IC3b Protein

IC3b (inactivated C3b) is derived from C3b. Conversion of C3b to iC3b destroys almost all of the functional binding sites present on C3b. C3b itself is produced by all three pathways of complement when native C3 is cleaved releasing C3a. iC3b is prepared by cleavage of C3b by factor I in the presence of factor H. Cleavage by factors H and I occurs rapidly when the C3b is free in solution and is slower when it is attached to a surface. Other cofactors for factor I also permit cleavage if C3b to iC3b and these include the two membrane proteins CR1 (CD35) and MCP (CD46) . Factor I can cleave C3b in two places in the alpha chain and if both sites are cleaved a small fragment (C3f, 2,000 Da) is released. Thus, iC3b may be a mixture of 176,000 and 174,000 molecular weight proteins. If the C3b precursor was attached to a surface, the iC3b remains on that surface.