SLAC Research cracks puzzle enzyme critical food supply

SLAC Research cracks puzzle of enzyme critical to food supply

2:32 AM, 18th November 2011
SLAC Research cracks puzzle of enzyme critical to food supply
From L to R: Serena DeBeer, Michael Roemelt and Frank Neese of the Max Planck Institute. The three are among the authors on a November 18, Science paper identifying a key atom inside the part of the nitrogenase enzyme where atmospheric nitrogen is converted into a form that living things can use.

MENLO PARK, US: If we could make plant food from nitrogen the way nature does, we’d have a much greener method for manufacturing fertilizer – a process that requires such high temperatures and pressures that it consumes about 1.5 per cent of the world’s energy.

Now, scientists working at Department of Energy’s SLAC National Accelerator laboratory have taken an important step towards understanding how nature performs this trick, by identifying a key atom that researchers had sought for more than a decade. The atom lies at the heart of an enzyme called nitrogenase, which plays a critical role in converting nitrogen in air into a form that living things can use. Scientists have long sought to determine structure of this enzyme. “The fascination with this enzyme is the fact that it enables this reaction to take place at room temperature and atmospheric pressure,” said Serena DeBeer, Chemist, Cornell University and Max Planck Institute for Bioinorganic Chemistry, who led the team at SLAC. In the Nov 18 issue of Science, two independent teams, using different approaches, identified the atom as carbon.

It had eluded scientists because of its sequestered location inside a cluster of metal atoms. The key in the team’s research was a technique called X-ray emission spectroscopy (XES), which co-author Uwe Bergmann of SLAC had developed.

 

A view of the critical cluster of atoms in the nitrogenase enzyme where atmospheric nitrogen is converted to ammonia. After more than a decade, scientists have finally identified the central atom as carbon. (C) Science

The researchers needed a trick to find the one important carbon inside the metal cluster. They used an intense beam of X-rays to knock innermost electrons out of iron atoms in the cluster. Normally other electrons from iron would fill this hole; but there was a tiny chance, much less than one in a thousand, that the hole would be filled by an electron belonging to a neighbouring atom and thus emit X-rays characteristic of the neighbour’s identity. It was this subtle feature in X-ray emission spectrum that revealed that a carbon atom, rather than a nitrogen or oxygen, was bound to the iron atoms in the cluster. “This was a simple but important question and we were able to give a straightforward answer,” said Bergmann.

The cluster of metal atoms is where nitrogen molecules from air are broken down and converted to ammonia and other compounds by microbes. Then plants take it up and spread it through the food chain. This is how we get roughly half of the nitrogen in our bodies; the rest comes from artificial fertilizers. Researchers knew a decade ago that the central atom in metal cluster must be nitrogen, oxygen or carbon. Each would affect the reaction differently.

“Because it’s sequestered in middle of a bunch of metal atoms and you’ve got no way to get your hands on it, it’s a really hard problem,” said Brian Hoffman, Chemist of Northwestern University, not involved in these studies. “What the team has done would appear to be a classic case where new technology leads to new science.”

(C) SLAC National Accelerator Laboratory News

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