Complex of aspartate carbamoyltransferase from Escherichia coli with its allosteric inhibitor, cytidine triphosphate: electron density at 5.9-angstroms resolution.

Following our earlier determination of the three-dimensional structure of aspartate carbamoyltransferase (EC 2.1.3.2; carbamoylphosphate: L-aspartate carbamoyltransferase) to 5.5-A resolution [S. G. Warren, B. F. P. Edwards, D. R. Evans, D. C. Wiley & W. N. Lipscomb (1973) Proc. Nat. Acad. Sci. USA 70, 1117-1121], we report here, from a different crystal form, the three-dimensional structure at 5.9 A of this enzyme complexed with its allosteric inhibitor, cytidine triphosphate. Location of the major binding site of this inhibitor within each of the six regulatory chains is made secure by comparison of these results with those obtained upon binding of 5-iodocytidine triphosphate to the enzyme. Conformational changes in the aspartate carbamoyltransferase molecule when this inhibitor binds are described briefly at 5.9-A resolution.