We all know that metals like copper, iron and zinc are needed to maintain human health. Molybdenum is also an essential nutritional requirement, used by several enzymes in the body to help metabolize carbon, nitrogen and sulfur compounds. Most other life forms use molybdenum in similar ways. But a one-celled organism that lives in deep-sea volcanic vents has developed an alternative metabolism that uses tungsten instead of molybdenum.
Called Pyrococcus furiosus, the name means "rushing fireball" and refers to the microorganism's quick rate of reproduction P. furiosus can double its numbers in just 37 minutes - and its preferred temperature of around 100 C (212 F), the boiling point of water.
Such high temperatures will kill most organisms, because extreme heat causes the body's proteins to break down. The proteins of hyperthermophilic archaea like P. furiosus are resistant to heat, however. Such hot water also tends to have very little dissolved oxygen, but this does not trouble P. furiosus because it is an anaerobic organism.
Tungsten, often described as the "metal from another world" because of its high melting point (3,422 C, or 6,192 F), is better known for its use in light bulbs than in life systems. But the element is very similar to molybdenum in many respects, and thus can be utilized by P. furiosus in similar ways.
Michael Adams and his team at the University of Georgia have purified four tungsten-containing enzymes used by P. furiosus. Adams says that the genome sequence of P. furiosus suggests it may contain a fifth tungstoenzyme, but that enzyme has not yet been characterized.
Enzymes are protein molecules that act as catalysts in biochemical reactions, spurring the reactions to work faster and more efficiently. An enzyme has a certain shape that only a particular chemical substance (the "substrate") can fit into. The enzyme is like a "lock" which only accepts a certain substrate "key." Once these two components come together, certain chemical bonds within the substrate molecule are activated.
Adams says two of the tungstoenzymes of P. furiosus are involved in amino acid metabolism, in much the same way as some molybdoenzymes. One of the tungstoenzymes is involved in carbohydrate metabolism, but is unlike molybdoenzymes because the reaction it catalyses is different. The function of the fourth purified tungstoenzyme is still unclear.
|This image shows a shadowed image of Pyrococcus furiosus with its many flagella.
While tungsten and molybdenum make up part of their respective enzymes in a similar fashion, the tungstoenzymes in P. furiosus are not the same shape as molybdoenzymes. Adams says this difference suggests that tungstoenzymes and molybdoenzymes probably evolved independently.
"The situation is similar to heme-containing enzymes," says Adams. Heme-containing enzymes, also known as hemoproteins, are a class of enzymes found in all mammalian cells (hemoglobin is one of the better known hemoproteins). "There are many different types of hemoproteins, with very different functions, and the various types can show little if any sequence similarity. Similarly, the molybdenum- and tungsten-containing families evolved separately, even though they contain a common cofactor."
There are many different types of molybdoenzymes they are used for various functions by everything from plants to animals to bacteria to archaea but all these different enzymes contain molybdenum at the same kinds of sites.
A very small number of these molybdoenzymes are also able to use tungsten. In these unusual enzymes, tungsten and molybdenum are both utilized at the same site. The first such enzyme was discovered by Lars Ljungdahl at the University of Georgia in 1983, in a thermophile called Clostridium thermoaceticum. Since this first discovery, more than a dozen of these enzymes have been isolated and characterized in bacteria and archaea. This type of enzyme shows genetic sequence similarity - and therefore is closely related - to molybdoenzymes.
The tungstoenzymes of P. furiosus are unique because they are not able to use molybdenum at all instead, they only use tungsten. These enzymes show no genetic or structural relationship to the huge class of molybdenum-containing enzymes - not even to the molybdoenzymes like Ljungdahl's that also contain tungsten.
"The tungstoenzymes of P. furiosus are very distinct evolutionarily from all molybdoenzymes and the few tungsten versions that are known," says Adams. "Yet even in the P furiosus enzymes, the way in which the tungsten is bound to the enzyme is very similar to the way it is in all molybdoenzymes - even though the rest of the enzyme structures are completely different."
"Molybdenum and tungsten enzymes take similar steps in the metabolic pathway," says Edward Stiefel, Professor of Chemistry at Princeton University. "They have the capability of playing the same roles. What is really interesting is that the rest of the proteins which make up the largest part of the entity are not at all similar. Thus, molybdenum and tungsten enzymes seem to point to a case of convergent evolution. Nature picked related elements to perform similar functions."
Many scientists believe studying life at deep-sea volcanic vents could teach us about early life on Earth. Because the hyperthermophilic archaea that colonize these vents are thought to be one of the slowest-evolving organisms, they may be the best living representatives of the Earth's earliest inhabitants.
|P. furiosus (the rushing fireball) is found in the marine sand surrounding sulfurous volcanoes.
Hydrothermal vents are rich in tungsten and scarce in molybdenum. The vents expel great quantities of sulfide, and molybdenum precipitates (turns into a solid) when exposed to sulfide. Tungsten, on the other hand, tends to remain soluble in the presence of sulfide.
Molybdenum becomes soluble when exposed to oxygen, so in normal seawater away from the anoxic, sulfidic vents - molybdenum is abundant.
Perhaps ancient tungsten-using organisms evolved into today's molybdenum-using creatures. Before oxygen became abundant on Earth, the oceans may have been full of sulfur and tungsten. The earliest sea creatures would've been able to use the tungsten much as P. furiosus does today, while molybdenum would have been in its inaccessible solid form. But once cyanobacteria began saturating the atmosphere and oceans with oxygen, molybdenum became soluble, eventually becoming more abundant in the oceans than tungsten. Organisms evolved to adjust to the difference, and molybdenum eventually replaced tungsten in most metabolic processes.
"Biology is very resourceful," says Stiefel. "You never know exactly how Nature is going to compensate, how it is going to replace one thing with something else."
Adams says his lab is currently trying to understand the physiological roles of the tungstoenzymes in P. furiosus.
"Our studies are aimed at understanding what the other three enzymes do in the cell, how they are regulated, and what the nature is of the fifth tungstoenzyme. To this end, we have recently constructed DNA microarrays containing all 2,200 genes of Pyrocccus and are using these to evaluate how all of these genes - and particularly those of the five tungstoenzymes - behave under a variety of growth conditions"
In a related project, Adams and his team are investigating the role and nature of tungsto- and molybdoenzymes in the hyperthermophile Pyrobaculum aerophilum.