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. Author manuscript; available in PMC: 2019 Apr 26.
Published in final edited form as: Nat Chem. 2017 Feb 6;9(7):698–707. doi: 10.1038/nchem.2714

Figure 5. Proposed mechanism of the radical SAM peptide epimerase YydG.

Figure 5

YydG generates a 5′-dA• radical which abstracts an amino acid Cα H-atom. A carbon-centred radical is generated and quenched by the thiolate H-atom of Cys223 leading to the formation of a D-amino acid residue. The SPASM [4Fe-4S] center likely assists the radical quenching by reducing the thiyl radical. Further reduction of Cys223 would be required to allow another catalytic cycle.