What are the minimum requirements for polymers in PDB entries?
Polypeptide systems with a chain length of 3 or greater are treated as polymers in PDB entries. Smaller systems are treated as independent ligands if they contain more than 10 atoms or as modifications if they have fewer than 10 atoms. Polynucleotide or polysaccharide systems with chain lengths of 2 or greater are treated as polymers. Each polymer chain is assigned a unique identifier (chain ID). For proteins and nucleic acids, SEQRES records for each chain are provided. Polysaccharides are not assigned SEQRES records or TER records. Covalently bound polymeric ligands are assigned chain IDs. Connections between polymeric groups are identified in LINK records. Example: Non-covalently bound polymeric ligands from PDB 1A1M: In this example, a MHC class I molecule is complexed (not covalently bound) with a peptide from the gag protein of HIV2. Since the peptide has more than 3 amino acids it is assigned its own chain ID (chain C) as well as SEQRES records.
