Binding of 5′-GTP to the C-terminal FeS cluster of the radical S -adenosylmethionine enzyme MoaA provides insights into its mechanism

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Abstract
The first step in molybdenum cofactor biosynthesis, the conversion of 5′-GTP to precursor Z, an oxygen-sensitive tetrahydropyranopterin is catalyzed by the S -adenosylmethionine (SAM)-dependent enzyme MoaA and the accessory protein MoaC. This reaction involves the radical-initiated intramolecular rearrangement of the guanine C8 atom. MoaA harbors an N-terminal [4Fe–4S] cluster, which is involved in the reductive cleavage of SAM and generates a 5′-deoxyadenosyl radical (5′-dA • ), and a C-terminal [4Fe–4S] cluster presumably involved in substrate binding and/or activation. Biochemical studies identified residues involved in 5′-GTP binding and the determinants of nucleotide specificity. The crystal structure of MoaA in complex with 5′-GTP confirms the biochemical data and provides valuable insights into the subsequent radical reaction. MoaA binds 5′-GTP with high affinity and interacts through its C-terminal [4Fe–4S] cluster with the guanine N1 and N2 atoms, in a yet uncharacterized binding mode. The tightly anchored triphosphate moiety prevents the escape of radical intermediates. This structure also visualizes the l -Met and 5′-dA cleavage products of SAM. Rotation of the 5′-dA ribose and/or conformational changes of the guanosine are proposed to bring the 5′-deoxyadenosyl radical into close proximity of either the ribose C2′ and C3′ or the guanine C8 carbon atoms leading to hydrogen abstraction.

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