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Propene Activation by the Oxo-Iron Active Species of Taurine/α-Ketoglutarate Dioxygenase (TauD) Enzyme. How Does the Catalysis Compare to Heme-Enzymes?
journal contribution
posted on 2006-08-02, 00:00 authored by Sam P. de VisserDensity 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 heme complexes. Since, the metal in TauD enzymes is five-coordinated, whereas
in heme-enzymes it is six-coordinated there are some critical differences in the valence molecular orbitals.
Thus, in oxo-iron heme catalysts one of the antibonding π* orbitals is replaced by a low-lying nonbonding
δ orbital resulting in a lower overall spin state. Moreover, heme-enzymes have an extra oxidation equivalent
located on the heme, which is missing in non-heme oxo-iron catalysts. As a result, the oxo-iron species of
TauD reacts via single-state reactivity on a dominant quintet spin state surface, whereas oxo-iron heme
catalysts react via two-state reactivity on competing doublet and quartet spin states.