Human C4 is a glycosylated protein originating from the human leukocyte antigen system. C4 is homologous to complement C3 and C5, sharing ~30% homology with them. Mature C4 is a trimer protein complex composed of three polypeptide chains: α chain, β chain, and γ chain, which provides a surface for interaction between the antigen-antibody complex and other complement components. In complement cascade, once the complement classical pathway or the lectin pathway were activated, the related activated proteases C1s or MASP-2 immediately cleave complement component 4 (C4) into two fragments C4a and C4b, simultaneously, cleave complement component 2 (C2) into two fragments C2a and C2b.
C4, a key member of the complement system, is cleaved into C4a and C4b upon catalyzed by C1s or MASP-2. C4b fragment is highly reactive and participates in the formation of the C3 convertases (C4bC2b) and C5 convertases (C4bC2bC3b) in the classical pathway and lectin pathway. C4a is an anaphylatoxin that mediates local inflammatory reactions. There are two common isoforms found in humans, C4A and C4B, whereas, only C4B in animals. The expression C4 directly determines the activation of the complement system. It has been reported that C4 abnormity is associated with a series of human diseases, such as complement dysfunction, immunodeficiency, systemic lupus erythematosus, and schizophrenia.
Fig.1 Role of C4 in the complement system and the structure of C4. (Kidmose, 2012)
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Reference
1. Kidmose, R.T.; et al. Structural basis for activation of the complement system by component C4 cleavage. PNAS. 2012, 109(38): 15425-15430.
Fig.1 Role of C4 in the complement system and the structure of C4. (Kidmose, 2012)