Crystallization and preliminary X-ray crystallographic studies of wild-type human ornithine transcarbamylase and two naturally occurring mutants at position 277

Wild-type human ornithine transcarbamylase (OTCase) and two mutants (R277Q and R277W) that cause `late-onset' hyperammonemia were crystallized and a preliminary structure determination was carried out. The unliganded wild-type enzyme crystallizes in the cubic space group I23, with unit-cell parameters a = b = c = 203.4 Å. R277Q crystallizes in two crystal forms under the same crystallization conditions. One crystal form is isomorphous to that of unliganded wild-type crystals, with unit-cell parameters a = b = c = 202.2 Å. The second form also belongs to a cubic space group, P4₃32, but has unit-cell parameters a = b = c = 139.8 Å. R277W crystals are isomorphous to the second crystal form of R277Q, with unit-cell parameters a = b = c = 138.7 Å. None of these crystal forms is isomorphous to other crystal forms of OTCase that have been studied. The structures in both crystal forms have been solved using molecular replacement. In the first crystal form there are two monomers in the asymmetric unit, corresponding to a solvent content of 75%. Because of its high molecular and crystal symmetry and the presence of non-crystallographic symmetry, this structure could not be solved with AMoRe or X-PLOR, but was solved successfully with COMO. There is only one monomer in the asymmetric unit in the second crystal form, corresponding to a solvent content of 62%. This structure was successfully solved with AMoRe.