Complement C8 is a three-subunit protein complex participating in the terminal complement system. Its α, β and γ chains are distinct components, unlike three copies of one identical chain. It joins the C5b-containing assembly after complement C7 and before recruitment of complement C9.[ref]
Structure of complement C6 suggests a mechanism for initiation and unidirectional, sequential assembly of membrane attack complex (MAC) by Alexander E. Aleshin and others. Introduction: C8αβγ heterotrimer and sequential assembly
Structural analysis of the assembled membrane attack complex identified the α and β chains as two different parts of the pore architecture. The γ chain supplied a separate structural feature used to orient the complex in the reconstruction. The resulting arrangement helped explain the interface for the first complement C9 molecule.[ref]
Structural basis of complement membrane attack complex formation by Marina Serna and others. Results: Molecular architecture of the MAC; MAC is an asymmetric pore
An original inventory example distinguishes subunits from complexes. A preparation with seven competent α chains, five β chains and nine γ chains can supply at most five complete αβγ units if each requires one of each and assembly is otherwise unrestricted. Adding all21 chains and dividing by three would give seven, incorrectly ignoring which subunit is limiting. Real assembly also depends on molecular competence and binding, so even five is only a count-based upper bound.
This distinction matters when interpreting a protein assay. An antibody recognizing one chain may detect that chain in incomplete or altered material as well as in a functional complement C8 complex. Establishing abundance of one subunit, correct assembly and membrane-damaging function requires different evidence.
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