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UC Davis, Stanford team proposes reaction mechanism for key step in assembly of hydrogen-generating bacterial catalyst

Researchers at the University of California, Davis, and Stanford University are proposing a reaction mechanism for one of the key steps in assembling the hydrogen-generating bacterial catalyst hydrogenase. They report on their work in a paper in the journal Science.

Hydrogenase-lg
This hydrogen-generating cluster of iron (brown) and sulfur (yellow) atoms, with side groups of carbon monoxide (gray/red) and cyanide (gray/blue), could be a key to future fuel sources. (Protein Data Bank/courtesy graphic) Click to enlarge.

The bacterial catalysts are based on precisely organized clusters of iron and sulfur atoms, with side groups of cyanide and carbon monoxide. Those molecules are highly toxic unless properly controlled, noted David Britt, professor of chemistry at UC Davis and co-author on the paper.

The cyanide and carbon monoxide groups were known to come from the amino acid tyrosine, Britt said. Jon Kuchenreuther, a postdoctoral researcher in Britt’s laboratory, used a technique called electron paramagnetic resonance (EPR) to study the structure of the intermediate steps. (EPR is a sophisticated spectroscopic technique that detects free radicals in chemical and biological systems.)

They found a series of chemical reactions involving a type of highly reactive enzyme called a radical S-adenosylmethionine (SAM) enzyme. The tyrosine is attached to a cluster of four iron atoms and four sulfur atoms, then cut loose leaving the cyanide and carbon monoxide groups behind.

This enzyme directs the radical chemistry, along with the production of normally poisonous CO and CN, along safe and productive pathways, Britt said.

Kuchenreuther, Britt and colleagues also used another technique, Fourier Transform Infrared to study how the iron-cyanide-carbon monoxide complex is formed. That work will be published separately.

It’s pretty interesting that bacteria can do this. We want to know how nature builds these catalysts—from a chemist’s perspective, these are really strange things. Together, these results show how to make this interesting two-cluster enzyme. This is unique, new chemistry.

—David Britt

Britt’s laboratory houses the California Electron Paramagnetic Resonance center (CalEPR), the largest center of its kind on the west coast.

Other authors on the paper are: at UC Davis, postdoctoral researchers William Myers and Troy Stich, project scientist Simon George and graduate student Yaser NejatyJahromy; and at Stanford University, James Swartz, professor of chemical engineering and bioengineering. The work was supported by grants from the US Department of Energy.

Resources

  • Jon M. Kuchenreuther, William K. Myers, Troy A. Stich, Simon J. George, Yaser NejatyJahromy, James R. Swartz, and R. David Britt (2013) “A Radical Intermediate in Tyrosine Scission to the CO and CN Ligands of FeFe Hydrogenase,” Science 342 (6157), 472-475 doi: 10.1126/science.1241859

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