Crystal structure of the Glu-239 → Gln mutant of aspartate carbamoyltransferase at 3.1-Å resolution: An intermediate quaternary structure

The structure of the unligated Glu 239 → Gln mutant of Escherichia coli aspartate carbamoyltransferase (EC 2.1.3.2) has been determined to 3.1-Å resolution and refined to a crystallographic residual of 0.22 in the space group P321. The unit-cell dimensions of the unligated enzyme are a = 122.3 Å, c = 147.1 Å. The c axis cell length is intermediate between the c axis lengths of the T (tense) (c = 142.2 Å) and R (relaxed) (c = 156.2 Å) state structures. Furthermore, the quaternary structure of the mutant enzyme is intermediate between the quaternary structures of the T form and the R form. The differences between the quaternary structures of the Glu-239 → Gln and T-form enzymes can be described as follows: the separation between the catalytic trimers increases by ≃ 1.5 Å along the threefold axis, and they each rotate in opposite directions ≃ 0.5° around the threefold axis, whereas the regulatory dimers rotate ≃ 2° around the twofold axes.