Iron-sulfur cluster-binding Twitch domain of GTP 3',8-cyclaseThe iron-sulfur cluster-binding Twitch domain is found at the C-terminus of GTP 3',8-cyclase (EC 220.127.116.11), which is also called molybdenum cofactor biosynthesis protein A (MoaA) in bacteria and archaea, molybdenum cofactor biosynthesis protein 1 (MOCS1) in most eukaryotes, and molybdenum cofactor biosynthesis enzyme CNX2 in plants. GTP 3',8-cyclase is a radical S-adenosylmethionine (SAM) enzyme that catalyzes the first step in molybdopterin biosynthesis, the cyclization of guanosine triphosphate to (8S)-3',8-cyclo-7,8-dihydroguanosine 5'-triphosphate, which is then converted to molybdopterin in subsequent steps. Radical SAM enzymes are characterized by a conserved CxxxCxxC motif, which coordinates the conserved iron-sulfur cluster that is involved in the reductive cleavage of SAM and generates a 5'-deoxyadenosyl radical, which in turn abstracts a hydrogen from the appropriately positioned carbon atom of the substrate. GTP 3',8-cyclase contains an additional iron-sulfur cluster at the C-terminal Twitch domain that is involved in substrate binding. The Twitch domain may be related to another iron-sulfur cluster-binding domain found at the C-terminus of some radical SAM enzymes, the SPASM domain, named after the biochemically characterized members, AlbA, PqqE, anSMEs, and MftC, which are involved in Subtilosin A, Pyrroloquinoline quinone, Anaerobic Sulfatase, and Mycofactocin maturation, respectively.