Complement B/C2 <p>These proteins belong to MEROPS peptidase family S1 (chymotrypsin family, clan PA(S)), subfamily S1A.</p><p>This family contains two mammalian proteins, complement C2 and complement factor B, which, respectively, have analogous roles in the classical and alternative pathways of complement activation. These proteins are composed of three regions, an N-terminal three-module complement control protein domain, a von Willebrand factor A domain, and a C-terminal serine protease domain. Briefly, they are activated by cleavage and function as the serine protease components of the C3/C5 convertases, which play similar roles in these pathways although composed of different proteins. Homologs in non-mammalian species are often more or less equally related to mammalian C2 and B and may be designated as complement B/C2. <taxon tax_id="7668">Strongylocentrotus purpuratus</taxon> (Purple sea urchin) has an atypical factor B with a five-module complement control protein domain.</p><p>The structures of the von Willebrand factor A and serine protease domains from human complement factor B (<db_xref db="SWISSPROT" dbkey="P00751"/>) have been analysed [<cite idref="PUB00022488"/>, <cite idref="PUB00024230"/>]. The A domain forms the classical vWF A domain fold, which consists of a central beta-sheet flanked on both sides by amphipathic alpha helices. It contains an integrin-like MIDAS (metal ion-dependent adhesion site) motif that adopts the open conformation typical of integrin-ligand complexes, with an acidic residue from another A domain (provided by a fortuitous crystal contact) completing the coordination of the metal ion. Although a closed conformation was not observed, modelling studies suggest that the A domain could adopt this conformation, implying that as with integrins, ligand-binding may induce conformational changes which transduce a signal to other domains in the protein [<cite idref="PUB00022488"/>]. The serine protease domain forms a chymotrypsin fold with several novel features [<cite idref="PUB00024230"/>]. Like other serine proteases it forms two beta-sheets, composed of six beta-strands each, surrounded by surface helices and loops. However, several novel deletions and insertions occur within these surface helices and loops, and differences in active site conformation also exist.</p>