Kinetic mechanism of catalytic subunits (c3) of E. coli aspartate transcarbamylase at pH 7.0.

In contrast to holo-enzyme (c6r6), catalytic subunits (c3) of Escherichia coli aspartate transcarbamylase (carbamoyl-phosphate:L-aspartate carbamoyltransferase, EC 2.1.3.2) do not exhibit allosteric interactions or inhibition effects that complicate kinetic investigations of substrate binding order. Equilibrium isotope-exchange kinetic probes of c3 at pH 7.0 and 30 degrees C produced kinetic saturation patterns consistent with a strongly preferred order random kinetic mechanism, in which carbamoyl phosphate binds prior to aspartate and carbamoyl aspartate is released before Pi. Weak substrate inhibition effects observed with c6r6 did not occur with c3, possibly due to decreased affinity for ligands at the dianion inhibition site.