Native, human, activated, C1r complement component. Generally, non-activated form composed of a single-chain glycoprotein. Present in normal human serum at 34 µg/ml. Non-activated C1r is found in circulating blood plasma as a dimer that associates with one C1q molecule and two C1s molecules to form the first component of complement (C1). Ensuing C1q binding to the classical complement pathway activators, each C1r proenzyme monomer is activated by cleavage into two disulfide-linked fragments of M.W. 60,000 and 35,000. The 35,000 peptide contains C1r enzymatic active site. Activated C1r continues activation via the classical pathway by cleaving, and thus activating C1s.
