In Douglas Adams' book, The Hitchhiker's Guide to the Galaxy, the giant super-computer, Deep Thought, gave the answer to Life, the Universe and Everything as 42. Perhaps it was thinking of molybdenum, an element which is absolutely essential to life on Earth, and perhaps the rest of the Universe.
Molydenum, whose Atomic Number is 42, is the middle member of the vertical triad, chromium, molybdenum, tungsten within the transition metals, in Group 6 of the Periodic Table. Its name is derived from 'molybdos', which is Greek for 'lead'. This is because, like graphite (pencil lead) - an allotrope of carbon - and lead ore, with which it was often confused, it was capable of leaving a mark on paper.
Its status as an element in its own right was first demonstrated by Carl Wilhelm Scheele in 1778, although it was not isolated until 1782, this being achieved by Peter-Jakob Hjelm.
Molybdenum is relatively scarce in the Earth's crust, being present at about 1.2 milligrams per kilogram, making it only about three times more abundant than gold. The principal ore is the sulphide, molybdenite, of which world production is about 80,000 tonnes per year, a high proportion coming from the USA. Industrially, however, it is usually produced as a by-product of copper production, the normal process being for the sulphide MoS2 to be 'roasted' in air to form the oxide MoO3; which is often used as an additive to steel and other alloys where it confers strength and corrosion resistance.
Molybdenum is silvery-white metal and soft when pure. However, it has a high melting point (2625°C), and molybdenum and molybdenum alloys have useful strength at elevated temperatures. Molybdenum therefore has application in high temperature steels, nuclear reactors, jet engines, lamp filaments, valves and electric furnaces. It forms very hard alloys which are highly resistant to corrosion, and so these are used in high-speed cutting tools and in heavy duty electrical contacts. It is also used as a catalyst for hydrodesulphurisation1 in the refining of petroleum.
Of all its inorganic compounds, perhaps the most important is the disulphide (MoS2) which is used as a solid lubricant additive either alone or added to oils and greases under the trade name, 'Molyslip'. This is able to operate effectively at high temperatures without igniting, as oils would. A further property of molybdenum disulphide as a lubricant is that, in contrast to graphite, it works in a dry atmosphere and in a vacuum, thus making it useful in aerospace applications.
Biological Role of Molybdenum
Molybdenum is the only second-row transition metal that is required by most living organisms, from lowly bacteria to human beings. Those few species that do not require molybdenum use tungsten, which lies immediately below molybdenum in the periodic table. Both these metals have a high chemical versality and an unusually high bioavailability and hence, over the course of biological evolution, have been incorporated into the active sites of enzymes. Molybdenum is thus an essential element for animals, plants and micro-organisms. In each case the metal atom plays the same role, acting as a donor or acceptor of electrons in redox systems2.
The average human takes in about 0.01 milligram a day (0.3g in a lifetime) and stores about 5 milligrams in the body. Traces of molybdenum are required by leguminous plants, such as clover, peas and beans, which have nitrogen-fixing bacteria living symbiotically in their root nodules. These bacteria possess enzymes called nitrogenases which convert atmospheric nitrogen gas into ammonia and subsequently into organic nitrogen compounds such as nucleic acids, amino acids, and proteins.
Higher animals access their nitrogen requirements by way of ingesting proteins from plant material or from other animals lower in the food chain.
Molybdenum nitrogenase3 has two highly complex metal centres: one containing iron and sulphur; the other iron, sulphur and molybdenum. Given that the 'Central Dogma of Molecular Biology' is that 'DNA makes RNA makes proteins', then it can be appreciated that if there is no molybdenum, then there will be no proteins and therefore no life! If soil lacks molybdenum, then the land is barren.The reduction of nitrogen to ammonia represents an extraordinary biochemical capacity. Molecular nitrogen (N2) contains a nitrogen-to-nitrogen triple bond whose Bond Dissociation Energy is 945 kJ mol-1. Compare this with the energy required to dissociate the carbon-to-carbon triple bond in ethyne, which is 808.3 kJ mol-1. Thus, strenuous conditions are required for reduction of molecular nitrogen in the laboratory, yet leguminous plants manage to reduce over a million tonnes of nitrogen per year4 under very mild conditions!
Although the number of molybdenum metalloenzymes can be counted in dozens, only three are known to be important in humans, these being:
Xanthine oxidase. This is present mainly in the liver. This enzyme catalyzes the oxidative hydroxylation of purines and pyridines, including conversion of hypoxanthine to xanthine, and also the further oxidation of xanthine to uric acid. This enzyme therefore plays a vital role in waste processing in animals.
Sulphite oxidase. This catalyses the oxidation of sulphite to sulphate, thus detoxifying the sulphite which arises from metabolism of sulphur-containing amino acids such as cysteine, from ingestion of hydrogen sulphite (bisulphite, HSO3-) preservative and from inhalation of sulphur dioxide, an atmospheric pollutant. Deficiency or absence of sulphite oxidase leads to neurological symptoms and early death.
Aldehyde oxidase, oxidises purines, pyrimidines and pteridines and is involved in the metabolism of nicotinic acid (niacin, part of the Vitamin B complex).
Molybdenum deficiency in humans is extremely rare and only happens in the most extreme circumstances. Indeed, the only documented case of acquired molybdenum deficiency occurred in a patient with Crohn's Disease on long-term Total Parenteral Nutrition (TPN)5 where molybdenum was not included. The patient developed rapid heart and respiratory rates, headache, night blindness, and eventually became comatose. Happily, the patient recovered after being given a molybdenum supplement to the TPN. Most human beings acquire their molybdenum requirements from their diet, important sources including legumes, green leafy vegetables, liver and nuts.
Molybdenum toxicity in grazing animals (ruminants) can occur when they graze on soil which is abundant in molybdenum but nutritionally deficient in copper. Ruminants tend to inadvertently ingest a lot of dirt, which contains molybdenum. This can then interfere with copper uptake, resulting in loss of hair growth. This was graphically described in the James Herriott book, Vet in Harness, where Mrs Dalby's stirks6 had the appearance of wearing spectacles. The treatment was to administer copper sulphate solution per os.