|Classical-complement-pathway C3/C5 convertase|
|PDB structures||RCSB PDB PDBe PDBsum|
C3 convertase (EC 184.108.40.206, Classical-complement-pathway C3/C5 convertase, C overbar 42 , C4b,2a, C5 convertase, C overbar 423, C4b,2a,3b, C42, C5 convertase, C423, C4b,2a,3b, complement C.hivin.4.hivin2, complement C3 convertase) belongs to family of serine proteases and is necessary in innate immunity as a part of complement system which eventuate in opsonisation of particles, release of inflammatory peptides, C5 convertase formation and cell lysis. C3 convertase exists in two forms (C3bBb and C4bC2a) but both of them cleave only C3, central molecule of complement system.
Complement system can be activated in three different pathways – classical, lectin and alternative pathway. All of these activation pathways lead to enzymatic cleavage of C3. Newly created fragments C3a and C3b are important in next steps of complement cascade development. The smaller fragment called C3a is released and stimulates inflammation through the chemoattractant activity. C3b, the larger fragment, becomes covalently attached to the microbial surface or to the antibody molecules through the thioester domain at the site of complement activation. After cleavage and binding to cell surface, the C3b fragment is ready to bind a plasma protein called Factor B. The Factor B (a zymogen) is cleaved by a plasma serine protease Factor D releasing a small fragment called Ba and generating a larger fragment called Bb that remains attached to C3b. Also Mg2+ ions are necessary for forming a functional C3 convertase. Thus, the alternative pathway C3 convertase is formed and is able to cleave C3 now.
Another form, the classical pathway C3 convertase contains different proteins of complement system – C4b and C2a. These fragments form during the classical or lectin pathway of the complement. The cleavage of C4 and C2 is mediated by serine proteases - C1 complex (C1q, C1r, C1s) in classical pathway and Mannose-binding lectin-associated serine proteases (MASPs: MASP1, MASP2, MASP3) in lectin pathway. C4 is homologous to C3, and C4b contains an internal thioester bond, similar to that in C3b, that forms covalent amide or ester linkages with the antigen-antibody complex or with the adjacent surface of a cell to which is antibody bound. C2 is cleaved by C1s to a smaller fragment called C2b and larger fragment called C2a that binds to C4b. The fragments C4a and C2b are released.
Once formed, both C3 convertases will catalyze the proteolytic cleavage of C3 into C3a and C3b (hence the name "C3-convertase"). C3b can then act as an opsonin or bind to activated bimolecular complex C4b2a to form a trimolecular complex, C5 convertase, which is a specific enzyme for C5.
Location on chromosome
The genes encoding C2, C3, C4 and factor B are located on chromosome 6 between the B locus of class I products and the D locus of class II products in the MHC.
Catalytic site of C3 convertase is in C2a and Bb subunits. Interesting is that catalytic subunits, once dissociated from their cofactors, never rebind to form an active enzyme.  The Bb subunit displays a two-domain structure.
- C3 convertases are unstable (half-life 10 – 20 min) – respectively they are deactivated upon spontaneous dissociation or by facilitated dissociation mediated by the regulators of complement activation proteins decay acceleration factor (DAF), complement receptor 1 (CR1), C4b-binding protein and Factor H. Convertase assembly is suppressed by the proteolytic cleavage of C3b (and C4b) as mediated by Factor I in the presence of membrane cofactor protein (MCP, CD46), C4b-binding protein, CR1, or a plasma-glycoprotein Factor H. These negative control processes are essential for the protection of self-tissue.
- Properdin (Factor P) is the only known positive regulator of complement activation that stabilizes the alternative pathway convertases (C3bBb). Properdin deficient individuals are sensitive to pyogenic infections. The properdin promotes the association of C3b with Factor B and inhibits the Factor H – mediated cleavage of C3b by Factor I.
C3 convertase cleaves C3 producing C3b which can form an additional C3 convertase. This positive-feedback effect is a unique feature of the alternative pathway of complement and results in the deposition of large numbers of C3b molecules on the surface of activating particles.  Nevertheless, this positive feedback mechanism can be regulated by binding of the control protein, nonproteolytic glycoprotein β1H, to C3b, which prevents uptake of factor B, facilitates decay-dissociation of Bb that is already bind to C3b and enhances proteolytic inactivation C3b by C3b inactivator (C3bINA - endopeptidase). Membrane-associated sialic acid promotes high-affinity binding of β1H to C3b without influencing the affinity of B for C3b.
Decay-accelerating factor (DAF) is another negative regulator of C3 convertase. It is a membrane protein and regulates also C5 convertase of the classical and alternative pathway. DAF protects host cells from damage by autologous complement. DAF acts on C2a and Bb and dissociates them rapidly from C4b and C3b – thereby preventing the assembly of the C3 convertase. 
C4 binding protein (C4-bp) interferes with the assembly of the membrane-bound C3 convertase of the classical pathway. C4-bp is a cofactor for the enzyme C3bINA. C4 binding protein inhibits the haemolytic function of cell-bound C4b. C4 binding protein and C3b inactivator control the C3 convertase of the classical pathway in a similar way to that described for β1H and C3b inactivator in the alternative pathway. 
C3b has different binding site for C3bINA, β1H, factor B and properdin. Binding β1H to C3b increases C3bINA binding, while factor B binding prevents C3bINA binding and is competitive with β1H binding.
Regulation of the amplification phase of the alternative pathway is exerted by multiple mechanisms:
- Intrinsic decay of C3 convertase
- Stabilization of C3 convertase by properdin
- Disassembly of this enzyme by serum glycoprotein β1H
- Inactivation of C3bby the C3b inactivator
- Protection of C3 convertase from the activation of these control proteins afforded by the surface properties of certain cells and other activators of the alternative pathway.
A convertase (of either type) with an additional 3b (C4b2a3b or C3bBbC3b) is known as "C5-convertase".
C3 convertase is, in classical terms, C4b2a; in the 1990s there was a motion put forward to change the nomenclature to C4b2b, however this was unsuccessful.
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