Hetero atoms

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In the PDB data file format for macromolecular models, each atom is designated either ATOM or HETATM (which stands for hetero atom). ATOM is reserved for atoms in standard residues of protein, DNA or RNA. HETATM is applied to non-standard residues of protein, DNA or RNA, as well as atoms in other kinds of groups, such as carbohydrates, substrates, ligands, solvent, and metal ions.


Standard Residue Ligands

Standard amino acids, nucleosides, nucleotides (or nucleobases) are designated HETATM when they are "ligand", that is, when they are isolated, free single monomeric residues, not part of a polypeptide/protein or polynucleotide chain. Examples are glycine in 6rs6, tryptophan in 6hip, and cytidine-5'-monophosphate "C" in 1kdo.


Standard amino acids in dipeptide ligands are also designated HETATM by convention of the PDB. An example is Ala-Pro in 2cyh. Dipeptide ligands are given the same chain ID as the chain to which they are bound. In contrast, tripeptides and longer polypeptides are designated ATOM and given unique chain IDs. An example is Tyr-Pro-Tyr in 1hqw.

Finding Models Containing a Residue

At RCSB.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 December, 2019, five entries contain PYL (explained in Non-Standard Residues).


See Also

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