Native human C4 is a naturally glycosylated (6.9%) polypeptide containing three disulfide-linked chains. C4 is central to the activation of both the classical and the lectin pathways of complement activation. Initiation of each pathway generates proteolytic enzyme complexes (C1q/C1r/C1s in the classical pathway and MBL/MASP1/MASP2 in the lectin pathway) which are bound to the target surface. These enzymes cleave a peptide bond in C4 releasing the anaphylatoxin C4a and activating C4b. Like C3, the thioester of metastable C4b is highly reactive and is capable of reacting with and covalently coupling C4b to amino or hydroxyl groups on the target surface. There are two variants of C4 that are common in man (C4A and C4B). Animals with only one type generally have C4B. The favored sites of attachment for variants C4A and C4B differ. Metastable C4b produced from C4A attaches primarily to amino groups while C4b produced from the C4B variant binds well to hydroxyl groups as well as amino groups (Law, S.K.A. and Dodds, A.W. (1997)). Surface-bound C4b forms the basis for formation of the C3/C5 convertase enzyme complex C4b,C2a. This enzyme activates C3, deposits C3b and thus converts itself from a weak C5 convertase to a highly efficient C5 convertase with a Km for C5 3000-fold lower than that of the C4b,C2a enzyme alone (Rawal N. and Pangburn M.K. (2003)). Surface-bound C4b is a weak opsonin and is recognized by receptors (CR1) on erythrocytes, lymphoid, and phagocytic cells. All of the complement activating functions of C4b are lost upon cleavage of the alpha chain generating C4c and C4d. The protease factor I cleaves C4b only when C4b is bound with one of the factor I cofactors: C4b binding protein (C4bBP), membrane cofactor protein (MCP) or complement receptor 1 (CR1).
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