FIGURE 13.

Structures of C3b and C3u in 50 mm and 137 mm NaCl buffers. A and B, best fit solution structures for C3u and C3b in 50 mm NaCl show that the TED (crimson) and MG1 (blue) domains are close to each other. C and D, best fit solution structures for C3u and C3b in 137 mm NaCl show that the TED (crimson) and MG1 (blue) domains have separated in this buffer. E, the C3b structure crystallized in 50 mm NaCl is similar to those in A and B. F, salt bridge interaction at Arg¹⁰² (MG1; blue) and Glu¹⁰³² (TED; crimson) is shown in green. The thioester (Cys¹⁰¹⁰ and Glu¹⁰¹³) is shown in orange. G and H, crystal structures for C5b in its complex with C6 (not shown) and the four superimposed monomers of active α₂-macroglobulin. In these structures, the TED (crimson) and MG1 (blue) domains are also separated.