The crystal structure of the Escherichia coli MobA protein provides insight into molybdopterin guanine dinucleotide biosynthesis

The molybdenum cofactor (Moco) is found in a variety of enzymes present in all phyla and comprises a family of related molecules containing molybdopte rin (MPT), a tricyclic pyranopterin with a cis-dithiolene group, as the inv ariant essential moiety, MPT biosynthesis involves a conserved pathway, but some organisms perform additional reactions that modify MPT. In eubacteria , the cofactor is often present in a dinucleotide form combining MPT and a purine or pyrimidine nucleotide via a pyrophosphate linkage. In Escherichia coli, the MobA protein links a guanosine 5'-phosphate to MPT forming molyb dopterin guanine dinucleotide. This reaction requires GTP, MgCl2,, and the MPT form of the cofactor and can efficiently reconstitute Rhodobacter sphae roides apo-DMSOR, an enzyme that requires molybdopterin guanine dinucleotid e for activity. In this paper, we present the crystal structure of MobA, a protein containing 194 amino acids. The MobA monomer has an alpha/beta arch itecture in which the N-terminal half of the molecule adopts a Rossman fold . The structure of MobA has striking similarity to Bacillus subtilis SpsA, a nucleotide-diphospho-sugar transferase involved in sporulation, The cocry stal structure of MobA and GTP reveals that the GTP-binding site is located in the N-terminal half of the molecule. Conserved residues located primari ly in three signature sequence motifs form crucial interactions with the bo und nucleotide, The binding site for MPT is located adjacent to the GTP-bin ding site in the C-terminal half of the molecule, which contains another se t of conserved residues presumably involved in MPT binding.