[meteorite-list] X-Ray Emission Crack Mystery Atom in Enzyme Critical for Life

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu, 17 Nov 2011 14:50:16 -0800 (PST)
Message-ID: <201111172250.pAHMoG0D000021_at_zagami.jpl.nasa.gov>

Chronicle Online e-News

X-ray emission cracks mystery atom in enzyme critical for life
http://www.news.cornell.edu/stories/Nov11/debeerCarbon.html

Nov. 17, 2011

By Anne Ju
amj8 at cornell.edu

Like a shadowy character just hidden from view, a mystery atom in the
middle of a complex enzyme called nitrogenase had long hindered
scientists' ability to study the enzyme fully.

But now an international team of scientists led by Serena DeBeer,
assistant professor of chemistry and chemical biology, has pulled back
the curtain using powerful synchrotron spectroscopy and computational
modeling to reveal carbon as the once-elusive atom.

The research was published online Nov. 17 in the journal Science.

"For chemists, one of the first steps you want to be able to take is to
actually model the site," DeBeer said. "It turns out that the chemistry
of how this cluster behaves will be different depending on what atom is
in the middle. This is the first step toward trying to unravel its
mechanism."

Why nitrogenase? In nature, all life requires the element nitrogen from
the atmosphere to form amino acids and build proteins. Bacteria need to
convert nitrogen to ammonia as a precursor to more complex biosynthetic
processes. The enzyme that catalyzes all this is nitrogenase, and it
does it by breaking one of the strongest bonds in chemistry -- the
nitrogen triple bond.

The chemical industry has figured out how to convert nitrogen to
ammonia in high-temperature and high-pressure industrial environments.
There's a fascination with understanding how the enzyme makes this same
process work in nature, DeBeer said.

DeBeer and colleagues honed in on a subset of atoms in the relatively
large enzyme, called the iron-molybdenum cofactor, which was thought to
be the site where dinitrogen (N2) gets converted to ammonia, and where
the mystery atom is situated inside.

The team used a method called X-ray emission spectroscopy (XES) at the
Stanford Synchrotron Radiation Light Source to excite the electrons in
the cofactor's iron cluster and to watch how electrons refilled the
spots, called "holes," they left behind. The holes were sometimes
filled by an electron belonging to a neighboring atom -- emitting X-ray
signatures with distinct ionization potentials that would distinguish
between different kinds of atoms.

This was how it was revealed that the cofactor contained a carbon atom,
rather than a nitrogen or an oxygen atom, that was bound to the iron
atoms in the cluster.

The paper's first author is Kyle M. Lancaster, a postdoctoral associate
in chemistry. DeBeer's collaborators are at the University of Bonn in
Germany, University of California-Irvine, Max Planck Institute and the
SLAC National Accelerator Laboratory at Stanford.

The research was supported by Cornell, University of Bonn, the Max
Planck Society and the National Institutes of Health.
Received on Thu 17 Nov 2011 05:50:16 PM PST