C9 Human

C9 binds to the C5b-8 complex and formson cell membranes the mature membrane attack complex. Each pathway of complement activation generates proteolytic enzyme complexes which binds the target surface. These enzymes cleave a peptide bond in the larger alpha chain of C5 releasing the anaphylatoxin C5a and activating C5b. This is the only proteolytic step in the assembly of the C5b-9 complex. Although C5b is unstable it remains bound to the activating complex for a few minutes during which it binds a single C6 from the surrounding fluid or it decays and is no longer capable of forming MAC.The C5b,6 complex may also remain connected to the C3/C5 convertase where the binding of a single C7 exposes a membrane-binding region and C5b,6,7 can enter into the bilipid layer of the target cell. Each C5b-7 complex can bind 1 molecule of C8 causing the complex to enter more firmly into the membrane. The C5b-8 complex can cause lysis without C9,though it will take longer andwill require many more complexes per cell than with C9. The main role of C8 is to catalyze the binding of C9 and each can bind another C9 initiating formation of a ring structure containing up to 18 C9molecules.C5b-9 complexes with 1 or more C9 are called the Membrane Attack Complex of complement.