EC - Complement factor I

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IntEnz Enzyme Nomenclature


Accepted name:
complement factor I
Other names:
C3b inactivator
C3b/C4b inactivator
complement C3b inactivator
complement C3b/C4b inactivator
complement C4b inactivator
complement C4bi
complement component C3b inactivator
conglutinogen-activating factor C
factor I
Systematic name:



Cleavage of complement subcomponent C3b requires its binding to cofactor factor H or complement receptor CR1; cleavage of iC3b requires complement receptor CR1; cleavage of C4b requires C4b-binding protein. In peptidase family S1 (trypsin family).

Links to other databases

Enzymes and pathways: NC-IUBMB , BRENDA , ExplorEnz , ENZYME@ExPASy , KEGG , MetaCyc , MEROPS , UniPathway
Protein domains and families: PROSITE:PDOC00124
Structural data: CSA , EC2PDB
Gene Ontology: GO:0004252
CAS Registry Number: 80295-66-5


  1. Nagasawa, S., Ichihara, C. and Stroud, R.M.
    Cleavage of C4b by C3b inactivator: production of a nicked form from C4b, C4b', as an intermediate cleavage product of C4b by C3b inactivator.
    J. Immunol. 125 : 578-582 (1980). [PMID: 7391570]
  2. Crossley, L.G.
    C3b inactivator and β1H.
    Methods Enzymol. 80C : 112-124 (1981). [PMID: 6210825]
  3. Müller-Eberhard, H.J.
    Molecular organization and function of the complement system.
    Annu. Rev. Biochem. 57 : 321-347 (1988). [PMID: 3052276]

[EC created 1981]