Molecular Replacement Using Known Structural Information

When identical or similar structures exist in different crystallographic environments, similarities between their diffraction patterns, which are directly related to their Fourier transforms, would be expected The technique of Molecular Replacement in protein X-ray crystallography (1 –7 ) exploits this similarity to determine phases. The dominant application is the case of identical or similar proteins crystallizing nonisomorphously in different space groups, and where one of the structures is already known. The proteins may have been cocrystallized with a cofactor, inhibitor, or other protein. There may be more than one subunit in the crystallographic asymmetric unit. The similarity may be only partial so that a fragment is used. Examples include related structures, site-directed mutants, structures with ligands, and Fab-antigen complexes.