Time evolution of the quaternary structure of Escherichia coli aspartate transcarbamoylase upon reaction with the natural substrates and a slow, tight-binding inhibitor.

Here, we present a study of the conformational changes of the quaternary structure of Escherichia coli aspartate transcarbamoylase, as monitored by time-resolved small-angle X-ray scattering, upon combining with substrates, substrate analogs, and nucleotide effectors at temperatures between 5 and 22 degrees C, obviating the need for ethylene glycol. Time-resolved small-angle X-ray scattering time courses tracking the T-->R structural change after mixing with substrates or substrate analogs appeared to be a single phase under some conditions and biphasic under other conditions, which we ascribe to multiple ligation states producing a time course composed of multiple rates. Increasing the concentration of substrates up to a certain point increased the T-->R transition rate, with no further increase in rate beyond that point. Most strikingly, after addition of N-phosphonacetyl-l-aspartate to the enzyme, the transition rate was more than 1 order of magnitude slower than with the natural substrates. These results on the homotropic mechanism are consistent with a concerted transition between structural and functional states of either low affinity, low activity or high affinity, high activity for aspartate. Addition of ATP along with the substrates increased the rate of the transition from the T to the R state and also decreased the duration of the R-state steady-state phase. Addition of CTP or the combination of CTP/UTP to the substrates significantly decreased the rate of the T-->R transition and caused a shift in the enzyme population towards the T state even at saturating substrate concentrations. These results on the heterotropic mechanism suggest a destabilization of the T state by ATP and a destabilization of the R state by CTP and CTP/UTP, consistent with the T and R state crystallographic structures of aspartate transcarbamoylase in the presence of the heterotropic effectors.