International
Tables for
Crystallography
Volume G
Definition and exchange of crystallographic data
Edited by S. R. Hall and B. McMahon

International Tables for Crystallography (2006). Vol. G, ch. 3.6, pp. 153-154

Section 3.6.6.1.3. Phasing via isomorphous replacement

P. M. D. Fitzgerald,a* J. D. Westbrook,b P. E. Bourne,c B. McMahon,d K. D. Watenpaughe and H. M. Bermanf

aMerck Research Laboratories, Rahway, New Jersey, USA,bProtein Data Bank, Research Collaboratory for Structural Bioinformatics, Rutgers, The State University of New Jersey, Department of Chemistry and Chemical Biology, 610 Taylor Road, Piscataway, New Jersey, USA,cResearch Collaboratory for Structural Bioinformatics, San Diego Supercomputer Center, University of California, San Diego, 9500 Gilman Drive, La Jolla, CA 92093-0537, USA,dInternational Union of Crystallography, 5 Abbey Square, Chester CH1 2HU, England,eretired; formerly Structural, Analytical and Medicinal Chemistry, Pharmacia Corporation, Kalamazoo, Michigan, USA, and fProtein Data Bank, Research Collaboratory for Structural Bioinformatics, Rutgers, The State University of New Jersey, Department of Chemistry and Chemical Biology, 610 Taylor Road, Piscataway, New Jersey, USA
Correspondence e-mail:  paula_fitzgerald@merck.com

3.6.6.1.3. Phasing via isomorphous replacement

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The data items in this category are as follows:

PHASING_ISOMORPHOUS [Scheme scheme42]

The bullet ([\bullet]) indicates a category key. The arrow ([\rightarrow]) is a reference to a parent data item.

Phases for many macromolecular structures are obtained from a previous determination of the same structure in the same crystal lattice. Examples of this are the determination of the structure of a point mutant or the determination of a structure in which a ligand is bound to an active site that was empty in the previous structure determination. In these cases, the new structure is essentially isomorphous with the parent structure, hence this method of phasing is termed `isomorphous phasing' in the mmCIF dictionary. It is not to be confused with multiple isomorphous phasing (MIR), a phasing technique that involves the use of heavy-atom derivatives. MIR phasing is discussed in Section 3.6.6.1.5[link].

Not much information is needed to characterize isomorphous phasing. The `parent' structure (the structure used to generate the initial phases for the present structure) is described in a free-text field and a second free-text field can be used to give details of the application of the method to the determination of the present structure (for instance, the removal of solvent or a bound ligand). In Example 3.6.6.3[link], the parent structure is the PDB entry 5HVP and the structure that is the subject of the present data block is identified as `HVP+CmpdA'. _phasing_isomorphous.method allows any formal techniques that were used in the application of the method to the present structure determination to be described, for example rigid-body refinement. Note that this data item is not to be used to reference a software package; this would be done using data items in the SOFTWARE category.

Example 3.6.6.3. Isomorphous replacement phasing of an HIV-1 protease structure described using data items in the PHASING_ISOMORPHOUS category.

[Scheme scheme43]








































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