Unknown amino acids and nucleic residues
From Proteopedia
Atomic coordinate file data entries in the Protein Data Bank (PDB) may include atoms of unknown amino acids, designated "UNK", or unknown nucleic residues, designated "N". Total such entries in March, 2025 are:
- UNK: 1,767 entries.
- N: 123 entries.
While one might expect unknown residues to be more common in entries released in the 20th century, in fact, they are far more common since the 21st century success of Electron cryomicroscopy. Cryo-EM is often used on samples with unknown components, and has a lower median resolution than does X-ray crystallography:
- 2.0 Å is the median resolution of the 192,742 X-ray crystallographic entries in the PDB in March, 2025, X of which have UNK, and 34 have N.
- 3.3 Å is the median resolution of the 25,538 Cryo-EM entries in the PDB in March, 2025 1,228 of which have UNK, and 89 have N.
|
Method |
Total Entries |
UNK Entries |
N Entries |
|
X-ray crystallography |
192,742 |
538 |
34 |
|
Cryo-EM |
25,538 |
1,228 |
89 |
Data in the above table are for March, 2025.
