Molybdenum transport in plants and animals

See also molybdoenzymes, molybdenum storage proteins, molybdo-pterin, Mo co-factor

Molybdenum transport as molybdate and comparison with sulfate. Preventing extracellular uptake by complexing.

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.

Biological transport

Multiplicity of Sulfate and Molybdate Transporters and Their Role in Nitrogen Fixation in Rhizobium leguminosarum bv. viciae Rlv3841

Rhizobium leguminosarum Rlv3841 contains at least three sulfate transporters, i.e., SulABCD, SulP1 and SulP2, and a single molybdate transporter, ModABC. SulABCD is a high-affinity transporter whose mutation prevented growth on a limiting sulfate concentration, while SulP1 and SulP2 appear to be low-affinity sulfate transporters. ModABC is the sole high-affinity molybdate transport system and is essential for growth with NO3- as a nitrogen source on limiting levels of molybdate (molybdate, a quadruple mutant with all four transporters inactivated, had the longest lag phase on NO3-, suggesting these systems all make some contribution to molybdate transport. Growth of Rlv3841 on limiting levels of sulfate increased sulB, sulP1, modB, and sulP2 expression 313.3-, 114.7-, 6.2-, and 4.0-fold, respectively, while molybdate starvation increased only modB expression (three- to 7.5-fold). When grown in high-sulfate but not low-sulfate medium, pea plants inoculated with LMB695 (modB) reduced acetylene at only 14% of the wild-type rate, and this was not further reduced in the quadruple mutant. Overall, while modB is crucial to nitrogen fixation at limiting molybdate levels in the presence of sulfate, there is an unidentified molybdate transporter also capable of sulfate transport.

Cheng, G., Karunakaran, R., East, A. K., and Poole, P. S.,Multiplicity of Sulfate and Molybdate Transporters and Their Role in Nitrogen Fixation in Rhizobium leguminosarum bv. viciae Rlv3841, Molecular plant-microbe interactions : MPMI, 2016, 29, 143-52.

Microbial ligand coordination: Consideration of biological significance

Siderophores are generally considered to be microbial chelating compounds secreted by bacteria to facilitate uptake of iron(III). Certain siderophores, however, have a high affinity for other transition metal ions, including manganese(III), copper(II), molybdenum(VI), and vanadium(V). A new class of microbial ligands produced by methanotrophs, called chalkophores, is produced to facility copper uptake.

This review considers the coordination of siderophores to Mn(III), Cu(II/I), Mo(VI) and V(V), and their stability constants relative to Fe(III), when available, as well as the coordination complexes of chalkophores to Cu( l). (C) 2015 Elsevier B.V. All rights reserved.

Springer, S. D., and Butler, A.,Microbial ligand coordination: Consideration of biological significance, Coordination Chemistry Reviews, 2016, 306, 628-635.

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.

Polyoxomolybdate storage protein

Nature's Polyoxometalate Chemistry: X-ray Structure of the Mo Storage Protein Loaded with Discrete Polynuclear Mo-O Clusters

Some N-2-fixing bacteria prolong the functionality of nitrogenase in molybdenum starvation by a special Mo storage protein (MoSto) that can store more than 100 Mo atoms.

The presented 1.6 angstrom X-ray structure of MoSto from Azotobacter vinelandii reveals various discrete polyoxomolybdate clusters, three covalently and three noncovalently bound Mo-8, three Mo5-7, and one Mo-3 clusters, and several low occupied, so far undefinable clusters, which are embedded in specific pockets inside a locked cage-shaped (alpha beta)(3) protein complex.

The structurally identical Mo-8 clusters (three layers of two, four, and two MoOn octahedra) are distinguishable from the [Mo8O26]4- cluster formed in acidic solutions by two displaced MoOn octahedra implicating three kinetically labile terminal ligands. Stabilization in the covalent Mo-8 cluster is achieved by Mo bonding to His alpha 156-N-epsilon 2 and Glu alpha 129-O-epsilon 1.

The absence of covalent protein interactions in the noncovalent Mo-8 cluster is compensated by a more extended hydrogen-bond network involving three pronounced histidines. One displaced MoOn octahedron might serve as nucleation site for an inhomogeneous Mo5-7 cluster largely surrounded by bulk solvent.

In the Mo-3 cluster located on the 3-fold axis, the three accurately positioned His140-N-epsilon 2 atoms of the alpha subunits coordinate to the Mo atoms. The formed polyoxomolybdate clusters of MoSto, not detectable in bulk solvent, are the result of an interplay between self- and protein-driven assembly processes that unite inorganic supramolecular and protein chemistry in a host-guest system.

Template, nucleation/protection, and catalyst functions of the polypeptide as well as perspectives for designing new clusters are discussed.

Kowalewski, Bjoern, Poppe, Juliane, Demmer, Ulrike, Warkentin, Eberhard, Dierks, Thomas, Ermler, Ulrich, and Schneider, Klaus, Nature's Polyoxometalate Chemistry: X-ray Structure of the Mo Storage Protein Loaded with Discrete Polynuclear Mo-O Clusters, Journal of the American Chemical Society, 2012, 134, 9768-9774.

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.

Transport across biomembranes

Metal Transport across Biomembranes: Emerging Models for a Distinct Chemistry

Transition metals are essential components of important biomolecules, and their homeostasis is central to many life processes. Transmembrane transporters are key elements controlling the distribution of metals in various compartments. However, due to their chemical properties, transition elements require transporters with different structural-functional characteristics from those of alkali and alkali earth ions. Emerging structural information and functional studies have revealed distinctive features of metal transport. Among these are the relevance of multifaceted events involving metal transfer among participating proteins, the importance of coordination geometry at transmembrane transport sites, and the presence of the largely irreversible steps associated with vectorial transport. Here, we discuss how these characteristics shape novel transition metal ion transport models.

Argueello, Jose M., Raimunda, Daniel, and Gonzalez-Guerrero, Manuel, Metal Transport across Biomembranes: Emerging Models for a Distinct Chemistry, Journal of Biological Chemistry, 2012, 287, 13510-13517.

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.

Bevers, L. E., Hagedoorn, P.L., Santamaria-Araujo, J.A., Magalon, A., Hagen, W.R., Schwarz, G., BIOCHEMISTRY, 47, 949-956, 2008.

Small Substrate Transport and Mechanism of a Molybdate ATP Binding Cassette Transporter in a Lipid Environment

Embedded in the plasma membrane of all bacteria, ATP binding cassette (ABC) importers facilitate the uptake of several vital nutrients and cofactors.

The ABC transporter, MolBC-A, imports molybdate by passing substrate from the binding protein MolA to a membrane-spanning translocation pathway of MolB.

To understand the mechanism of transport in the biological membrane as a whole, the effects of the lipid bilayer on transport needed to be addressed.

Continuous wave-electron paramagnetic resonance and in vivo molybdate uptake studies were used to test the impact of the lipid environment on the mechanism and function of MolBC-A. Working with the bacterium Haemophilus influenzae, we found that MolBC-A functions as a low affinity molybdate transporter in its native environment.

In periods of high extracellular molybdate concentration, H. influenzae makes use of parallel molybdate transport systems (MolBC-A and ModBC-A) to take up a greater amount of molybdate than a strain with ModBC-A alone.
In addition, the movement of the translocation pathway in response to nucleotide binding and hydrolysis in a lipid environment is conserved when compared with in-detergent analysis. However, electron paramagnetic resonance spectroscopy indicates that a lipid environment restricts the flexibility of the MolBC translocation pathway.

By combining continuous wave-electron paramagnetic resonance spectroscopy and substrate uptake studies, we reveal details of molybdate transport and the logistics of uptake systems that employ multiple transporters for the same substrate, offering insight into the mechanisms of nutrient uptake in bacteria

Rice, A. J., Harrison, A., Alvarez, F. J. D., Davidson, A. L., and Pinkett, H. W., Small Substrate Transport and Mechanism of a Molybdate ATP Binding Cassette Transporter in a Lipid Environment, Journal of Biological Chemistry, 2014, 289, 15005-15013.