Error: Pathway image not found.
Alternative Complement Pathway
Last Updated: 2019-09-22
The alternative complement pathway is one of the three complement pathways, the other two being classical and lectin. These pathways work innately to opsonize pathogens and kill them. The alternative pathway is activated with the complement C3 protein is cleaved spontaneously in the blood. The C3b component is then free to covalently bond to the surface of pathogens or apoptotic cells, acting as a tag for other parts of the immune system. Complement factor B is cleaved into factors Ba and Bb, and factor Bb can then bind to complement factor C3b on the surface of the pathogen along with a water molecule. This complex is known as fluid-phase C3 convertase, and it cleaves many more C3 proteins into C3a and C3b. Properdin is another compound that is important for complement activation, and it binds to the C3bBb complex, stabilizing it and forming the C3bBbP complex. This complex then can bind another C3b protein, and it then functions as a C5 convertase, splitting C5 into C5a and C5b. At this point, the remainder of the pathway is the same between the alternative and classical pathways. The complement C5b protein binds to and forms a complex with component C6, followed by C7, C8 and C9. Multiple molecules of C9 end up binding to this complex, and this is what forms the membrane attack complex pore that allows for uncontrolled diffusion of the cell’s contents, and if enough pores are formed, the cell will be killed.
Alternative Complement Pathway References
Fredslund F, Jenner L, Husted LB, Nyborg J, Andersen GR, Sottrup-Jensen L: The structure of bovine complement component 3 reveals the basis for thioester function. J Mol Biol. 2006 Aug 4;361(1):115-27. doi: 10.1016/j.jmb.2006.06.009. Epub 2006 Jun 21.Pubmed: 16831446
Huemer HP, Larcher C, van Drunen Littel-van den Hurk S, Babiuk LA: Species selective interaction of Alphaherpesvirinae with the "unspecific" immune system of the host. Arch Virol. 1993;130(3-4):353-64. doi: 10.1007/bf01309666.Pubmed: 8390825
Cai G, Satoh T, Hoshi H: Isolation from fetal bovine serum of a fragment b of complement factor B-like protein improving a long-term survival of human endothelial cells. Arch Biochem Biophys. 1997 Sep 1;345(1):150-5. doi: 10.1006/abbi.1997.0234.Pubmed: 9281322
This pathway was propagated using PathWhiz - Pon, A. et al. Pathways with PathWhiz (2015) Nucleic Acids Res. 43(Web Server issue): W552–W559.
Propagated from SMP0063815
Highlighted elements will appear in red.
Enter relative concentration values (without units). Elements will be highlighted in a color gradient where red = lowest concentration and green = highest concentration. For the best results, view the pathway in Black and White.
Visualize Compound Data
Visualize Protein Data