FIGURE 4.

Coordination geometry of the DNPEP binuclear center. A, stereoview of the as-isolated ZnZn-DNPEP structure showing a four-coordinate distorted tetrahedral geometry for both zinc ions. The metal-metal distance is ∼3.3 Å. The electron density maps indicate that the bridging solvent is mobile and can move perpendicularly to the metal-metal axis to form hydrogen bonding interactions with Met⁴³⁵ and Glu²⁹⁷. B, stereoview of the ZnMg-DNPEP structure showing a distorted tetrahedral geometry for the site A zinc and an octahedral geometry for the magnesium at site B. The metal-metal distance is ∼3.4 Å. The position of the bridging solvent is substantially different compared with the ZnZn-DNPEP structure, and it is seen to form a hydrogen bonding interaction with the nearby Glu²⁹⁷ residue.