Substrate binding domain of molybdate-binding proteins, the type 2 periplasmic binding protein fold.
Molybdate binding domain ModA. Molybdate transport system is comprised of a periplasmic binding protein, an integral membrane protein, and an energizer protein. These three proteins are coded by modA, modB, and modC genes, respectively. ModA proteins serve as initial receptors in the ABC transport of molybdate mostly in eubacteria and archaea. Bacteria and archaea import molybdenum and tungsten from the environment in the form of the oxyanions molybdate (MoO(4) (2-)) and tungstate (WO(4) (2-)). After binding molybdate with high affinity, they interact with a cognate membrane transport complex comprised of two integral membrane domains and two cytoplasmically located ATPase. This interaction triggers the ligand translocation across the cytoplasmic membrane energized by ATP hydrolysis. In contrast to the structure of the two ModA homologs from Escherichia coli and Azotobacter vinelandii, where the oxygen atoms are tetrahedrally arranged around the metal center, the structure of Pyrococcus furiosus ModA/WtpA (PfModA) has revealed a binding site for molybdate and tungstate where the central metal atom is in a hexacoordinate configuration. This octahedral geometry was rather unexpected. The ModA proteins belong to the PBP2 superfamily of periplasmic binding proteins that differ in size and ligand specificity, but have similar tertiary structures consisting of two globular subdomains connected by a flexible hinge. They have been shown to bind their ligand in the cleft between these domains in a manner resembling a Venus flytrap.
Jiyao W et al.(2023). "The conserved domain database in 2023.",