C5-convertase

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C5-convertase is an enzyme involved in the complement system. Its primary function is to cleave C5 protein to C5a and C5b. C5a is a smaller product and diffuses into the plasma, whereas C5b remains and initiates the formation of membrane attack complex (MAC).

There are two forms of C5-convertase, one derived from the classical pathway and the other from the alternate pathway. In both cases, the main difference between C3 and C5-convertase is the presence of C3b. Thus, the form derived from the classical complement pathway consists of surface bound C4b, C2a, and C3b forming the active C4b2a3b complex, which is the C5 covertase, while the derivation of the alternative complement pathway consists of two C3b and one Bb and referred to as C3bBb3b (C5 convertase).

C5 is then converted by either of these into C5a and C5b. C5b complexes with C6 and C7 to settle on the cell surface, serving to recruit C8 to insert into the cell membrane and trigger the binding and polymerization of C9, thus forming the membrane attack complex, which can create a pore in cell membranes in order to kill pathogens.