Substrate binding domain of ModA/WtpA from Azotobacter vinelandii and its closest homologs;the type 2 periplasmic binding protein fold.
This subfamily contains domains found in ModA proteins that serve as initial receptors in the ABC transport of molybdate 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 shown that a binding site for molybdate and tungstate is 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.
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