Large Differences Are Observed Between the Crystal and Solution Quaternary Structures of Allosteric Aspartate Transcarbamylase in the R State

Solution scattering curves evaluated from the crystal structures of the T and R states of the allosteric enzyme aspartate transcarbamylase from Escherichia coli were compared with the experimental x-ray scattering patterns. Whereas the scattering from the crystal structure of the T state agrees with the experiment, large deviations reflecting a significant difference between the quaternary structures in the crystal and in solution are observed for the R state. The experimental curve of the R state was fitted by rigid body movements of the subunits in the crystal R structure which displace the latter further away from the T structure along the reaction coordinates of the T-->R transition observed in the crystals. Taking the crystal R structure as a-reference, it was found that in solution the distance between the catalytic trimers along the threefold axis is 0.34 nm larger and the trimers are rotated by 11 degrees in opposite directions around the same axis; each of the three regulatory dimers is rotated by 9 degrees around the corresponding twofold axis and displaced by 0.14 nm away from the molecular center along this axis.