Soluble C3-convertase, also known as C4b2a, catalyzes the proteolytic cleavage of C3 into C3a and C3b as part of the classical complement system as well as the mannan-binding lectin pathway. C3a is an anaphylotoxin, and C3b serves as an opsonizing agent. Factor I can cleave C3b into C3c and C3d, the latter of which plays a role in enhancing B cell responses. In the alternative complement pathway, C3 is cleaved by iC3Bb, another form of C3-convertase.
Structure
Several crystallographic structures of C3 have been determined and reveal that this protein contains 13 domains.[6][7][8][9]
Clinical use
Levels of C3 in the blood may be measured to support or refute a particular medical diagnosis. For example, low C3 levels are associated with some types of kidney disease such as post-infectious glomerulonephritis and shunt nephritis.
^ Sahu A, Lambris JD (April 2001). "Structure and biology of complement protein C3, a connecting link between innate and acquired immunity". Immunol. Rev.180: 35–48. doi:10.1034/j.1600-065X.2001.1800103.x. PMID 11414361.
^ Janssen BJ, Huizinga EG, Raaijmakers HC, Roos A, Daha MR, Nilsson-Ekdahl K, Nilsson B, Gros P (September 2005). "Structures of complement component C3 provide insights into the function and evolution of immunity". Nature437 (7058): 505–11. doi:10.1038/nature04005. PMID 16177781.
^ Janssen BJ, Christodoulidou A, McCarthy A, Lambris JD, Gros P (November 2006). "Structure of C3b reveals conformational changes that underlie complement activity". Nature444 (7116): 213–6. doi:10.1038/nature05172. PMID 17051160.
^ Wiesmann C, Katschke KJ, Yin J, Helmy KY, Steffek M, Fairbrother WJ, McCallum SA, Embuscado L, DeForge L, Hass PE, van Lookeren Campagne M (November 2006). "Structure of C3b in complex with CRIg gives insights into regulation of complement activation". Nature444 (7116): 217–20. doi:10.1038/nature05263. PMID 17051150.
^ Fredslund F, Jenner L, Husted LB, Nyborg J, Andersen GR, Sottrup-Jensen L (August 2006). "The structure of bovine complement component 3 reveals the basis for thioester function". J. Mol. Biol.361 (1): 115–27. doi:10.1016/j.jmb.2006.06.009. PMID 16831446.
