C5konvertasit

C5konvertasit (C5 convertase) is a multi‑protein enzyme complex of the complement system that catalyzes the proteolytic cleavage of complement protein C5 into C5a and C5b. It represents a pivotal step linking complement activation to generation of the potent anaphylatoxin C5a and to assembly of the membrane attack complex (MAC) via C5b.

C5 convertases form on target surfaces after activation of earlier complement components. Two major types are recognized: the classical/lectin pathway C5 convertase, formed from C4b2a with additional C3b (often designated C4b2a3b), and the alternative pathway C5 convertase, formed from C3bBb with an additional C3b (C3bBbC3b). Their activity depends on surface-bound C3b molecules that allow binding and positioning of C5 for cleavage by the convertase complex.

Functionally, C5a is a strong chemoattractant and activator of neutrophils and other immune cells, while C5b initiates MAC formation that can lyse target cells. Regulation of C5 convertase formation and stability is achieved by complement regulators such as factor H, factor I, and membrane proteins (CD46, CD55), limiting host tissue damage.

Dysregulation of C5 cleavage contributes to inflammatory and hemolytic disorders; therapeutic inhibitors of C5 (for example eculizumab and related agents) have been developed to block C5 activation in specific clinical settings. C5 convertase remains a focus of research into complement modulation and immune pathology.