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Incoming links: Complement factor I in IUBMB Enzyme Nomenclature
Complement factor I 2026-11-05
Complement factor I is a regulatory serine protease of the complement system. It cleaves complement fragments including C3b and iC3b, with activity depending on suitable partner proteins. Its official enzyme classification is EC 3.4.21.45.[ref]
Complement factor I in IUBMB Enzyme Nomenclature. accepted name, Reaction and Comments
For cleavage of C3b, the substrate binds a partner such as complement factor H or complement receptor 1. Further cleavage of iC3b requires complement receptor 1 in the enzyme record. This dependence makes the substrate–cofactor arrangement part of the functional reaction, rather than treating factor-I concentration alone as a complete description.[ref]
Complement factor I in IUBMB Enzyme Nomenclature. Comments, substrate-specific cofactor requirements
A 2009 structural and functional study examined the first four domains of complement factor H. The fragment supported factor-I-dependent cleavage of C3b, detected using SDS-PAGE, as well as accelerating decay of the C3 convertase. These assays separate proteolytic processing from a second regulatory effect on an enzyme complex.[ref]
A useful control comparison therefore holds complement factor I and substrate constant while adding or omitting the required partner. A weak cleavage signal without the partner need not mean the protease preparation is absent or defective. The experiment tests the assembled reaction system.