Part of the enzyme active site, with a high concentration of mutations leading to lethal phenotypes. Several aromatic residues surround the active site and potentially are involved in binding of the Tris ligand. Tris (white) is held in position by the catalytic nucleophile, D196 (blue) via a zinc ion (orange). R220 (green) is directly involved in ligand binding, along with H72 (purple) and Y380 (magenta). The aromatic side chains of F320 (red) and Y84 (yellow) interact to hold the structure together. These interactions are shown in dotted lines.