C5-convertase

C5 convertase is an enzyme complex that is involved in the complement of the immune system. Wherein the active subunit ( C2a ) is a serine protease which catalyzes the hydrolysis of C5 protein in C5a and C5b.

Design and function

There are two known forms of C5 convertase. A mold is built on the so-called classical pathway of complement components C4b to, C2a and C3b that the C4b2a3b complex - form - the C5 convertase. The component C4b therefore arises from the cleavage of the complement component C4 by the serine protease C1s. C4b exposed to a highly reactive thioester, which can easily form a covalent bond with nucleophiles. The complement component C2 binds to C4b. By C1s C2b component is cleaved from C2. It remains a C4b2a complex - the C3 convertase - left. This enzyme is able in his turn complement component C3 into C3a and C3b to split. Also C3b has - in analogy to C4b - a highly reactive thioester.

Via the alternative pathway C5 convertase is formed from two complement components C3b and Bb type component as C3bBb3b. The Bb component is obtained by cleavage of factor B by factor D into the components Ba and Bb. The component C3b is obtained by cleavage of C3 with the help of C3 convertase ( = C4b2a ). The fragments C3a and C3b arise.

C5 convertase cleaves the C5 complement component C5a in the two components (11 kDa) and C5b (190 kDa). While C5a is a multi-functional anaphylatoxin with chemotactic properties, which is detectable mainly in blood serum, C5b is on the cell membrane of a target cell is the starting point for the formation of the membrane attack complex.