JBC, Vol. 252, Issue 22, 8136-8141, Nov, 1977
Asymmetry of binding and physical assignments of CTP and ATP sites in aspartate transcarbamoylase
P. Suter and J. P. Rosenbusch
The allosteric effectors of aspartate transcarbamoylase from Escherichia
coli, CTP and ATP, associate with both the regulatory and the catalytic
moieties of the enzyme. Studies with isolated, active subunits yield one
binding site per regulatory dimer and one per catalytic trimer.
Investigations of effector association with hybrid enzymes, containing
either the three regulatory dimers or the two catalytic trimers in
inactivated forms, indicate that the data obtained with isolated subunits
can be used to analyze the binding patterns of these ligands to the native
hexamer. Thus, the nonlinear Scatchard plots, characteristic of the binding
of CTP and ATP to the native enzyme, can be interpreted in terms of three
effector molecules associating with the regulatory subunits, and two
binding to the catalytic moiety of the enzyme. Results with native protein
in the presence of saturating concentrations of active site ligands support
these assignments. The differences between the binding isotherms of CTP and
ATP to the enzyme are due to their different affinities to the two types
of subunits. The apparent half-of-the-site saturation of the regulatory
moiety of aspartate transcarbamoylase supports the concept that this
protein has a tendency to exist in an asymmetric state.
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