Molybdenum in Biology - An Essential Trace Element
Molybdenum transport in plants and animals
See also molybdoenzymes, molybdenum storage proteins, molybdo-pterin, Mo co-factor
It is likely that molybdenum is taken up and transported in plants and animals in the form of the simple molybdate ion [MoO4]2-. In sheep [Scaife, 1956] molybdenum in the blood and urine is readily dialysable and is entirely anionic. A study of molybdenum toxicity in the microorganism Salmonella typhimurium has indicated that molybdate and sulfate are transported in the same system [McKillen and Spencer, 1970]. In this organism uptake of molybdenum and molybdenum toxicity are prevented by complexing of extracellular molybdate with L-cysteine and reduced glutathione. However, it has also been suggested that in the microorganism Aspergillus nidulans molybdenum is taken up in a phosphorylated entity and a carbohydrate [Arst et al., 1970; Arst and Cove, 1970]. The following chelating agents have been reported to be effective against molybdenum toxicity in mice (presumably because they coordinate with the molybdate ion): ethylenediaminetetraacetate, diethylenetriaminepentaacetate, unithiol and deoxycholate [Chem. Abs,1971]
Scaife, J. F., New Zealand J. Sci. Technol., 1956, 38A ,293.
McKillen, M. N.and Spencer, B., Biochem. J., 1970, 118 , 27.
Arst, H. N., MacDonald, D. W. and Cove, D. J., Mol. Gen. Genet., 1970, 108 , 129.
Arst, H. N. and Cove, D. J., Mol. Gen. Genet., 1970, 108 , 146.
Chem. Abs., 1971, 74 , 99958u.
Molybdate and tungstate storage protein
The release of molybdate from the molybdenum storage protein (MoSto), depends on temperature and pH value. A tungsten-containing storage protein ("WSto") was synthesized in vivo by growing cells. It was also constructed in vitro by a metal anion exchange using the isolated MoSto protein. The X-ray crystal structure of the W-loaded protein form showed that the protein contains different types of polyoxotungstates, the formation of which is templated by the different protein pockets.
Schemberg, J., Schneider, K., Fenske, D., and Muller, A., Azotobacter vinelandii metal storage protein: "Classical" inorganic chemistry involved in Mo/W uptake and release processes, Chembiochem, 2008, 9, 595-602.
Molybdate binding protein
A periplasmic protein (ModA) that is capable of binding molybdate and tungstate as part of the ABC-type transporter is required for the uptake of micronutrients. The crystallographic structure of the Xanthomonas axonopodis ModA protein with bound molybdate consists of two nearly symmetrical domains separated by a hinge region where the oxyanion-binding site lies. For the three groups of molybdate-binding proteins, bacterial phytopathogens, enterobacteria and soil bacteria, the ligand-binding hydrogen bonds are mostly conserved. Hydrophobic interactions in the active site are discussed. Two new residues, Ala(38) and Ser(151), are shown to be part of the ligand-binding pocket.
periplasmic (space) = space between the cell wall and the boundary membrane of the cell (=plasmalemma) which regulates the passage of molecules between the cell and its surroundings.
Balan, A., Santacruz-Perez, C., Moutran, A., Ferreira, L. C. S., Neshich, G., and Barbosa, J. A. R. G., Crystallographic structure and substrate-binding interactions of the molybdate-binding protein of the phytopathogen Xanthomonas axonopodis pv. citri, Biochimica et Biophysica Acta-Proteins and Proteomics, 2008, 1784, 393-399.
Mo cofactor biosynthesis
Molybdenum and tungsten enzymes catalyze redox reactions in the global carbon, nitrogen, and sulfur cycles. Except in nitrogenases both metals are associated with a unique metal-binding pterin (MPT). It is synthesized by a conserved multistep biosynthetic pathway which ends with the insertion and thereby biological activation of molybdenum or tungsten. The biogenesis of W-containing enzymes, mostly found in archaea, is poorly understood. The function of the Pyrococcus furiosus MoaB protein that is homologous to bacteria (such as MogA) and eukaryotic proteins (such as Cnx1) involved in the final steps of Mo cofactor synthesis is described. Metal and nucleotide specificity for MPT adenylylation is well conserved between W and Mo cofactor synthesis. MogA, Cnx1G, and MoaB proteins exhibit the same adenylyl transfer activity essential for metal insertion in W or Mo cofactor maturation.
