target-Factor B

Complement Factor B is a highly specific serine protease that is tightly regulated. The proenzyme factor B is made up of three N-terminal complement control protein (CCP) domains and connected via a 45-residue linker to a VWA domain and a C-terminal serine protease (SP) domain, which carries the catalytic center. Fragment Bb contains the VWA and SP domains, fragment Ba contains the CCP1 through CCP3 and the linker.

The activate process of Factor B has several steps: first, it binds to surface-bound C3b or its fluid-phase counterpart C3(H₂O); second, it is cleaved into fragments Ba (residues 1-234) and Bb (residues 235-739) via factor D; third, fragment Ba dissociates from the complex, leaving behind the alternative pathway C3 convertase complex C3b-Bb, which cleaves C3 into C3a and C3b. The C3b-Bb complex is very unstable. Besides, a similar C3 convertase complex is generated upon activation of the classical (antibody-mediated) and lectin-binding pathways, made up of C4 and C2, which are homologous to C3 and factor B, respectively. After activated, Factor B catalyzes the central amplification step of complement activation to initiate inflammatory responses, cell lysis, phagocytosis, and B-cell stimulation.

Fig. 1 Structure of factor B.¹

Reference

1. From Wikipedia: By Emw - Own work, CC BY-SA 3.0 https://commons.wikimedia.org/wiki/File:Protein_CFB_PDB_1dle.png