The kinetically favored H-transfer pathways between Cys493 and Gly829 in either model result in the retention of the same H-atom in the glycyl radical, contradicting the experimental evidence. Therefore, an occasional kinetically unfavorable transfer step to the si-side needs to occur, which leads to the observed H/D isotope exchange reactions of the glycyl radical. The same principles applied for the H/D-transfer reaction in substrate-and product-bound enzyme, while in contrast, the calculated rates for apo-BSS suggest that neither H-transfer to nor H/D exchange of the glycyl radical should be observable in a reasonable time frame because of their slow kinetics.