Metal cation influence on activity and regulation of aspartate carbamoyltransferase

At saturating carbamoylphosphate and nonsaturating aspartate concentrations, Mg2+, Ca2+, Sr2+, Ba2+, Mn2+, Al3+, and Gd3+ inhibit aspartate carbamoyltransferase (carbamoylphosphate:L-asparate carbamoyltransferase, EC 2.1.3.2) from EScherichia coli. When nucleotide triphosphates are present, these inhibitory effects are displaced to higher concentrations of cation. At lower levels of cation and saturating carbamoylphosphate concentration, Mg2+, Mn2+, Al3+, and Gd3+ partially relieve allosteric inhibition by GTP but have little influence on activation by ATP and inhibition by CTP. At nonsaturating carbamoylphosphate concentrations, however, Mg2+, Mn2+, Al3+, and Gd3+ increase enzymatic activity to 170% over the level when GTP alone is present. In addition, Mg2+, Mn2+, and Al3+ show enhancement of ATP activation by 120-130% but only slight relief of CTP inhibition. We suggest that three modes of action by the metal can account for the observed kinetic behavior. (i) In the absence of nucleotide, metals inhibit catalytic activity either by a direct interaction with the enzyme or indirectly by complexing carbamoylphosphate. (ii) The metal-nucleotide complex interacts allosterically with the enzyme to enhance enzymatic activity relative to that produced by the free nucleotide, as noted above. (iii) By chelating to nucleotides, the metal diminishes their tendency to bind competitively at the carbamoylphosphate portion of the active site, as shown particularly by experiments on the catalytic subunit.