Complement system serine protease (None standard; individual proteases (C1r, C1s, MASP-1, MASP-2, MASP-3, Factor B, Factor D, Factor I, C2) have their own abbreviations)

Complement serine proteases are specialized enzymes integral to all three activation pathways of the complement system: classical, lectin, and alternative. They function as part of proteolytic cascades, where recognition events trigger sequential protease activation, amplifying immune responses. Key members include C1r and C1s (classical pathway), MASP-1, MASP-2, and MASP-3 (lectin pathway), Factor B, Factor D, and Factor I (alternative pathway), and C2. These enzymes utilize a conserved catalytic triad (histidine, serine, aspartate) for peptide bond cleavage, with many regulated by specific inhibitors such as serpins (e.g., C1-INH). Their tight regulation is critical, as dysregulation leads to overactive complement responses implicated in inflammatory, autoimmune, and infectious diseases. Because they play central roles in the complement cascade—including C3 and C5 convertase formation—they are notable targets for drugs modulating immune responses.

Other names

Complement serine endopeptidasesComplement proteaseC1rC1sMASP-1MASP-2MASP-3Factor BFactor DFactor IC2

Inhibition of protease catalytic activity (prevent complement activation); Inhibition of convertase formation; Blocking substrate cleavage (e.g., preventing C3/C5 cleavage)