Next, the ligands that were in close proximity to the metal ions in the retrieved PDB structures were identified. For this purpose, a cutoff of 2.6 Å between the metal ion and the surrounding heteroatoms was chosen to allow some tolerance around the typical distance of heteroatoms coordinated to these ions (e.g. the distance between Mg and O is 2.1-2.2 Å and between Zn and N is 2.0-2.2 Å). In total, 948 (for Mg 2+ ), 689 (for Mn 2+ ) and 2,160 (for Zn 2+ ) unique ligands had heteroatoms within 2.6 Å of each metal ion (complete list in S2 of the Supporting Information). Predominantly, all three metal ions were coordinated by only one heteroatom from the ligand. Some of the ligands were found to coordinate to more than one of the metal ions in different structures. Overall, 73 ligands interacted with all investigated ions, while 104 interacted with both Mg 2+ and Mn 2+ , 39 with both Mn 2+ and Zn 2+ , and 58 with both Mg 2+ and Zn 2+ (Figure ). In total, the set contained 3,450 unique chelating ligands across all investigated metal ions. When inspecting the retrieved ligands, it became evident that many ligands contained a phosphate group. This was especially true for magnesium-chelating ligands where over half had a phosphate group or a derivative thereof. This trend was also reflected in the five most frequent ligands found for each metal ion (Figure ). All of them contained a phosphate group in the case of Mg 2+ , 4 in the case of Mn 2+ and none in the case of Zn 2+ . There was also some overlap among the five most frequent ligands with adenosine-5'diphosphate (ADP), adenosine-5'-triphosphate (ATP), and phosphoaminophosphonic acid-adenylate ester (ANP) being among the most frequent ligands for both Mg 2+ and Mn 2+ .