The proximity of fGln123 to the HisH active site is another structural feature influenced by the binding of the allosteric effector, PrFAR. It is expected that fGln123 assists in recruiting and stabilizing the glutamine substrate, but it is unclear to what extent, if any, this proximity influences the chemical coordinate. The Inactive fGln123 oxyanion hole contributions (Figure ) are nearly identical to Active, with an average difference of less than 0.2 kcal/mol per residue in each TS (all numerical values are listed in Table ). Therefore, the oxyanion hole energetics are not influenced by the proximity of fGln123. To further investigate the role of fGln123, the interaction energy between fGln123 and the HisH active site is calculated via the ALMO-EDA procedure for each QM model (Computational Methods). The total interaction energy (Total INT) in ES is much more favorable when fGln123 is closer to the active site. Active-and Inactive Val51-ES have similar fGln123 interaction energies of -29.2 and -30.6 kcal/mol, which are much lower than the Inactive fGln123 and Inactive values (-5.4 and -13.1 kcal/mol, respectively) (Figure ). The difference in fGln123 interaction energy between Inactive fGln123 and Inactive can be reasoned by the presence of a hydrogen bond between the sidechain NH2 group of fGln123 and the backbone carbonyl oxygen of Val140 (Figure ) that is present in the Inactive model but absent in the Inactive fGln123model.