= C3 convertase
{c}
A <C3 convertase> is an enzymatic complex of the <complement system> that cleaves <Complement C3> into <C3a> and <C3b>. Different activation routes assemble different C3-cleaving complexes. The important shared activity is selective substrate cleavage, rather than a universal single protein carrying the name.<Enzyme Nomenclature by the IUBMB>[EC 3.4.21.43, classical-pathway C3 cleavage reaction]{ref}
Assembly and lifetime matter because a catalytic complex can process many substrate molecules. In an idealized constant-rate example, $1\,\mathrm{nM}$ active complex turning over at $2\,\mathrm{s^{-1}}$ produces $20\,\mathrm{nM}$ cleavage products in ten seconds, provided substrate remains abundant. Each consumed C3 molecule supplies one C3a fragment and one C3b fragment; the enzyme is not consumed on every turnover.
A 2009 study of <complement factor H> combined structural and functional measurements. The tested factor-H fragment accelerated decay of preassembled C3 convertase and supported <complement factor I>-dependent C3b cleavage in a separate assay.<Structure of complement fragment C3b-factor H and implications for host protection by complement regulators by Jin Wu and others>[Figure 1d–e, convertase decay and cofactor assays]{ref}
These observations separate dismantling an active assembly from cutting its protein components. Both can regulate complement activity, but they are not the same measured reaction. Comparing complex lifetime, substrate cleavage and component abundance therefore gives more mechanistic information than measuring total complement protein alone.
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