In the reverse process, the hydrogen transfer from R-GlyHD to the thiyl radical occurs with a barrier of 95.6 kJ/mol, which is 0.2 kJ/mol lower than for H/H (KIE 0.9), while the barrier for hydrogen transfer from S-GlyDH is at 72.4 kJ/mol, i.e. 0.2 kJ/mol higher than for the respective H/H exchange (KIE 1.1)(Fig. ). The removal of a deuteron from GlyD2 is significantly slower compared to the process when only protium is involved, and the energy difference of 5.5 kJ/mol results in a KIE of 9 (for values see Table and S6 and Fig. ).