Interactions of metal-nucleotide complexes with aspartate carbamoyltransferase in the crystalline state

We report the results of crystallographic difference maps at 3.0-A resolution of complexes of metal-nucleoside triphosphates with aspartate carbamoyltransferase (carbamoylphosphate:L-aspartate carbamoyltransferase, EC 2.1.3.2) from Escherichia coli. The complexes Gd3+-ATP, Al3+-ATP, and Gd3+-CTP bind to the allosteric effector domain of the enzyme in nearly the same orientation as the metal-free nucleotides. The result is consistent with kinetic observations of nearly identical allosteric efficacy of ATP and CTP and their complexes with cations. The effector Gd3+-GTP, however, binds in a distinctly different conformation and location than does 8-bromoguanosine 5'-triphosphate, reported in a separate investigation [Honzatko, R. B. & Lipscomb, W.N. (1982) J. Mol. Biol. 160, 265-286]. The difference in the binding modes of Gd3+-GTP and the bromo derivative suggests a possible mechanism for the relief of allosteric inhibition of GTP due to metal cations. We observe no binding of metal-nucleoside triphosphates in the region of the phosphate crevice of aspartate carbamoyltransferase, consistent with the reduced ability of metal nucleotides to compete with carbamoyl phosphate for the active site. However, a single Gd3+ ion binds in the region of the active site as evidenced by strong density. The binding of cations near the active site probably causes the inhibition of catalysis observed in kinetics experiments reported earlier [Honzatko, R.B., Lauritzen, A.M. & Lipscomb, W.N. (1981) Proc. Natl. Acad. Sci. USA 78, 898-902].