Complement C2 is a single chain serum glycoprotein (110 kDa), which serves as the catalytic subunit of C3 and C5 convertases in the classical and lectin pathways. During complement activation, C2 is cleaved by classical (C1s
) or lectin (MBL-associated serine protease-2; MASP-2
) proteases into two fragments: C2b and C2a. C2a, a serine protease, in complex with C4b fragment of complement factor C4
, generates the C3 (C4b2a) or C5 (C4b2a3b) convertase. C3 convertase is very short-lived and cleaves complement C3
into C3a and C3b fragments (selective cleavage of Arg-|-Ser bond in C3 alpha-chain). C3 convertase requires the presence of magnesium and decays over time at physiologic temperatures. However, continuous activation of complement pathways shifts the substrate preference from C3 to C5
by formation of C5 convertase (formed by addition of C3b fragment to C3 convertase i.e. C4b2a3b). C5 convertase cleaves complement C5 to become activated into C5a and C5b fragments (selective cleavage of Arg-|-Xaa bond in C5 alpha-chain) and by a series of additional steps, promotes lysis of bacteria and damaged cells by pore or membrane attack complex (MAC) formation. Deficiency of C2 has been reported to be associated with certain autoimmune diseases. Single nucleotide polymorphisms (SNPs) in the C2 gene have been associated with altered susceptibility to age-related macular degeneration.