Substrate Orientation in 4-Oxalocrotonate Tautomerase and Its Effect on QM/MM Energy Profiles

The tautomerization of 2-oxo-4E-hexendioate by 4-oxalocrotonate tautomerase has been studied by quantum
mechanical/molecular mechanical (QM/MM) methods using three models, A−C, with different substrate
orientations. The computed QM/MM energy profiles are rather different. Various energy partitioning analyses
indicate the origin of these differences and the role of the active site residues for different substrate orientations.
The proposed new model C is preferred over the previously used models A and B because it combines favorable
substrate binding geometries with reasonable barriers and is consistent with the experimental evidence from
mutation studies concerning the catalytic ability of specific residues in the binding site, especially R11‘.