* Complement scientists have decided to designate the smaller of all C fragments with an ‘a’, and the larger with a ‘b’ and hence more recent literature may refer the larger C2 fragment as C2b. Complement Technology, Inc. uses the current Uniprot names which follow the original naming practice.
The larger fragment of complement protein C2 is C2a that results from the activation of the classical and lectin pathways. CompTech prepares the C2a fragment from complement protein C2 which was purified from normal human serum. C2a forms the proteolytic subunit of the C3 and C5 convertase of both pathways. Initiation of each pathway generates proteolytic enzyme complexes which are bound to the target surface (C1q/C1r/C1s in the classical pathway and MBL/ Ficolin/ MASPs in the lectin pathway). C1s and MASP in these complexes activate both C4 and C2. They cleave a peptide bond in C4 depositing C4b on the surface. They also cleave C2 into two fragments, the larger catalytic fragment C2a (73 kDa) and the smaller fragment C2b (34 kDa). The larger fragment binds to C4b and forms the C3/C5 convertase enzyme complex C4b,C2a (Nagasawa S. & Stroud, R. M. 1977), which activates C3, cleaving it to C3b and C3a. Deposition C3b on or near the C4b,C2a site converts the C3/C5 converatse, which is a weak C5 convertase, to a highly efficient C5 convertase (C4b,C2a,C3b) with a Km for C5 3000-fold lower than that of the C4b,C2a enzyme alone (Rawal N. and Pangburn M.K. 2003).
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