Venom complement C3 homolog family

General Activity
In mammals, complement component C3 is the most important protein of the complement system as it is activated by all three pathways (classical, lectin, and alternative pathways), and its various activation products are crucial for mediating, directly or indirectly, virtually all biological functions of complement.

In venom, complement C3 homolog is a structural and functional analog of complement component C3b, the activated form of C3. The best described protein is the cobra venom factor (CVF) . It binds factor B (CFB) , which is subsequently cleaved by factor D (CFD) to form the bimolecular complex CVF/Bb. This complex is a C3/C5 convertase that cleaves both complement components C3 and C5. Unlike endogenous C3b/Bb, CVF/Bb is a stable complex which is completely resistant to the actions of complement inhibitory factors H (CFH) and I (CFI) . Therefore, CVF continuously activates complement, resulting in the depletion of complement activity.

Venom complement C3 homologs are heterotrimers composed of alpha, beta and gamma chains that are processed from a single precursor.