Assessment of the allosteric mechanism of aspartate transcarbamoylase based on the crystalline structure of the unregulated catalytic subunit

The lack of knowledge of the three-dimensional structure of the trimeric, c atalytic (C) subunit of aspartate transcarbamoylase (ATCase) has impeded un derstanding of the allosteric regulation of this enzyme and left unresolved the mechanism by which the active, unregulated C trimers are inactivated o n incorporation into the unliganded (taut or T state) holoenzyme. Surprisin gly, the isolated C trimer, based on the 1.9-Angstrom crystal structure rep orted here, resembles more closely the trimers in the T state enzyme than i n the holoenzgme:bisubstrate-analog complex, which has been considered as t he active, relaxed (R) state enzyme, Unlike the C trimer in either the T st ate or bisubstrate-analog-bound holoenzyme, the isolated C trimer larks 3-f old symmetry, and the active sites are partially disordered. The flexibilit y of the C trimer, contrasted to the highly constrained T state ATCase, sug gests that regulation of the holoenzyme involves modulating the potential f or conformational changes essential for catalysis, Large differences in str ucture between the active C trimer and the holoenzyme:bisubstrate-analog co mplex: call into question the view that this complex represents the activat ed R state of ATCase.