C3, as noted above, is homologous to C4: it is a major component of the alternative, as well as the classical pathway. C3 is synthesized as a single polypeptide chain, and processed on secretion into two disulfide-linked chains, a (110 kDa) and (3 (75 kDa). C3 is cleaved and activated by C4b2a (or by the alternative pathway C3 convertase, C3bBb). Cleavage releases the anaphylatoxic fragment C3a (77 amino acids) from the N-terminus of the a chain. The rest of the molecule, C3b, has like C4b (Figure 4), an exposed reactive thiolester, and will react with surface hydroxyl or amine groups on the complement activator, or with water. Covalent binding to an activator is called the 'acceptor binding reaction' of C3b to distinguish it from noncovalent binding to specific receptors. Again only about 10% of the 'nascent' C3b generated binds to the activator: the rest reacts with water and diffuses away. The half-life of the reactive thiolester is <100 ps, and so nascent C3b can diffuse only a short distance before it reacts with a nucleophile. The suface-bound C3b is therefore clustered around the site of proteolytic activation: such clusters are likely to be important in promoting adhesion to C3 receptors. Among the C3b molecules which become activated, one may become bound to the C4b portion of the activating enzyme. This forms a C4b2a3b complex. The covalently associated C4b and C3b molecules form a binding site for C5, which orients C5 for cleavage by the protease domain of C2a, thus initiating the lytic phase of the complement system. C4b2a3b is referred to as the C5 convertase of the classical pathway.

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