Alternate locations

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Some PDB files have alternate locations (altlocs) for some atoms. Each atom that has altlocs is marked with an altloc identifier, typically A and B. At sufficiently high resolution, two positions may be visible in the electron density map for certain atoms. If these have approximately equal densities, each alternate location will be assigned an occupancy of 50%. Sometimes 3 or even more alternate locations may be distinguished (see examples in the following section).


Visualizing alternate locations

FirstGlance in Jmol tells you how many atoms have alternate locations (when you are in more details mode), and the range of occupancies assigned to them. It tabulates summaries of alternate locations and occupancies, with links that highlight subgroups. It can animate alternate locations. Examples (from FirstGlance in Jmol):

The links below display the model in FirstGlance. Right click the link to open it in a new browser tab. Look in the Molecule Information Tab for alternate locations.

  • Glu3 in 3HYD. The atom with the largest distance between its alternate locations is the sidechain epsilon oxygen 1 (OE1), with 1.66 Å between alternate locations. Here are its ATOM records from the PDB file. The vertical bar points to the column with the altloc A and B designators. Each location is assigned 50% occupancy ("Occ" column).
             |            X       Y      Z       Occ   Temp
ATOM  49  OE1AGLU A   3   9.326   0.609  10.555  0.50  4.93 
ATOM  50  OE1BGLU A   3   8.211   1.826  10.769  0.50  8.58
  • 9ins is insulin. Eight amino acid sidechains on the surface have two alternate rotamer locations each. Occupancies range from 40-60%.
  • 1BSZ: Arg155 in chain A has 3 sidechain positions, each given an occupancy of 33%.
  • 1ALX has one methanol in 5 alternate locations, each with 20% occupancy; another methanol in 4 alternate locations, each with 25% occupancy; as well as other moieties with 3, 2, and 1 locations/atom. Surprisingly, the two alternate locations of residue 11 in chain A are different amino acids: Tyr and Trp.
  • 1izh is an unusual example. Its ligand is modeled in two completely different, overlapping orientations, indicated as alternate locations A and B.

Models with the most alternate locations

The OCA Database and Browser highlights the structure with the most 'altlocs'. See Believe It or Not! within Proteopedia for that Protein Data Bank entry and other information mined from OCA.

PDB Format

The altloc identifier occupies the 17th column in ATOM/HETATM records: PDB format. See the example above.

CIF Format

The PDB to PDBx/mmCIF Data Item Correspondences shows that altloc information is in _atom_site.label_alt_id records in mmCIF files.

See Also

Proteopedia Page Contributors and Editors (what is this?)

Eric Martz, Wayne Decatur

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