Biochemical and structural data on PFL and its activating enzyme suggest that the generation of the glycyl radical by the respective activating enzyme is strictly stereospecific for all GRE, and a reversible glycyl radical deactivation process is only known for PFL , but not for any other GRE. Moreover, one of the two conserved Cys residues of PFL has already known to be involved in the H/D exchange of the glycyl radical in PFL . Therefore, the observed H/D-exchange of BSS occurs most likely during the H-transfer reactions between Gly829 and Cys493, which occur in equilibrium in both directions. Because the sulfhydryl hydrogen of Cys493 is exchangeable with solvent, this would lead to H/D exchange at the glycyl radical if the H transfer reaction between Cys and Gly occasionally occurred in the less preferred enantiomeric orientation. Therefore, the observed H/D exchange at C of the glycyl radical can be explained if the energetic barriers of proR-and proS-directed hydrogen transfer from Cys493 to the glycyl radical are not too far apart, making the processes kinetically comparable.