= Complement factor H
<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>.<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]{ref}
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>.<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)]{ref}
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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