= Complement factor I
<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.<Complement factor I in IUBMB Enzyme Nomenclature>[accepted name, Reaction and Comments]{ref}
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.<Complement factor I in IUBMB Enzyme Nomenclature>[Comments, substrate-specific cofactor requirements]{ref}
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.<Structure of complement fragment C3b-factor H and implications for host protection by complement regulators by Jin Wu and others>[Figure 1d–e, functional characterization]{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.
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