Taurine/alpha-ketbglutarate dioxygenase is one of the most studied a-ketoglutarate-dependent dioxygenases (alpha KGDs), involved in several biotechnological applications. We investigated the key step in the Catalytic cycle of the aKGDs, the hydrogen transfer process, by a quantum mechanics/molecular mechanics approach (B3LYP/CHARMMA22). 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 the quantuth tunneling of the hydrogen atom to increase the rate constant by a factor of 40 at 5 degrees C: As a copsequence, a quite high kinetic jsotope effect close to 60 is obtained, which is consistent withf the experimental value.