Native, human, activated C1s complement component. Present in human serum at 31 µg/ml. Nonactivated C1s is found in circulating blood plasma as a Ca2+-dependent dimer in association with one C1q molecule and two C1r molecules to form the first component of complement (C1). Ensuing C1q binding to classical complement pathway activators, each C1r protein is cleaved to form activated C1r enzyme. Dimeric C1r enzyme cleaves, and thus activates, each C1s molecule into two disulfide-linked fragments of M.W. 59 kDa and 28 kDa. The 28 kDa peptide contains the C1s enzymatic active site. Activated C1s continues activation via classical pathway by cleaving, and thus activating, C2 and C4.
