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 p reliminary structure determination was carried out. The unliganded wild-typ e enzyme crystallizes in the cubic space group I23, with unit-cell paramete rs a = b = c = 203.4 Angstrom. R277Q crystallizes in two crystal forms unde r the same crystallization conditions. One crystal form is isomorphous to t hat of unliganded wild-type crystals, with unit-cell parameters a = b = c = 202.2 Angstrom. The second form also belongs to a cubic space group, P4(3) 32, but has unit-cell parameters a = b = c = 139.8 Angstrom. R277W crystals are isomorphous to the second crystal form of R277Q, with unit-cell parame ters a = b = c = 138.7 Angstrom. 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 fi rst crystal form there are two monomers in the asymmetric unit, correspondi ng to a solvent content of 75%. Because of its high molecular and crystal s ymmetry and the presence of non-crystallographic symmetry, this structure c ould not be solved with AMoRe or X-PLOR, but was solved successfully with C OMO. There is only one monomer in the asymmetric unit in the second crystal form, corresponding to a solvent content of 62%. This structure was succes sfully solved with AMoRe.