Most metalloproteins retrieved from the PDB contained Zn 2+ , followed by Mg 2+ and Mn 2+ (Table ). This is a reflection of the distribution of these metal ions among enzymes. While both Zn 2+ and Mg 2+ mostly figure among the three most frequent metal ions in each EC class, Mn 2+ is much less common and usually not among the top 4. However, in many more proteins containing Zn 2+ or Mg , the small molecule ligand does not interact with the metal ion compared to proteins containing Mn 2+ , resulting in a rather even distribution of the number of ligands per metal ion in this study.