Subsequently, the C-terminus of the GRE refolds, the activating enzyme dissociates, and the GRE is now in its active, radical-containing state. Because of mesomeric interchange with the electrons of the peptide bond, the glycyl radical is highly stabilized, but only under strictly anaerobic conditions . Any contact of an activated GRE with molecular oxygen results in irreversible destruction by oxygenolytic cleavage of the peptide bond at the site of the glycyl radical, as demonstrated for PFL and BSS . This apparently complicated indirect method of initiating a radical reaction is especially advantageous for catabolic reactions occurring multiple times: instead of the high cost of converting one SAM cofactor to methionine and 5'-deoxyadenosine with every reaction of a SAM radical enzyme, this occurs only once per catabolic GRE during its activation. The introduced stable glycyl radical is continuously recycled, allowing a multitude of reactions to proceed without re-activation. The general mechanism of GRE is initiated by substrate binding, which closes the active site and triggers a cascade of hydrogen atom transfer steps, leading to a succession of radical intermediates in the reaction mechanism. Starting with the glycyl radical, a more reactive thiyl radical is generated at the conserved Cys, which then reacts with the bound substrate to create an enzyme-bound substrate radical. To be able to do that, the active site Cys simultaneously needs access to the glycyl radical outside and to the bound substrate within the active site cavity (Fig. . insert) and therefore needs to pass the border of this cavity. This substrate radical then undergoes the intended conversion reaction, which is typically not possible in the non-radical state, producing an enzyme-bound product radical species. Finally, the hydrogen atom transfer reactions are repeated in a reverse cascade via the thiyl and glycyl radical intermediates, generating the product in the active site. These reactions are believed to require a tightly closed enzyme-substrate complex to protect the radical intermediates from reacting with interfering molecules from the solvent. Only when the stable glycyl radical state is reached again, the active site can safely be opened to release the product and bind new substrate(s).