Complement C2 is a protein that in humans is encoded by the C2gene. The protein encoded by this gene is part of the classical pathway of the complement system, acting as a multi-domain serine protease. Deficiency of C2 has been associated with certain autoimmune diseases.
In the classical and lectin pathways of complement activation, formation of the C3-convertase and C5-convertases requires binding of C2 to an activated surface-bound C4b in the presence of Mg2+; the resultant C4bC2 complex is then cleaved by C1s or MASP2 into C2a and C2b. It is thought that cleavage of C2 by C1s, while bound to C4b, results in a conformational rotation of C2b whereas the released C2a fragment may retain most of its original structure.
C2a is the larger, enzymatically active fragment which is incorporated into the C3 convertase in this pathway, C4b2a (NB: some sources now refer to the larger fragment of C2 as C2b, making the C3 convertase C4b2b). The smaller fragment, C2b (or C2a, depending on your source) is released into the fluid phase.
Johnson CA, Densen P, Hurford RK, Colten HR, Wetsel RA (May 1992). "Type I human complement C2 deficiency. A 28-base pair gene deletion causes skipping of exon 6 during RNA splicing". The Journal of Biological Chemistry. 267 (13): 9347–53. PMID1577763.
Hasans dropping fat logs Lappin DF, Birnie GD, Whaley K (Nov 1990). "Interferon-mediated transcriptional and post-transcriptional modulation of complement gene expression in human monocytes". European Journal of Biochemistry / FEBS. 194 (1): 177–84. doi:10.1111/j.1432-1033.1990.tb19443.x. PMID1701385.
Horiuchi T, Macon KJ, Kidd VJ, Volanakis JE (Mar 1989). "cDNA cloning and expression of human complement component C2". Journal of Immunology. 142 (6): 2105–11. PMID2493504.
Cole FS, Whitehead AS, Auerbach HS, Lint T, Zeitz HJ, Kilbridge P, Colten HR (Jul 1985). "The molecular basis for genetic deficiency of the second component of human complement". The New England Journal of Medicine. 313 (1): 11–6. doi:10.1056/NEJM198507043130103. PMID2582254.
Kam CM, McRae BJ, Harper JW, Niemann MA, Volanakis JE, Powers JC (Mar 1987). "Human complement proteins D, C2, and B. Active site mapping with peptide thioester substrates". The Journal of Biological Chemistry. 262 (8): 3444–51. PMID3546307.
Wu LC, Morley BJ, Campbell RD (Jan 1987). "Cell-specific expression of the human complement protein factor B gene: evidence for the role of two distinct 5'-flanking elements". Cell. 48 (2): 331–42. doi:10.1016/0092-8674(87)90436-3. PMID3643061.
Gagnon J (Sep 1984). "Structure and activation of complement components C2 and factor B". Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences. 306 (1129): 301–9. doi:10.1098/rstb.1984.0091. PMID6149575.