Fetler L, Vachette P. The allosteric activator Mg-ATP modifies the quaternary structure of the R-state of Escherichia coli aspartate transcarbamylase without altering the T<-->R equilibrium.
J Mol Biol 2001;
309:817-32. [PMID:
11397099 DOI:
10.1006/jmbi.2001.4681]
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Abstract
The allosteric enzyme aspartate transcarbamylase from Escherichia coli (ATCase) displays regulatory properties that involve various conformational changes, including a large quaternary structure rearrangement. This entails a major change in its solution X-ray scattering curve upon binding substrate analogues. We show here that, in the presence of the nucleotide effector ATP, known to stimulate the enzyme activity, the scattering profiles show a marked dependence on the metal bound to ATP. Whereas ATP has no major effect on the scattering pattern of ATCase, a saturating concentration of Mg-ATP notably modifies the scattering profile of the enzyme, either in the absence or in the presence of the bisubstrate analogue N-(phosphonacetyl)-l-aspartate (PALA). The transition with PALA in the presence of this metal-nucleotide complex remains concerted. Furthermore, Mg-ATP, as already observed with ATP, has no detectable direct effect on the T to R transition. The experimental scattering curves in the presence of Mg-ATP were fitted by a modeling approach using rigid body movements of the regulatory subunits and the catalytic trimers in the crystal structures. While the differences observed in the T-state in the presence of Mg-ATP are essentially attributed to the binding per se of the nucleotide, the solution structure of the R-state complexed to Mg-ATP is even more extended along the 3-fold axis than the previously described R solution structure, which is already more stretched out along the same axis than the crystal R structure. Based on the crystal structure of the enzyme in the R-state complexed with free ATP, a proposal is made to account for the effect of magnesium.
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