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.

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.

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.”

What Next?

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