Complement component 1R is a protein involved in the complement system.The complement complexes have multiple subunits: one carrying the active site, one that binds to the cell surface, and one that binds to substrate. The steps in the complement cascade have similarities to those in the coagulation cascade. Furthermore, many enzymes have the same pattern of activation and multimeric structure with specific function of the several subunits. Both C1r and C1s are enzymatic, as is C2, as noted earlier. In the alternate pathway, factors B and D are enzymatic but deficiency states are unknown. C1r is a single-chain glycoprotein with a molecular weight of about 83,000. Upon activation, it is cleaved into an A chain and a B chain which are held together by a disulfide bond. The B chain contains the catalytic portion of the enzyme.