Component C2 which is part of the classical pathway of the complement system is cleaved by activated factor C1 into two fragments: C2b and C2a. C2a, a serine protease, then combines with complement factor C4b to generate the C3 or C5 convertase. Selective cleavage of Arg-|-Ser bond in complement component C3 alpha-chain to form C3a and C3b, and Arg-|-Xaa bond in complement component C5 alpha-chain to form C5a and C5b. The MIDAS-like motif in the VWFA domain binds divalent metal cations. Belongs to the peptidase S1 family.