C4b-binding protein

C4b-binding protein (C4BP) is a protein complex involved in the complement system where it acts as inhibitor. C4BP has an octopus-like structure with a central stalk and seven branching alpha-chains. The main form of C4BP in human blood is composed of 7 identical alpha-chains and one unique beta-chain, which in turn binds anticoagulant, vitamin K-dependent protein S.

C4BP is a large glycoprotein (500 kDa) with an estimated plasma concentration of 200 micrograms/mL synthesized mainly in the liver.

The genes coding for C4BP α-chain (C4BPA) and β-chain (C4BPB) are located in the regulators of complement activation (RCA) gene cluster on the long arm of chromosome 1 in the vicinity of other complement inhibitors.