C5b (source code)
= C5b
{c}
<C5b> is the larger <protein> fragment produced by cleavage of <complement C5>. It initiates assembly of the terminal <complement system> components that form the <membrane attack complex>. Its companion cleavage product <C5a> has a different signaling role. The unprocessed precursor, <C5a> and <C5b> should therefore be treated as distinct molecular species.<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: C5 cleavage and terminal-complement assembly]{ref}
The initial assembly recruits <complement C6> and further components. <C5b> supplies an initiating platform rather than constituting a complete membrane pore by itself. In a 2012 structural investigation, the measured full-length <complement C6> structure supported a proposed mechanism for sequential assembly around that platform. The paper's model and its directly determined <crystal structure> are different kinds of evidence.<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; Discussion and Figure6: proposed initiation pathway]{ref}
For original molecular accounting, cleavage of ten intact <complement C5> molecules can yield ten <C5b> fragments and ten <C5a> fragments if every cleavage is successful and the products are retained. That does not create twenty <complement C5> molecules: the fragments divide the precursor's material. An assay recognizing a sequence shared by the precursor and <C5b> would need an additional way to distinguish processing states.
Likewise, detecting ten <C5b> fragments does not demonstrate ten finished <membrane attack complexes>. Recruitment of partners and productive assembly remain necessary. Measuring the amount of a fragment, formation of a complex and a functional endpoint such as <hemolysis> addresses three related but distinct questions.
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