To assess how well different screening libraries are suited for hit discovery for metalloenzymes containing either Mg 2+ , Mn 2+ , or Zn 2+ , substructure searches with the identified metal-chelating groups were conducted. For this purpose, we used the Life Chemicals chelator focused library (6,428 compounds) and the Otava chelator fragment library (1,784 fragments) (Table ). In these libraries, less than 21% of the extracted metal-chelating groups were found, and none of them contained phosphorus. When considering only groups without a phosphorus atom, 35% were found in the Life Chemicals library and 27% in the Otava library. In total, 255 chelating groups were matched in Life Chemicals compounds, while 196 groups were found in Otava fragments (full list in S6 of the Supporting Information). Out of those, 169 metal-chelating groups were found in both libraries, 86 groups only in the Life Chemical library, and 27 only in the Otava library. The most frequent metal-chelating group was a carbonyl group which was found in over 70% of the compounds in both libraries (Table ). Most of the extracted chelating groups were found in less than 1% of t he library compounds. Next, we investigated how well the metal-containing groups were represented in large chemical spaces. For this purpose, we used WuXi AppTec's GalaXi space (12 billion virtual products, 2023-03) and Enamine Ltd 's REAL space (34 billion virtual products, 2022-11, Table ). The substructure searches in these large spaces were carried out using SpaceMACS. To save computational time, the maximum number of returned hits was limited to 100,000 virtual products. In total, 766 metal-chelating groups were found in the virtual products of the REAL space and 282 of the GalaXi space (full list in S7 of the Supporting Information). All but t wo of the groups that were found in the GalaXi space were also contained in the REAL space. The maximum number of hits (100,000 virtual products) was returned for 21% of the all metal-chelating groups when searching REAL space and 17% when searching the GalaXi space (Table ). Almost half of the phosphorus containing chelating groups (229 groups) were found in REAL space. However, 19 2 of these groups were not well represented with less than 10 virtual products as a match and only 5 groups had over 10,000 matches. None of the phosphorus containing chelating groups were found in GalaXi space. When considering the metal-chelating groups without a phosphorus atom only, 73% were found in the REAL space and 39% in the GalaXi space. While in REAL space different numbers of hits were returned for different groups, in GalaXi space mostly at least 10,000 matches were returned or none.