FIGURE 9.

The copper centers of FetP are in differing conformations. Copper sites A (A and B) and B (C and D) are shown as ball and stick representations. The two conformations of the copper cation and its ligands at each site are shown side by side to highlight the changes in ligand identity and geometry in each conformation. The copper cation is shown as an orange sphere and is always bound to His⁴⁴, Met⁹⁰, and His⁹⁷ from the same subunit and His¹²⁷* from the other subunit. Similar to the copper cation, Met⁹⁰ is also observed in two conformations. In CuA1 (A), copper is additionally bound to Glu⁴⁶ (CuA1-O distance 2.7 Å) in a degenerated octahedral arrangement with five copper ligands. The three His-N atoms all lie on the same plane, and the Cu-N bonds form nearly right angles (97–105°), whereas the Glu⁴⁶-O-Cu-Met⁹⁰-S atoms almost form a straight line. In CuA2, the copper atom is in the center of a tetrahedral complex (B), similar to that of CuB2 (D).