Small ligand information in the PDB
Every chemical compound in the PDB is stored in tables with its description, the PDB structures in which the compound is present and the amino acids pre-calculated to be in contact with the ligand.
Ligand searches are allowed using a three letter code as well as keywords that match chemical names or synonyms. All chemical information, including the three letter codes, comes from the mmCIF dictionaries
If you want to know how firestar considers the biological relevance of a specific compound, please check this page