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Capture and Quality Control Mechanisms for Adenosine-5′-triphosphate Binding

Li, Li, Martinis, Susan A., Luthey-Schulten, Zaida
Journal of the American Chemical Society 2013 v.135 no.16 pp. 6047-6055
Gibbs free energy, active sites, adenosine triphosphate, aminoacyl tRNA ligases, binding capacity, fluorescence emission spectroscopy, histidine, mutagenesis, nucleosides, quality control
The catalytic events in members of the nucleotidylyl transferase superfamily are initiated by a millisecond binding of ATP in the active site. Through metadynamics simulations on a class I aminoacyl-tRNA synthetase (aaRSs), the largest group in the superfamily, we calculate the free energy landscape of ATP selection and binding. Mutagenesis studies and fluorescence spectroscopy validated the identification of the most populated intermediate states. The rapid first binding step involves formation of encounter complexes captured through a fly casting mechanism that acts upon the triphosphate moiety of ATP. In the slower nucleoside binding step, a conserved histidine in the HxxH motif orients the incoming ATP through base-stacking interactions resulting in a deep minimum in the free energy surface. Mutation of this histidine significantly decreases the binding affinity measured experimentally and computationally. The metadynamics simulations further reveal an intermediate quality control state that the synthetases and most likely other members of the superfamily use to select ATP over other nucleoside triphosphates.