Complement component 1r (C1r)

C1r is a serine protease and a key subcomponent of the C1 complex, which initiates the classical pathway of the complement system. It circulates in blood as an inactive zymogen and becomes activated through autocatalysis when C1 binds to an activating ligand. Upon activation, active C1r cleaves and activates C1s, which then cleaves C4 and C2. The activity of C1r is tightly regulated by C1-inhibitor (C1-INH), preventing inappropriate activation of the complement pathways. Dysregulation or deficiency in C1-INH leads to diseases such as hereditary angioedema. Bacterial proteins can also interact with C1r, potentially modulating immune evasion strategies.

Other names

C1r esterase

C1-INH binds to and irreversibly inactivates C1r, preventing its proteolytic activity.