6at1

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spinqualitylabelsall models 6at1, resolution 2.6Å Show:Asymmetric Unit Biological Assembly Drag the structure with the mouse to rotate   Export Animated Image

STRUCTURAL CONSEQUENCES OF EFFECTOR BINDING TO THE T STATE OF ASPARTATE CARBAMOYLTRANSFERASE. CRYSTAL STRUCTURES OF THE UNLIGATED AND ATP-, AND CTP-COMPLEXED ENZYMES AT 2.6-ANGSTROMS RESOLUTION

Overview

The crystal structure of Escherichia coli aspartate carbamoyltransferase, complexed with adenosine 5'-triphosphate (ATP) has been solved by, molecular replacement and has been refined to a crystallographic residual, of 0.17 at 2.6-A resolution by using the computer program X-PLOR. The unit, cell dimensions of this crystal form are a = b = 122.2 A and c = 143.3 A, and the space group is P321. Although the c-axis unit cell dimension is, approximately 1 A longer than the corresponding dimension of the, CTP-ligated P321 crystal form (c = 142.2 A), the ATP-ligated enzyme adopts, a T-like quaternary structure. The base moiety of ATP interacts with, residues Glu10, Ile12, and Lys60 while the ribose is near Asp19 and Lys60;, the triphosphate entity is bound to Lys94, although His20 and Arg96 are, nearby. We observe a higher occupancy for ATP in the allosteric site of, the R1 regulatory chain in comparison to the occupancy of the R6, allosteric site. These crystallographically independent sites are related, by a molecular 2-fold axis. There are other violations of the, noncrystallographic symmetry that are similar to those observed in the, refined CTP-ligated aspartate carbamoyltransferase structure. These, infringements on the molecular symmetry might be the result of, intermolecular interactions in the crystal. To ensure the most meaningful, comparison with the ATP-ligated structure, we refined the previously, reported CTP-bound and unligated structures to crystallographic residuals, between 0.17 and 0.18 using X-PLOR. These X-PLOR refined structures are, not significantly different from the initial structures that had been, crystallographically refined by a restrained least-squares method. After, making all possible comparisons between the CTP- and ATP-ligated and the, unligated T-state structures, we find that the most significant, differences are located at the allosteric sites and in small changes in, the quaternary structures. At the allosteric site, the binding of CTP and, ATP successively enlarges the nucleotide binding cavity, particularly in, the vicinity of the base. The changes in the quaternary structure can be, characterized by an increase in the separation of the catalytic trimers by, approximately 0.5 A as ATP binds to the unligated T structure. On the, basis of these structural studies, we discuss the relationships between, the conformational differences in the allosteric site and the small, changes in the quaternary structure within the T form to the possible, mechanisms for CTP inhibition and ATP activation.

About this Structure

6AT1 is a Protein complex structure of sequences from Escherichia coli with ZN as ligand. This structure superseeds the now removed PDB entry 4ATC. Active as Aspartate carbamoyltransferase, with EC number 2.1.3.2 Full crystallographic information is available from OCA.

Reference

Structural consequences of effector binding to the T state of aspartate carbamoyltransferase: crystal structures of the unligated and ATP- and CTP-complexed enzymes at 2.6-A resolution., Stevens RC, Gouaux JE, Lipscomb WN, Biochemistry. 1990 Aug 21;29(33):7691-701. PMID:2271528

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