Propene activation by the oxo-iron active species of taurine/α- ketoglutarate dioxygenase (TauD) enzyme. How does the catalysis compare to heme-enzymes?

Density functional calculations on the oxygenation reaction of propene by a model for taurine/α-ketoglutarate dioxygenase (TauD) enzyme are presented. The oxo-iron active species of TauD is shown to be a powerful and aggressive oxidant, which is able to hydroxylate C−H bonds and epoxidize CC bonds with low barriers. In the case of propene oxygenation, the hydroxylation and epoxidation mechanisms are competitive on a dominant quintet spin state surface. We have compared the mechanism and thermodynamics of TauD with oxo-iron heme catalysts, such as the cytochromes P450, and found some critical differences. The TauD model is found to be much more reactive toward oxygenation of substrates than oxo-iron complexes in a heme environment with much lower reaction barriers. We have analyzed this and assigned this to the strength of the O−H bond formed after hydrogen abstraction from a substrate, which is at least 10 kcal mol-1 stronger in five-coordinated oxo-iron nonheme complexes than in six-coordinated oxo-iron ...