C5Konvertasen

C5Konvertasen, also known as C5 convertases in the complement system, are proteolytic enzyme complexes that cleave the complement component C5 into C5a and C5b. They act as the trigger for the terminal pathway of the complement cascade, leading to inflammatory responses and the formation of the membrane attack complex (MAC).

There are two main forms of C5 convertase. In the classical and lectin pathways, the C5 convertase is C4b2a3b, formed when the C3 convertase C4b2a associates with an additional C3b molecule. In the alternative pathway, the C5 convertase is C3bBbP, formed when C3b binds to the C3 convertase C3bBb and is stabilized by properdin (P). These complexes enable the proteolytic cleavage of C5 to C5a and C5b.

C5a is a potent anaphylatoxin that promotes inflammation by recruiting and activating immune cells. C5b initiates the assembly of the MAC (C5b-9), which can create pores in target membranes and contribute to cell lysis. The activities of C5 convertases are tightly regulated by various proteins to prevent excessive inflammation and tissue damage. Regulators include factors such as CD55 and CD46 on host cells, and fluid-phase regulators like Factor H and Factor I, which modulate convertase formation and C5 cleavage.

Dysregulation or deficiencies in C5 convertase activity can contribute to disease. Overactivation of the terminal pathway is implicated in inflammatory conditions, while impaired C5 cleavage can increase susceptibility to certain bacterial infections, notably Neisseria meningitidis. Therapeutically, agents that inhibit C5 activation (for example, monoclonal antibodies against C5) can block downstream effects of C5 convertases in some diseases.