Substrate saturation curves for WT-, C115A-, D179A-, and D179N-PaHisGS obeyed Michaelis-Menten kinetics (Fig. ), and data fit to equation (1) produced the apparent steady-state kinetic parameters in Supplementary Table . The Michaelis constant for ATP (KATP) increased less than 2-fold for C115A-PaHisGS, and the Michaelis constant for PRPP (KPRPP) actually decreased between 2-and 3-fold for D179A-and D179N-PaHisGS, suggesting C115 and D179 make negligible contributions to substrate binding. The kcat values for the mutants were reduced only ~4-fold in comparison with the WT-PaHisGS, pointing to these residues' modest importance in catalysis. Over the course of the ATPPRT-catalysed reaction, the 6-NH2 group must donate a proton to yield the 6-NH group of PRATP, and in the case of PaATPPRT, this proton abstraction happens on the enzyme. Based on their respective positions in the active site (Fig. ), both C115 and D179 were candidates to act as general base for this proton abstraction, but the small catalytic effect of their replacements for residues that cannot participate in acid-base catalysis does not support such role, leaving the identity of the general base still elusive.