Use of molecular replacement in the structure determination of the P21212 and the P21 (Pseudo P21212) crystal forms of oxidized uteroglobin

The structure of the symmetrical dimer of oxidized rabbit Uteroglobin, as determined from the crystal form in space group C222₁, has been used as a model to determine the general parameters of this protein in two other crystal forms; namely, a symmetrical dimer in P2₁2₁2 and an asymmetrical dimer in P2₁ with non-crystallographic symmetry approaching P2₁2₁2. Independently, the structure in P2₁2₁2 was solved by multiple isomorphous replacement.After exchanging data, the analysis was carried out in two different laboratories with different methods of molecular replacement. The result was the same for both approaches, and it could be shown further that the packing of molecules in both crystal forms analysed is so similar that they can be considered pseudoisomorphous, i.e. distinguished only by the fact that two out of three symmetry operators are crystallographically perfect in one case and molecular and approximate only in the other.The principal fold of the polypeptide chain is the same in all crystal forms considered so far, but there is evidence for differences in the detail, which will be worked out later with progressing refinement.