The copper-dependent formylglycine-generating enzyme (FGE) catalyzes the oxygen-dependent oxidation of specific peptidyl-cysteine residues to formylglycine. Our QM/MM calculations provide a very likely mechanism for this transformation. The reaction starts with dioxygen binding to the tris-thiolate Cu I center to form a triplet Cu II -superoxide complex. The rate-determining hydrogen atom abstraction involves a triplet-singlet crossing to form a Cu II À OOH species that couples with the substrate radical, leading to a Cu Ialkylperoxo intermediate. This is accompanied by proton transfer from the hydroperoxide to the S atom of the substrate via a nearby water molecule. The subsequent OÀ O bond cleavage is coupled with the CÀ S bond breaking that generates the formylglycine and a Cu IIoxyl complex. Moreover, our results suggest that the aldehyde oxygen of the final product originates from O 2 , which will be useful for future experimental work.

The copper-dependent formylglycine-generating enzyme (FGE) catalyzes the oxygen-dependent oxidation of specific peptidyl-cysteine residues to formylglycine. Our QM/MM calculations provide a very likely mechanism for this transformation. The reaction starts with dioxygen binding to the tris-thiolate Cu I center to form a triplet Cu II -superoxide complex. The rate-determining hydrogen atom abstraction involves a triplet-singlet crossing to form a Cu II À OOH species that couples with the substrate radical, leading to a Cu Ialkylperoxo intermediate. This is accompanied by proton transfer from the hydroperoxide to the S atom of the substrate via a nearby water molecule. The subsequent OÀ O bond cleavage is coupled with the CÀ S bond breaking that generates the formylglycine and a Cu IIoxyl complex. Moreover, our results suggest that the aldehyde oxygen of the final product originates from O 2 , which will be useful for future experimental work.