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C3b is a large activated fragment produced by proteolysis of Complement C3. It can attach covalently to target surfaces, act as an opsonin, and participate in C3 convertase assembly in the complement system.^[ref]

Processing changes its functions. Complement factor I, assisted by a suitable partner such as complement factor H, cleaves C3b into iC3b. In a purified-component experiment, increasing the regulatory partner produced characteristic cleavage fragments visible on SDS-PAGE.^[ref]

Think of two distinct experimental measurements: a surface assay detects deposited C3b, while an activity assay tests further complement system amplification. Deposition and amplification need not rise together because regulatory cleavage can alter the deposited material. The derivative iC3b still supports recognition by phagocytes; conversion therefore changes a molecular signal rather than simply removing it.^[ref]

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.^[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.^[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.

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.

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]

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]

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.

Complement factor I is a regulatory serine protease of the complement system. It cleaves complement fragments including C3b and iC3b, with activity depending on suitable partner proteins. Its official enzyme classification is EC 3.4.21.45.^[ref]

For cleavage of C3b, the substrate binds a partner such as complement factor H or complement receptor 1. Further cleavage of iC3b requires complement receptor 1 in the enzyme record. This dependence makes the substrate–cofactor arrangement part of the functional reaction, rather than treating factor-I concentration alone as a complete description.^[ref]

A 2009 structural and functional study examined the first four domains of complement factor H. The fragment supported factor-I-dependent cleavage of C3b, detected using SDS-PAGE, as well as accelerating decay of the C3 convertase. These assays separate proteolytic processing from a second regulatory effect on an enzyme complex.^[ref]

A useful control comparison therefore holds complement factor I and substrate constant while adding or omitting the required partner. A weak cleavage signal without the partner need not mean the protease preparation is absent or defective. The experiment tests the assembled reaction system.

iC3b is a processed fragment of C3b formed through cleavage by complement factor I with a suitable partner such as complement factor H. It functions as an opsonin: deposition on a target supports recognition and uptake by phagocytes, including through complement receptor 3. ^[ref]

Processing does not simply erase every biological effect of the original protein. It changes the interactions available to the resulting fragment. In a 1983 experiment, cultured monocytes on appropriately coated surfaces showed complement-associated phagocytosis without the appreciable hydrogen peroxide release seen with the IgG-coated comparison.^[ref]

The comparison separates target uptake from an oxidative response. Measuring either alone would miss that separation; receptor engagement, ingestion and production of reactive substances are distinct experimental readouts. The older paper writes the fragment as C3bi; iC3b is the corresponding modern notation.

Jin Wu is the first author of a 2009 structural study of C3b bound to the first four domains of complement factor H. The paper's contribution statement credits Jin Wu with purifying Complement C3 and C3b, crystallizing the complex, collecting data and determining and analysing the protein structure.^[ref]

The study combined that structure with team experiments testing C3 convertase decay and complement factor I-dependent cleavage. These connected a binding arrangement to two distinct regulatory effects in the complement system.^[ref]

The division of measurements matters. A structure shows how partners fit together; an activity assay asks what changes when they interact. Agreement between these forms of evidence supports a mechanism more strongly than a binding image alone.