Two active mutants of aspartate transcarbamoylase from Escherichia coli have been purified from strains which produce large quantities of enzyme. Each enzyme was isolated from a different spontaneous revertant of a pyrimidine auxotrophic strain produced by mutagenesis with nitrogen mustard. Both enzymes exhibit allosteric properties with one having significantly less and the other more cooperativity than wild-type enzyme. Isolated catalytic subunits had different values of Km and Vmax. Studies on hybrids constructed from mutant catalytic and wild-type regulatory subunits (and vice versa) indicate that catalytic chains encoded by pyrB and not the regulatory chains encoded by pyrI were affected by the mutations. Differential scanning calorimetry experiments support these conclusions. Both mutant enzymes undergo ligand-promoted conformational changes analogous to those exhibited by wild-type enzyme: a 3% decrease in the sedimentation coefficient and a 5-fold increase in the reactivity of the sulfhydryl groups of the regulatory chains. Interactions between catalytic and regulatory chains in the mutants are weaker than those in the wild-type enzyme. The gross conformational changes of the mutants upon adding the bisubstrate ligand, N-(phosphonacetyl)-L-aspartate, in the presence of the substrate, carbamoylphosphate, and the activator, ATP, correlate with differences in cooperativity. The mutant with lower cooperativity is more readily converted from the low-affinity, compact, T-state to the high-affinity, swollen, R-state than is wild-type enzyme; this conversion for the more cooperative enzyme is energetically less favorable
