[AbHisZ] in the experiment; thus, equation ( ) does not hold. WT-PaATPPRT are known to form the Michaelis complex in crystallo, likely due to a highly unfavourable on-enzyme equilibrium for the forward reaction. The active site interactions are very similar between the two structures, except the electron density for Mg 2+ in R56A-PaATPPRT was not well defined, so the metal was not modelled in, and the hydrogen bond between E163 and the PRPP 3ʹ-OH is absent in R56A-PaHisGS (Fig. ). The structures are also very similar to those of the respective WT enzymes. Therefore, any structural or conformational differences that may lead to allosteric rescue of R56A-PaHisGS catalysis are not captured in the static view of the crystal structure. been observed in other allosteric systems, and it can also be seen here that PaHisZ binding increases order in the system.