  TauD dioxygenase is one of the most studied $\alpha$-ketoglutarate dependent
dioxygenases ($\alpha$KGDs), involved in several biotechnological applications.
We investigated the key step in the catalytic cycle of the $\alpha$KGDs, the
hydrogen transfer process, by a QM/MM approach (B3LYP/CHARMM22). Analysis of
the charge and spin densities during the reaction demonstrates that a concerted
mechanism takes place, where the H atom transfer happens simultaneously with
the electron transfer from taurine to the Fe=O cofactor. We found quantum
tunneling of the hydrogen atom to increase the rate constant by a factor of 40
at 5\textcelsius{}. As a consequence a quite high KIE value of close to 60 is
obtained, which is consistent with the experimental value.
