First, we compared the calculated rates for the preferential pathway via S-Gly829 (red arrow in Fig. ) with those of the alternate pathway via R-Gly829. It turns out that the value of k S H/D is calculated as 2 s -1 while that of k R H/D equals 2.1*10 -4 s -1 , which means that the pathway proceeds exclusively through S-GlyDH while R-GlyHD is of no consequence. This shows that despite the 5400-fold kinetic preference for the transfer of D via the re face, one of the expected rare D transfers via the si face of the glycyl radical is a prerequisite for the H/D exchange. The same approach can be applied to analyzing the experiment when the deuterated enzyme is incubated in H2O. For the backward exchange of D to H in the glycyl radical, the preferred pathway proceeds via the re-face attack (purple arrows in Fig: ) with a calculated value of k R D/H of 0.29 s -1 while the alternative pathway, starting with the transfer of H to the si face, is associated with a k S D/H value of 2.4*10 -5 s -1 , almost 12,000-fold slower than the process via the re-face.