| Abstract: | The binding of the bisubstrate ligand N-(phosphonacetyl)-L-aspartate (PALA) to the active sites of both the free catalytic subunit of aspartate transcarbamoylase and the intact holoenzyme causes conformational changes which have been studied extensively. However, no kinetic information has been available about the sequence of events occurring during the formation or dissociation of the complexes. Stopped flow kinetics, 31P saturation transfer NMR spectroscopy, and presteady-state kinetics were used to monitor the interaction of PALA with the catalytic subunit (or a derivative containing nitrotyrosyl chromophores which served as spectral probes). The various experimental approaches lead to a mechanism that includes a rapid binding of PALA with an "on" rate of about 10(8)M-1s-1 and an "off" rate of 28 s-1, followed by a much slower isomerization of the complex with a forward rate constant of 0.18 s-1. Analysis of the presteady-state bursts of enzyme activity when the protein is added to a mixture of substrates and PALA and of the lag in activity when the PALA complex with catalytic subunit is added to substrates yielded a rate constant for the reverse isomerization of 0.018s-1. Thus, the conformational change subsequent to PALA binding leads to a 10-fold increase in the equilibrium constant for complex formation. Stopped flow kinetic measurements of the spectral change resulting from mixing the complex of PALA and nitrated protein with native enzyme showed a slow process with a t1/2 of about 11 s, whereas 31P saturation transfer NMR experiments yielded at t1/2 of about 260 ms for the dissociation of PALA from the complex. This apparent disparity is understood in terms of the two-step binding scheme where rapid dissociation of the initial ligand X enzyme complex is measured by the NMR technique and the slow isomerization of the complex is responsible for the bulk of the stopped flow signal. |
