FIGURE 3.

Crystal structure of IsdH^N2-N3 bound to Hb. A, four IsdH^N2-N3 receptors bind to one Hb tetramer in the asymmetric unit of the crystal (PDB 4IJ2). Three of the IsdH^N2-N3 receptors are complete, comprising IsdH^N2, linker, and IsdH^N3 domains. The position of the IsdH^N3 domain in the fourth receptor molecule is not well defined (the expected location is marked by a dashed ellipse). B, stereo diagram showing the 2F_o − F_c electron density map for one full IsdH^N2-N3 receptor contoured at 1 σ. C, detail of the IsdH^N2-N3·α (blue/orange) and IsdH^N2-N3·β (teal/yellow) interaction complexes. Residues from the loop regions, which are not modeled in the structure, are shown with dashed lines. D, the IsdH^N3·α interface with F_o − F_c difference map shown at 3 σ (green), indicating electron density that is not accounted for by atoms of the IsdH^N2-N3·Hb model. The structure of heme-bound IsdH^N3 (PDB 2Z6F, red) is superimposed to indicate the expected binding position of the heme group (lines), which coincides with a peak of F_o − F_c difference electron density in the IsdH^N2-N3·Hb complex (green). Heme transfer from globin to the IsdH receptor requires a relatively small translation of only 12 Å between the E and F helices of the globin (dashed arrow).