Changes in the x-ray solution scattering of aspartate transcarbamylase following the allosteric transition.

Aspartate transcarbamylase (Escherichia coli) has been studied by X-ray solution scattering in the s range 0.002 to 0.06 Å^?1. The spectra display sharp maxima and minima whose positions and amplitudes show considerable changes upon ligation with the transition state analogue N-(phosphonacetyl)-l-aspartate. The magnitude of the change in diffraction pattern is so large that X-ray solution scattering should be a useful technique for studying the proportions of different quaternary forms in solutions of this enzyme. In particular, the kinetics of the allosteric transition appear to be within the reach of X-ray diffraction experiments.Some structural parameters of the allosteric transition were obtained from the diffraction patterns. The radius of gyration of the native enzyme is 45.9 ± 0.5 Å, and after ligation it increases to 48.4 ± 1.0 Å. At the same time, the peak of the pair distribution function is shifted from 58 Å to 63 Å. These changes indicate that the molecule swells after the allosteric transition to the R form. However, the maximum distance (from the pair distribution function) does not increase after ligation, and may even decrease slightly. Some probable subunit movements during allosteric activation are discussed.