Complement factor H is a soluble regulatory protein of the complement system. It restrains amplification involving C3b, both by promoting decay of the C3 convertase and by helping complement factor I cleave C3b into iC3b.[ref]
Structure of complement fragment C3b-factor H and implications for host protection by complement regulators by Jin Wu and others. Abstract and Introduction: decay-acceleration and cofactor activities
A 2009 study combined a crystal structure with functional assays of the first four domains of complement factor H. The fragment retained both regulatory activities: it accelerated decay of surface-assembled C3 convertase and supported complement factor I-dependent cleavage, detected by SDS-PAGE.[ref]
Structure of complement fragment C3b-factor H and implications for host protection by complement regulators by Jin Wu and others. Figure 1d–e; Results, characterization of FH(1–4)
The two assays answer different questions. Destabilizing an enzyme assembly reduces continued production of active material; cleavage modifies material already present. Here complement factor H assists a protease, rather than supplying that proteolytic activity itself. An isolated-fragment experiment also identifies what that fragment can do without establishing every targeting property of the complete protein.
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