Allosteric rescue of catalytically impaired ATP phosphoribosyltransferase variants links protein dynamics to active-site electrostatic preorganisation.

ATP phosphoribosyltransferase catalyses the first step of histidine biosynthesis and is controlled via a complex allosteric mechanism where the regulatory protein HisZ enhances catalysis by the catalytic protein HisG S while mediating allosteric inhibition by histidine. Activation by HisZ was proposed to position HisG S Arg56 to stabilise departure of the pyrophosphate leaving group. Here we report active-site mutants of HisG S with impaired reaction chemistry which can be allosterically restored by HisZ despite the HisZ:HisG S interface lying ~20 Å away from the active site. MD simulations indicate HisZ binding constrains the dynamics of HisG S to favour a preorganised active site where both Arg56 and Arg32 are poised to stabilise leaving-group departure in WT-HisG S . In the Arg56Ala-HisG S mutant, HisZ modulates Arg32 dynamics so that it can partially compensate for the absence of Arg56. These results illustrate how remote protein-protein interactions translate into catalytic resilience by restoring damaged electrostatic preorganisation at the active site.