Glycyl radical activating enzymes: Structure, mechanism, and substrate interactions

• Radical SAM enzymes use SAM and a [4Fe–4S] cluster for diverse catalytic reactions.
• GREs catalyze reactions through production of glycyl, thiyl, and substrate radicals.
• PFL-AE has provided insights into active site of radical SAM enzymes.
• PFL-AE established how activating enzymes interact with their substrate GREs.

The glycyl radical enzyme activating enzymes (GRE–AEs) are a group of enzymes that belong to the radical S-adenosylmethionine (SAM) superfamily and utilize a [4Fe–4S] cluster and SAM to catalyze H-atom abstraction from their substrate proteins. GRE–AEs activate homodimeric proteins known as glycyl radical enzymes (GREs) through the production of a glycyl radical. After activation, these GREs catalyze diverse reactions through the production of their own substrate radicals. The GRE–AE pyruvate formate lyase activating enzyme (PFL-AE) is extensively characterized and has provided insights into the active site structure of radical SAM enzymes including GRE–AEs, illustrating the nature of the interactions with their corresponding substrate GREs and external electron donors. This review will highlight research on PFL-AE and will also discuss a few GREs and their respective activating enzymes.