Molybdenum in Biology - An Essential Trace Element
Essential role of molybdenum
Molybdenum is an essential trace element for several enzymes important to animal and plant metabolism: mammalian xanthine oxidase/xanthine dehydrogenase, aldehyde oxidase, sulfite oxidase, formate dehydrogenase, nitrate reductase and nitrogenase. Molybdenum functions as an electron carrier in those enzymes that catalyse the reduction of nitrogen and nitrate. Molybdenum is essential to plants being necessary for plant production, even though present in plant tissue at a level much lower (0.5 ppm dry matter basis) than the critical levels for other essential elements.
Molybdenum is essential to humans. Molybdenum is needed for at least three enzymes. Sulfite oxidase catalyses the oxidation of sulfite to sulfate, necessary for metabolism of sulfur amino acids. Sulfite oxidase deficiency or absence leads to neurological symptoms and early death. Xanthine oxidase catalyses oxidative hydroxylation of purines and pyridines including conversion of hypoxanthine to xanthine and xanthine to uric acid. Aldehyde oxidase oxidises purines, pyrimidines, pteridines and is involved in nicotinic acid metabolism. Low dietary molybdenum leads to low urinary and serum uric acid concentrations and excessive xanthine excretion.
Turnlund, J.R., Keyes, W.R., Peiffer, G.L., Chiang, G., Molybdenum Absorption, Excretion, And Retention Studied With Stable Isotopes In Young Men During Depletion And Repletion, American Journal Of Clinical Nutrition , 1995, 61 ,1102-1109.
The metabolic function of molybdenum (and other elements) has been reviewed.
Spears, J.W., Reevaluation of the metabolic essentiality of the minerals -Asian-Australasian Journal Of Animal Sciences , 1999, 12 ,1002-1008.
Molybdenum and the origin of life - molybdenum in pre-biotic chemistry
See also nitrogenase.
Stepwise oxygenation of the Proterozoic ocean ― molybdate as a marker
Oxygenation of the Earth's atmosphere is thought to have proceeded in two broad steps near the beginning of the Proterozoic eon (2,500 million years ago) and its end (542 million years ago). The oxidation state of the Proterozoic ocean between its beginning and its end and the timing of deep-ocean oxygenation have important implications for the evolutionary course of life on Earth. A new perspective on ocean oxygenation based on the authigenic accumulation of molybdenum in sulfidic black shales is presented.
By 2,650 Myr ago accumulation of authigenic molybdenum from sea water is already seen in shales. The small magnitudes of these enrichments reflect weak or transient sources of dissolved molybdenum before about 2,200 Myr ago, consistent with minimal oxidative weathering of the continents.
At roughly 2,150 Myr ago, more than 200 million years after the initial rise in atmospheric oxygen, in deposited shales enrichments appear which are indicative of persistent and vigorous oxidative weathering.
After about 1,800 Myr ago expansion of sulfidic conditions maintained a mid- Proterozoic molybdenum reservoir at below 20 per cent of the modern concentration, which in turn may have acted as a nutrient feedback limiting the spatiotemporal distribution of euxinic ( sulfidic) bottom waters and perhaps the evolutionary and ecological expansion of eukaryotic organisms(10).
By 551 Myr ago, molybdenum contents reflect a greatly expanded oceanic
reservoir due to oxygenation of the deep ocean and corresponding decrease in
sulfidic conditions in the sediments and water column.
Scott, C., Lyons, T. W., Bekker, A., Shen, Y., Poulton, S. W., Chu, X., and Anbar, A. D., Tracing the, Nature, 2008, 452, 456-4U5.
See also
Pearce, C. R., Cohen, A. S., Coe, A. L., and Burton, K. W., Molybdenum isotope evidence for global ocean anoxia coupled with perturbations to the carbon cycle during the early Jurassic, Geology, 2008, 36, 231-234.
Molybdenum in pre-biotic chemistry-the nitrogen cycle
The nitrogen cycle provides essential nutrients to the biosphere, but
its antiquity in modern form is unclear. In a drill core though homogeneous
organic- rich shale in the 2.5- billion- year- old Mount McRae Shale,
Australia, nitrogen isotope values vary from +1.0 to +7.5 per mil (parts per
thousand) and back to +2.5 parts per thousand over similar to 30 meters. These
changes evidently record a transient departure from a largely anaerobic to an
aerobic nitrogen cycle complete with nitrification and denitrification.
Complementary molybdenum abundance and sulfur isotopic values suggest that
nitrification occurred in response to a small increase in surface- ocean oxygenation.
These data imply that nitrifying and denitrifying microbes had already evolved
by the late Archean and were present before oxygen first began to accumulate in
the atmosphere
