In conclusion, the very high barrier of removing H from R-GlyHD (95.6 kJ/mol) suggests that deuterium exchange is correlated with the rare si-directed hydrogen transfer events, which produce an S-GlyDH intermediate (Fig. red pathway). The calculated back-transfer of H from S-GlyDH is much faster than that of D (difference of the barriers 28.6 kJ/mol), so an H/D exchange at the glycyl radical will be inevitable if the first step occurs.