C protein activated C1s is a subcomponent of the first component of C (C1). Activated C1s is a glycosylated single-polypeptide zymogen of 85 kD. Activation of the proenzyme C1s occurs through cleavage by the active form of C1r. The activated protease, activated C1s, consists of a disulfide-linked H chain and a L chain. Activated C1s is a serine protease and its catalytic site is located in the L chain. Activation of the classical C pathway is triggered by activated C1s which cleaves C4 and C2 to form the C3 convertase, C4bC2a.Synonyms: C1 esterase, Complement 1s, Complement C1s, Complement C1s subcomponent, Complement component 1 subcomponent s