Inactive Val51 differs from Active only in the backbone dihedrals of Val51; the N-H points away from the active site, mimicking the inactive conformation, but the oxyanion strand position is consistent with Active. The Gly52 N-H contributes -10.8 kcal/mol in Inactive Val51-TS2, which is less than the contributions from Val51 in Active (-14.7 kcal/mol), but the difference in energy between TS2 and ES in Inactive Val51 is smaller (∆E ‡ =9.8 kcal/mol, vs. (∆E ‡ =14.5 kcal/mol in Active) (Figure ). This results from a higher energy Inactive Val51-ES relative to Active-ES (∆E=5.3 kcal/mol), while the TS2 structures have approximately the same energy (∆E=0.7 kcal/mol). The atom positions frozen during geometry optimizations are the same between Inactive Val51 and Active; therefore, their structure energies can be directly compared, unlike the other models. The Inactive Val51 oxyanion hole stabilizes TS4 more than in any other model (nearly twice that of Active-TS4). This is because a third lone pair of electrons is detected in the NBO analysis of this structure, whereas the other structures only have two electron lone pairs in TS4. Regardless, Inactive Val51-TS4 is 3.4 kcal/mol higher in energy than Active-TS4, consistent with less efficient catalysis than in the Active construct.