Assuming a partial reversibility of the BSS reaction between the product and the product radical, we have investigated the same process for the enzyme:product model (E:P) by modeling the steps of the BSS mechanism in reverse. Thus, the process starts with the bound product in the E:P complex, Gly829 in the radical state (spin density at C 0.91), and Cys493 in the sulfhydryl form with an SH group. The Cys residue of the E:P complex is localized between the G-loop hosting Gly829 and a tightly packed arrangement of the side chains of residues Met494, Leu766, and Gln707. The H transfer occurs in analogy to the E:S complex, involving either an attack on the si (proS) or re (proR) face of the glycyl radical. As expected, the calculated geometries of the transition states are almost identical to those observed for is divided between the C atom of Gly829 and the S atom of Cys493 for both the proR-(0.61 and 0.28, respectively) and the proS-directed reaction (0.68 and 0.32, respectively). The geometric and electronic similarities to the E:S complexes correspond also with the very similar calculated values of the energy barriers between E:P and the respective TS EP , which were 38.8 kJ/mol for proR-TS EP and 61.9 kJ/mol for proS-TS EP . The similarity even extended to the prediction that the first intermediate containing the thiyl radical of Cys493 is energetically more stable (by 33 kJ/mol) than the E:P state with the glycyl radical (Table ).