This article was published in Scientific American’s former blog network and reflects the views of the author, not necessarily those of Scientific American
"Have you ever read Ulysses?"
The question catches me off guard. I am interviewing Michael Russell, a geochemist working at NASA's Jet Propulsion Laboratory. Russell was originally trained as an ore prospector, but several twists and turns in his scientific career brought him where geology, chemistry and biology intersect: the origin of life. Decades of research on ancient rocks and modern biochemistry culminated into the hypothesis that life emerged as a self sustaining web of chemical reactions, in hydrothermal vents on the ocean floor. According to Russell and others, life was breathed into these carbonate chimneys by simple molecules seeping from the ocean floor.
We have only covered a handful of molecules and minerals when Russell brings up Ulysses. Confused, I admit there exists a James Joyce shaped gap in my literary knowledge. "Sorry", I say, "I haven't." Russell laughs. "That's all right, I mean who has? But I've finally finished it, and the one thing I got out of it was the word ineluctable. So we now have a paper out that is called 'On the Ineluctable Requirement for Molybdenum in the Origin of Life'."
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Ineluctable. There's no time to let the word sink in; the conversation is moving forward at a dizzying speed. We skirt past supernovas, the problems with primordial soup and endless stacks of turtles. Not until the dust has settled do I look up the relevant passage in Ulysses. None the wiser, I turn to the dictionary. Inevitable. Essential. Unavoidable. Words rarely used to describe molybdenum. Why, of all obscure elements known to man, would molybdenum be necessary for life?
The answer lies with the electrons that whirl around the molybdenum core. Like many other metals, molybdenum is eager to take an extra electron under its wings or give a spare one away. Life has eagerly exploited this ability to juggle electrons around. Cells not only incorporate molybdenum ions into their enzymes, but also zinc, copper, iron and nickel. Many of these metal containing proteins shuttle electrons between molecules, as if they are playing a massive game of hot potato, changing, breaking and building molecules along the way. Electrons truly are what makes life go round. Or, as the Hungarian Nobel prize winner Albert Szent-Györgyi put it: "Life is nothing but an electron looking for a place to rest".
Some electrons find cold and empty beds in their search. Take the methanogens. These bacteria and archaea make a living by stripping electrons from hydrogen (H2) and attaching them onto carbon dioxide (CO2) in several steps, generating methane (CH4) and water (H2O) in the process. That might sound easy enough, but carbon dioxide is not a thankful molecule: it's stable as it is, and very reluctant to accept additional electrons.
This is where molybdenum comes in. Or rather could come in, for while molybdenum plays a role in the conversion of carbon dioxide to methane, the mechanism that Russell proposes has not yet been proven for this particular reaction. Russell's argument boils down to a single point: molybdenum can ease difficult electron transfers because it usually has not one, but two electrons to give away. There's nothing stopping molybdenum from donating these electrons to two different molecules. In this way, the molybdenum ion could compensate for the effort of imposing one electron onto a stubborn naysayer (such as carbon dioxide), by donating the other one to a more willing recipient.
The forking of electrons works because the electron that rolls 'downhill' releases energy that is channelled into pushing the other electron 'up the slope'. Some molybdenum enzymes are known to perform this trick, but no one really knows how widespread such crossed electron transfers really are in biochemistry. In an article published last year, Russell and Wolfgang Nitschke write that electron bifurcation 'is an old, but almost forgotten friend of research'.
How old? In Russell's most recent paper (the one with 'ineluctable' in the title), he and his team suggest as old as life itself. Previous investigations into the age of the molybdenum family were based on genetic sequences alone, and pointed towards a more recent origin. But comparisons between bare genes can paint a misleading picture. Genetic sequences are to proteins what recipes are to cooking: shallow descriptions that lack the finer subtleties of texture and form. This is why Russell and his colleagues compared the three dimensional structure of different molybdenum proteins instead. Structure is more conserved than sequence, which is perfect for exploring ancient relationships.
They found that the roots of molybdenum enzymes run deep. According to structure, most molybdenum proteins can be sorted into two piles: those of archaea and bacteria. The oldest divide in life. Archaea are microorganisms, just like bacteria, but their biochemistry differs like day and night. According to Russell, the presence of molybdenum proteins in both archaea and bacteria means they were also present in the last common ancestor of all life on earth. His conclusion: to master the molecules of metabolism, including both electron lovers and haters, life needed molybdenum (and tungsten, which is chemically similar).
Does the ancient origin of molybdenum enzymes really prove that electron bifurcation by molybdenum was 'ineluctable' for the origin of life? No. When it comes to life's earliest beginnings, some degree of speculation is inevitable. "Some of the time, you're just going to be wrong", says Russell. "We try to approach the truth, but we have to face up with the fact that we cannot be right all the time." While molybdenum might not be the key to our origins, it still holds a clue to the larger riddle, a tiny puzzle all in itself.
Russell has one more argument to persuade me that molybdenum really does hold the answer to life, the universe and everything. Near the end of our conversation, he asks whether I have read the Hitchhiker's Guide to the Galaxy. "The atomic number of Molybdenum is 42." I can almost hear the grin on the other end of the line.
Images:
Lost City courtesy Deborah Kelley (University of Washington)
Molybdenum crystal and cube by Alchemist-hp.
Superimposed molybdenum enzymes from reference.
Reference:
Schoepp-Cothenet, B., van Lis, R., Philippot, P., Magalon, A., Russell, M., & Nitschke, W. (2012). The ineluctable requirement for the trans-iron elements molybdenum and/or tungsten in the origin of life Scientific Reports, 2 DOI: 10.1038/srep00263