Activation of Anthranilate Phosphoribosyltransferase from Sulfolobus solfataricus by Removal of Magnesium Inhibition and Acceleration of Product Release

Anthranilate phosphoribosyltransferase from the hyperthermophilic archaeon Sulfolobus solfataricus (ssAnPRT) is encoded by the sstrpD gene and catalyzes the reaction of anthranilate (AA) with a complex of Mg2+ and 5'-phosphoribosyl-alpha 1-pyrophosphate (Mg center dot PRPP) to N-(5'-phosphoribosyl)-anthranilate (PRA) and pyrophosphate (PPi) within tryptophan biosynthesis. The ssAnPRT enzyme is highly thermostable (half-life at 85 degrees C = 35 min) but only marginally active at ambient temperatures (turnover number at 37 degrees C = 0.33 s(-1)). To understand the reason for the poor catalytic proficiency of ssAnPRT, we have isolated from an sstrpD library the activated ssAnPRT-D83G + F149S double mutant by metabolic complementation of an auxotrophic Escherichia coli strain. Whereas the activity of purified wildtype ssAnPRT is strongly reduced in the presence of high concentrations of Mg2+ ions, this inhibition is no longer observed in the double Mutant and the ssAnPRT-D83G single mutant. The comparison of the crystal Structures of activated and wild-type ssAnPRT shows that the D83G mutation alters the binding mode of the substrate Mg center dot PRPP. Analysis of PRPP and Mg2+-dependent enzymatic activity indicates that this leads to a decreased affinity for a second Mg2+ ion and thus reduces the concentration of enzymes with the inhibitory Mg-2 center dot PRPP complex bound to the active site. Moreover, the turnover number of the double mutant ssAnPRT-D83G + F149S is elevated 40-fold compared to the wild-type enzyme, which can be attributed to an accelerated release of the product PRA. This effect appears to be mainly caused by an increased conformational flexibility induced by the F149S Mutation, a hypothesis which is supported by the reduced thermal stability of the ssAnPRT-F149S single mutant.