Download W for Wolfram: Molybdenum(Mo) and Tungsten(W) in Biology and

W for Wolfram: Molybdenum(Mo) and Tungsten(W) in Biology and the Environment
1. Metals are crucial to biological systems but toxic in their free state; their use and
storage in cells are highly controlled and regulated with specialised transport
proteins.
2. The bioavailability of the element in the geosphere/biosphere interface
during the initial development of the metalloprotein as well as the pressure to
evolve multiple biochemical pathways to secure the viability of critical
cellular functions, led to the diversity of metals that are used in life in the
same chemical reaction.
3. Mo-containing enzymes are found in all aerobic organisms but W-containing
enzymes are only found in obligate or strict anaerobes that are typically
hyperthermophilic because tungsten compounds are oxygen sensitive
4. The environment of early Earth, before the appearance of oxygen gas, was
hot, anaerobic and highly reducing, therefore Mo and W were present as
MoS2 and WS2, with tungsten sulphides being more soluble in water and thus
would have been more available for organisms.
5. W, not Mo was also probably acquired first my living organisms because WS bonds, that of which are typically found in W-containing enzymes, are
more stable than Mo-S bonds.
6. As Earth’s crust cooled, and as its atmosphere became aerobic due to
photosynthic organisms, Mo and W became present as MoO4 and WO4, with
molybdenum oxides being more soluble in water, thus becoming more
available for organism.
7. Intracellular redox poise of aerobic organisms were higher than anaerobic
organisms, therefore Mo-containing active sites in enzymes became more
suitable for life because they had higher redox potentials.
8. Tungsten is relatively scarce in Nature, but certain types of natural waters,
including hot spring waters and deep sea hydrothermal vents are enriched
with tungsten.
9. Deep sea vents originate from convective currents of sea water through
newly formed oceanic crust at seafloor spreading centres and eject
superheated fluids highly enriched with sulphides and various metals like
tungsten, therefore tungsten is present in vent fluids at very much higher
concentrations than sea water.
10. However, the hydrothermal cycling process of sea water at the vents causes
molybdenum to be removed instead, and molybdenum levels of vent fluids
are lower than that of sea water.
11. Vents reach more than 200m in diameter and 50m in height, and support
complex ecosystems that ultimately depend on chemolithoautotrophic microorganisms that use vent fluids as sources of minerals, sulphur and CO2.
12. Tungsten is known to be toxic in plants like rye grass because they replace the Mo
in nitrate reductase with W.
13. Certain plants like tomato plants can discriminate against the uptake of W in the
presence of Mo, and some plants are able to accumulate and sequester W in
peripheral tissues, without any detrimental effects.
14. Tungsten is moderately toxic to mammals but research is sketchy, however high
levels of tungsten have been linked to childhood leukaemia in a few tungsten
mining towns in the US.
15. The predominant health hazard of W to humans relates to those in the tungsten
carbide industry where an alloy of WC2 and Chromium, “hard metal”, is produced;
hard metal dust induces lung and skin diseases, and prolonged exposure causes
neuropsychological problems as well.