C3konvertasen

C3 konvertasen (C3 convertases) are a set of enzyme complexes in the complement system that catalyze the proteolytic cleavage of C3 into C3a and C3b, thereby initiating opsonization, inflammation, and amplification of the cascade. They act at a central step that links recognition of a target with downstream defense mechanisms.

There are two main types of C3 convertases corresponding to the complement pathways. In the classical and lectin pathways, the C3 convertase is the C4b2a complex, formed when C4b deposited on a surface binds the proteolytic fragment C2a. This complex cleaves C3 to generate C3a, an inflammatory mediator, and C3b, which serves as an opsonin and as a component of additional convertases. In the alternative pathway, the C3 convertase is C3bBb, formed when C3b binds Factor B and Factor D cleaves B to Bb; this complex is stabilized on surfaces by properdin and regulated by factors such as Factor H and factor I.

C3 convertases also participate in generating C5 convertases. When a C3 convertase associates with another C3b molecule, it becomes a C5 convertase (C4b2a3b in the classical/lectin pathway or C3b2Bb in the alternative pathway), which cleaves C5 into C5a and C5b, advancing to the terminal membrane attack complex.

Regulation of C3 convertases is tight and involves factors that promote decay of the complexes (such as DAF/CD55 and C4BP for classical/lectin, and Factor H for alternative) and proteolytic inactivation of C3b (Factor I). Dysregulation can lead to increased susceptibility to infection or inflammatory tissue damage, and components of the convertases are targets in therapeutic strategies.