Responsible for cleaving the alpha-chains of C4b and C3b in the presence of the cofactors C4-binding protein and factor H respectively. Inactivates complement subcomponents C3b, iC3b and C4b by proteolytic cleavage. Heterodimer of a light and heavy chains; disulfide-linked. The fully processed and mature protein circulates as a zymogen, and is allosterically activated by substrate-induced remodeling of the active site (By similarity). Plasma. Belongs to the peptidase S1 family.