Figure 6.

The Zn²⁺-binding site. (A) Electron density at the Zn²⁺-binding site (gray mesh representing 2m |Fo| − D|Fc| map at 1 σ). Cross-eyes stereo image showing the Zn²⁺ ion coordination (dark gray sphere with blue dashed lines) with the His₂₇₇ side chain, one acetate ion (ACT), one chloride ion (green sphere), and two waters (red spheres) that are also at hydrogen-bonding distances (cyan dashed lines) from Nε2 of the His₁₆₆ side chain. The other acetate oxygen is at hydrogen-bonding distance (cyan dashed line) from one of the Zn²⁺ coordinating waters; light green dashes, non-bonding distances; sticks representation: carbon in gray, nitrogen in blue, oxygen in red. (B) Surface representation of two neighboring monomers (light blue and yellow) highlighting the positions of the Zn and Fe sites with their respective ligands and channels and the bridging cysteine persulfide. The zinc site is located at the end of a tunnel leading to the active site Css₃₁ of the neighboring monomer.