Figure 1.

(A) General mechanism of glycyl radical enzymes (GREs). A partner radical S-adenosylmethionine (SAM) activating enzyme (AE) first abstracts a hydrogen atom from the α-carbon of a conserved active site glycine residue on the GRE to generate a carbon-centered radical. The glycine-centered radical is proposed to abstract a hydrogen atom from an essential active site cysteine residue to generate a thiyl radical intermediate that initiates reaction with the substrate. Further reaction of the substrate-based radical generates a product-based radical, which then abstracts a hydrogen atom from the essential cysteine residue to regenerate the thiyl radical. (B) Previously characterized activities of 4-hydroxyproline dehydratase (HypD) and Δ¹-pyrroline-5-carboxylate reductase (P5CR) from Clostridioides difficile 70-100-2010.