 Complement C3

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.^[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?^[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.^[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.