C1r B chain is a serine protease that combines with C1q and C1s to form C1, the first component of the classical pathway of the complement system. Selective cleavage of Lys(or Arg)-|-Ile bond in complement subcomponent C1s to form the active form of C1s (EC 3.4.21.42). C1 is a calcium-dependent trimolecular complex of C1q, C1r and C1s in the molar ration of 1:2:2. C1r is a dimer of identical chains, each of which is activated by cleavage into two chains, A and B, connected by disulfide bonds (By similarity). The iron and 2-oxoglutarate dependent 3-hydroxylation of aspartate and asparagine is (R) stereospecific within EGF domains. Belongs to the peptidase S1 family.