In total, 1,223 different metal-chelating groups were extracted from the retrieved ligands (S3 in Supporting Information). A large number of groups contained phosphorus, mainly in the form of phosphate derivatives, in particular the groups derived from Mg 2+ binding ligands. Magnesium is known to bind to phosphocompounds such as nucleotides and phospholipids, and the former were well represented among the top five ligands found for Mg 2+ and Mn 2+ containing proteins (Figure ). As the phosphate derivatives contained in these ligands interacted with different atoms with the metal ions in the different structures in the PDB, a large number of metal-chelating groups (521 out of 1,223) was generated from these ligands. The other groups mainly contained carboxyl, primary alcohol, primary amines, and thiol groups at the higher-level nodes in the hierarchical tree and got more complex when descending to lower level ones.