The PDB assigns EC (Enzyme Commission) numbers to structures and chains in the archive. These assignments are based on UniProtKB/GenBank/KEGG/author specified mapping of the enzyme to EC numbers. Enzyme Classification is based on the recommendations of the Nomenclature Committee of the International Union of Biochemistry and Molecular Biology (IUBMB). It describes each enzyme for which an EC number has been provided (EC). 

Here you can browse an EC name, view the number of associated PDB structures, and search for the specific associated structures. In addition to searching for an enzyme in the tree by term, you can search for the enzyme by either partial or full EC number.