the black and white of me: Other catalyst for hydrogen…

Hmm interesting… once again, man gets technology from nature.

Iron and carbon monoxide are the crucial ingredients that nature uses to process hydrogen, according to researchers. Resolving the structure of the last of the three known hydrogenase enzymes has excited chemists, who are keen to follow nature's clear advice and develop their own hydrogen catalysts for energy applications.

The dream of replacing oil with hydrogen is in danger of stalling without a cheap and clean way to make it and release its stored chemical energy. The best synthetic catalysts use platinum to do those jobs — which involve splitting hydrogen molecules into ions, or recombining the ions to make molecules again. But this rare, expensive metal is hardly the answer to sustainable living.

Nature manages the process with a cheap metal – iron. There are three iron-containing hydrogenase enzymes to choose from. All three are found in unrelated organisms that have evolved their enzymes separately. Two have been found in bacteria in soil and oil wells; the third is used to provide energy for certain microbes living around hydrothermal vents, by combining hydrogen and carbon dioxide to make methane.

The first two contain a pair of metal atoms at their active site — either iron–iron, or iron–nickel — buried deep inside the enzymes' structures. But researchers in Germany, led by Seigo Shima at the Max Planck Institute for Terrestrial Microbiology in Marburg, and Ulrich Ermler at the Max-Planck Institute for Biophysics in Frankfurt, have now achieved what others had failed to do — figure out the detailed structure of the third hydrogenase, known as [Fe] hydrogenase.

Mix and match

The active site in [Fe] hydrogenase is light sensitive, so previous crystal structures had shown just the bare skeleton of the enzyme, leaving biochemists in the dark about exactly how it worked.

To get a high-quality crystal of the enzyme, Shima and Ermler's groups first took the active part of the hydrogenase from Methanothermobacter marburgensis, and separately extracted the enzyme without its active component from another organism, Methanocaldococcus jannaschii.

The active site of [Fe] hydrogenase hooks up with carbon monoxide, water and an unknown ligand (Unk).SCIENCE

They then reconstituted the whole active enzyme by carefully mixing the two together in a dark, air-free environment. This gave enough material to grow a crystal of the intact enzyme, the structure of which is published in Science¹.

[Fe] hydrogenase's active site has just one iron atom, linked to two CO groups and two other organic groups. The active site also contains an as-yet uncharacterized ligand, and another vacant site which, in the team's structure, is occupied by water (see graphic, right).

The structure shows that the enzyme splits hydrogen in a different way to the other two hydrogenases. Normally, a molecule of hydrogen is split by a metal at the centre of the enzyme, leaving positive and negative hydrogen ions. The positive ion is whipped away, whereas the negatively charged hydride has its two electrons removed to make another positively charged ion.

Once, twice, three times an enzyme

But [Fe] hydrogenase has an active site that sits near the edge of the enzyme. When the hydrogen molecule is broken up, the negative hydride is quickly grabbed by an organic molecule that also sits near the enzyme's surface. This mopped-up hydride is eventually used by the host organism to make methane.

"It works quite differently, it's very surprising," says Tom Rauchfuss, a catalysis expert at the University of Illinois at Urbana-Champaign. But the three hydrogenases have an obvious similarity — all three active sites contain an iron atom stuck to a CO group. "This is almost a religious moment," says Rauchfuss. "This is nature saying three times: I like iron and CO

Both metal and ligand seem to be crucial, says Juan Fontecilla-Camps at Joseph Fourier University in Grenoble, France. "These active sites are the only ones known that have CO bound to metal," he says. "The iron–CO unit is unique to hydrogen metabolism."

John Peters, an expert on the iron–iron hydrogenase enzyme at Montana State University in Bozeman, reckons that organic chemists will now look for ways to make the molecule that mops up the freshly minted hydride ion, whereas inorganic chemists will try to produce a range of structures based on iron and CO. Between them, they might just take nature's hint and create a catalyst to keep the hydrogen dream alive.