Figure 2: Three-dimensional structures of the catalytic domain models for thermolysin ((a): PDB 1KEI), peptidyl-Lys metallopeptidase ((b): PDB 1GE6), and carboxypeptidase A ((c): PDB 1YME). The zinc ion is shown as a green sphere, and amino acid side chains are shown as sticks colored red for oxygen, blue for nitrogen, and silver for carbon. Hydrogen bonds are indicated by dashed lines.