The transfer of deuterium from Cys493 to the re face of the glycyl radical is associated with a barrier of 43.2 kJ/mol (Fig. E:S), representing a slight increase by 2.8 kJ/mol compared to that for H/H transfer, which results in an intrinsic KIE of 3.0. The reverse reaction, i.e. the transfer of a D atom from R-GlyHD back to the thiyl radical, proceeds with a barrier of 77.4 kJ/mol. The predicted intrinsic KIE associated with this transfer is higher than in the forward process (7.9), due to a higher increase of the energy barrier (5.2 kJ/mol) upon replacing protium for deuteron.