= Complement C3
{c}
<Complement C3> is a central <protein> of the <complement system>. <Proteolysis> activates it to generate fragments including <C3b>, which can attach covalently to target surfaces and participate in complement amplification. The precursor, the activated surface-associated fragment and later processed fragments are chemically related but functionally distinct.<Structure of complement fragment C3b-factor H and implications for host protection by complement regulators by Jin Wu and others>[Introduction: C3 activation, C3b attachment and regulation]{ref}
A 2005 structural study compared native <Complement C3> with its major processed fragment <C3c>. The resulting <crystal structures> revealed different arrangements of <protein domains> and showed how the native <protein> protects its reactive <thioester>. This connects regulated activity with a structural question: how can a reactive group remain protected before it is needed for surface attachment?<Structures of complement component C3 provide insights into the function and evolution of immunity by Bert J. C. Janssen and others>[Abstract; Figures1–2 and4: C3/C3c comparison and thioester protection]{ref}
The comparison was C3 versus <C3c>, not a direct photograph of C3 becoming <C3b>. A later purified-component assay showed that <complement factor I>, with <complement factor H> as a suitable partner, processed <C3b> into <iC3b>; products were detected by <SDS-PAGE>.<Structure of complement fragment C3b-factor H and implications for host protection by complement regulators by Jin Wu and others>[Figure1e: cofactor-dependent C3b cleavage]{ref}
Following the fragment names is therefore part of following the mechanism. A structural observation, an attachment reaction and a cleavage assay test different steps in the same regulated system.
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