C1s proenzyme is a glycoprotein produced primarily by hepatocytes but also by other cells. The proenzyme is produced as a single chain polypeptide of 688aa. Upon activation the peptide is cleaved between R437 and I438 giving the activated enzyme consisting of two disulfide-linked chains, A chain (52.2kD) and B chain (27.7kD). The Ca dependent catalytic subunit of the C1 complex (C1q, C1r, C1s) comprises of two C1s and two C1r molecules and this serine protease tetramer is responsible for the initial steps in the classical complement pathway, cleavage of C4 and C2.