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 th e T and R states of the allosteric enzyme aspartate transcarbamylase f rom Escherichia coli were compared with the experimental x-ray scatter ing patterns. Whereas the scattering from the crystal structure of the T state agrees with the experiment, large deviations reflecting a sig nificant difference between the quaternary structures in the crystal a nd 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 obse rved in the crystals. Taking the crystal R structure as a reference, i t was found that in solution the distance between the catalytic trimer s along the threefold axis is 0.34 nn larger and the trimers are rotat ed by 11 degrees in opposite directions around the same axis; each of the three regulatory dimers is rotated by 9 degrees around the corresp onding twofold axis and displaced by 0.14 nm away from the molecular c enter along this axis. Proteins 27:110-117 (C) 1997 Wiley-Liss, Inc.