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 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 into a conformational rotation of C2a whereas the released C2b 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, C4b2. C2b is released into the fluid phase.
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