3CSU: Catalytic Trimer Of Escherichia Coli Aspartate Transcarbamoylase

The lack of knowledge of the three-dimensional structure of the trimeric, catalytic (C) subunit of aspartate transcarbamoylase (ATCase) has impeded understanding of the allosteric regulation of this enzyme and left unresolved the mechanism by which the active, unregulated C trimers are inactivated on incorporation into the unliganded (taut or T state) holoenzyme. Surprisingly, the isolated C trimer, based on the 1.9-A crystal structure reported here, resembles more closely the trimers in the T state enzyme than in the holoenzyme:bisubstrate-analog complex, which has been considered as the active, relaxed (R) state enzyme. Unlike the C trimer in either the T state or bisubstrate-analog-bound holoenzyme, the isolated C trimer lacks 3-fold symmetry, and the active sites are partially disordered. The flexibility of the C trimer, contrasted to the highly constrained T state ATCase, suggests that regulation of the holoenzyme involves modulating the potential for conformational changes essential for catalysis. Large differences in structure between the active C trimer and the holoenzyme:bisubstrate-analog complex call into question the view that this complex represents the activated R state of ATCase.
PDB ID: 3CSUDownload
MMDB ID: 10359
PDB Deposition Date: 1999/4/22
Updated in MMDB: 2012/11
Experimental Method:
x-ray diffraction
Resolution: 1.88  Å
Source Organism:
Similar Structures:
Biological Unit for 3CSU: trimeric; determined by author and by software (PISA)
Molecular Components in 3CSU
Label Count Molecule
Proteins (3 molecules)
Protein (Aspartate Carbamoyltransferase)(Gene symbol: pyrB)
Molecule annotation
Chemicals (2 molecules)
* Click molecule labels to explore molecular sequence information.

Citing MMDB