Complement C5: Proteins and Enzymes

C5 is synthesised in the liver as a single polypeptide chain. Before secretion the molecule is glycosylated and secreted into plasma as a 190kDa glycoprotein consisting of a disulphide linked alpha-chain (115kDa) and beta-chain (75kDa). C5 occurs in plasma at about 75mg/L. It is activated via either the classical or the alternative complement pathway, by protoelytic cleavage of C5a (11.5kDa) from the N-terminal of the alpha-chain. C5a is a potent anaphylotoxin and stimulates directed migration of neutrophils, eosinophils, basophils and monocytes. The other fragment, C5b,initiates the assembly of the membrane attack complex that mediates the cytolysis of viral and bacterial pathogens.