Any residue of protein or nucleic acid that is not included in the list of Standard Residues is considered Non-Standard. The atomic coordinates for atoms in non-standard residues are given in records of type HETATM in the PDB file format.
- SEP and TPO in 1bkx
- 1MA, 2MG, 5MC, 5MU, 7MG, H2U, M2G, OMC, OMG, PSU, and YG in 1evv
- PSU and several others in tRNA
- MSE in 2ab5 (see Selenomethionine)
Although phosphoserine and phosphothreonine are given as non-standard residues SEP and TPO in 1bkx, an alternative convention may also be followed. For example, a non-standard phenylethane amino acid sidechain in 1b07 is named GLY (GLY 5 on chain C in ATOM records) plus PYL (sequence number 1005 in chain C in HETATM records).
The former scheme of designating modified standard nucleotides with plus signs (+A, +C, +G, +I, +T, +U) was discontinued in the PDB remediation project, effective August 1, 2007. The unremdiated files can still be obtained, see Getting Unremediated PDB Files.
At PDB.Org, using the Advanced Search and query type Chemical ID, you can find all entries in the database that contain a particular chemical component. For example, in March, 2010, five entries contain PYL (explained in Non-Standard Residues).
A complete list of all compounds in the PDB is available, including hydrogens, 3D structures, and bond orders, in the Chemical Components Dictionary of the Worldwide Protein Data Bank. This includes all Standard Residues as well as Non-Standard Residues, carbohydrate adducts, Ligands and Hetero Groups. It is updated weekly for newly released entries.