Temperature value

From Proteopedia

Revision as of 20:50, 12 December 2009

Definition

In crystallography, uncertainty in the positions of atoms increases with disorder in the protein crystal. Disorder may have two components, static and dynamic. First, some regions of the molecule may adopt different conformations in different copies of the molecule, each molecule's conformation being stable (static disorder). Second, some regions of every copy of the molecule may be subject to thermal motion, meaning vibration about the rest position^[1]. Large thermal motions cease when the crystal is frozen very rapidly with liquid nitrogen, which is the usual procedure prior to irradiation. Typically, protein crystals are kept well below freezing while being irradiated, although irradiation may warm the crystal permitting some thermal motion. Disorder in a frozen protein crystal may include both static disorder and dynamic disorder, the latter being represented by the "snapshot" of thermal disorder frozen in a moment in time.

Some regions of the molecule may have higher average disorder, and others lower average disorder. Typically, the ends of chains have higher average disorder, and hence their positions are less certain than are residues in the core of a tightly packed domain, where disorder is less. The disorder for each atom is quantitated in its temperature value.

Coloring by Temperature

spinqualitylabelsall models Colored by Temperature Show:Asymmetric Unit Biological Assembly Drag the structure with the mouse to rotate   Export Animated Image

Visualizing relative disorder or uncertainty in atomic positions is done by coloring by temperature value. Atoms with low temperature values are colored blue, while atoms with high temperature values are colored red. Light blue, white, and pink atoms represent the scale of intermediate temperature values. The temperature values themselves are relative, not absolute, and hence the colors are also relative. The most important information from coloring by temperature is to identify the red residues whose positions are least certain. Their positions should be taken only as rough approximations, and this is especially worth knowing if any residues in areas of special interest are red.

While the temperature colors of two separate models cannot be rigorously compared (since the temperature values themselves are relative, not absolute), high resolution models tend to have fewer red atoms than do models with modest resolution. For example, a (1kzk) has , while the (1uwb) has . Also, many of the surface sidechains are missing (due to disorder) in 1uwb. Missing sidechains are labeled S- in FirstGlance in Jmol (linked under the molecule on all PDB code-titled pages in Proteopedia).

Missing Residues

Often the very ends of chains, or surface loops, may be so disordered as to prevent assigning any atomic positions at all, leading to missing residues. FirstGlance in Jmol (linked beneath the molecule on every PDB code-titled page in Proteopedia) has a Gaps button that explains how to detect and visualize missing residues. Alternatively, at the PDB, the Sequence tab provides a graphic representation of the sequence that indicates gaps in two ways. First, the thin black line underneath the sequence is broken; second, touching a residue above breaks in the line reports "no identifier from ATOM record (no structural data available)". However, it is easy to overlook breaks in the line, while the method in FirstGlance in Jmol, which relies upon the SEQRES to Coordinates alignment server of Wang and Dunbrack, offers a reliable way of finding every gap.

Data Format

In the PDB file format, each atom is given not only X, Y, and Z Cartesian coordinates, but two additional values immediately following called occupancy and temperature value (also known as the isotropic B value). If the end of a chain adopts either of two stable positions with equal probability, each position has 50% occupancy. The temperature factor is provided to quantitate the level of thermal motion. However, these two components of disorder cannot be distinguished with crystal diffraction data alone. Therefore, the occupancy is often given as 1.0 (100%), while the degree of "blur" in the electron density map, representing both components of disorder, is reported in the temperature value field. These values are mapped to colors when a crystallographic result is colored by temperature.

Uncertainty in NMR Models

NMR models provide no information in the temperature value fields of PDB files. Rather, the variation between the models gives some indication of uncertainty or flexibility.

See Also

• Resolution
• Quality assessment for molecular models
• NMR Ensembles of Models

Content Donors

Large portions of this page were adapted from the Glossary of ProteinExplorer.Org, with the permission of the principal author, Eric Martz.

References Cited

1. ↑ Rhodes, G. 2006. Crystallography Made Crystal Clear, 3rd ed. Academic Press.