1.

CutC reaction and activation. (a) CutC catalyzes the degradation of choline to trimethylamine (TMA) and acetaldehyde; TMA is subsequently oxidized by flavin-containing monooxygenase 3 (FMO3) to form the disease-associated metabolite trimethylamine-N-oxide (TMAO). (b) Mechanism of glycyl radical formation in glycyl radical enzymes (GREs): The GRE-activating enzyme transfers an electron from an iron–sulfur cluster to S-adenosylmethionine (SAM), leading to reductive cleavage of SAM into methionine and a 5′-deoxyadenosyl radical (5′-dAdo^•). This radical abstracts a hydrogen atom from the active-site glycine, generating the catalytically essential glycyl radical. Subsequent hydrogen transfer between the glycyl radical and a nearby cysteine residue forms a thiyl radical, which initiates substrate C–H bond cleavage during the GRE catalytic cycle. (c) Crystal structure of CutC in complex with choline (PDB ID 5FAU), illustrating the substrate-binding site and key catalytic residues.