Totally Radical: Unique Chemistry Needed to Make a Biological Hydrogen Catalyst

Key steps revealed in assembling the active site of a hydrogen-generating catalyst in bacteria.

EPR spectra (A) with generated with isotope labeled tyrosines show that a tyrosine radical (B) is cleaved to form an oxobenzyl radical (C) as an intermediate in forming the CO and CN Fe ligands in FeFe hydrogenase.
Image courtesy of University of California, Davis
EPR spectra (A) with generated with isotope labeled tyrosines show that a tyrosine radical (B) is cleaved to form an oxobenzyl radical (C) as an intermediate in forming the CO and CN Fe ligands in FeFe hydrogenase.

The Science

Using state-of-the-art magnetic resonance approaches, researchers revealed details of how potentially poisonous chemicals, carbon monoxide and cyanide, are safely incorporated into the active site of a bacterial hydrogen-generating catalyst.

The Impact

Understanding the mechanisms and chemistry bacteria use to make hydrogen may allow scientists to imitate the chemical process in the laboratory and possibly lead to new approaches for easier, more economical production of hydrogen fuel.

Summary

Microbial hydrogenase enzymes generally use iron to catalyze the reversible formation of hydrogen from protons and electrons. Key to hydrogenase enzyme efficiency is a set of iron-coordinating ligands that include carbon monoxide (CO) and cyanide (CN). The CO and CN-ligands are derived from the amino acid tyrosine in a reaction catalyzed by a radical S-adenosylmethionine enzyme termed HydG but scientists did not know the intermediate steps of how the tyrosine is broken to form these necessary ligands. Using electron paramagnetic resonance (EPR) spectroscopy, BES-funded researchers at University of California, Davis detected and characterized the HydG reaction intermediates, finding a radical intermediate of tyrosine bound to a cluster of 4 iron atoms and 4 sulfur atoms in the active site of the hydrogenase. Their results show that the HydG enzyme breaks a bond tethering a side chain carbon on the tyrosine, leaving the leftover components, CO and CN groups, now linked to the iron. While radicals can be considered a problem in other systems, in this case, the HydG enzyme harnesses the powerful radical to drive the chemical reaction. Keeping the CO and CN molecules bound to Fe also avoids the inherent toxicity of free CO and CN within the cell. Using complementary state-of-the-art approaches, the researchers are continuing to study this unique, new chemistry in hopes of revealing further details in how the hydrogenase is constructed and functions.

Contact

R. David Britt
Department of Chemistry, University of California, Davis, Davis CA 95616
[email protected]

Funding

This work was funded by the Division of Chemical Sciences, Geosciences, and Biosciences (R. David Britt award no. DE-FG02-11ER16282) and the Division of Material Sciences and Engineering (James R. Swartz award no. DE-FG02-09ER46632) of the Office of Basic Energy Sciences of the U.S. Department of Energy.

Publications

Kuchenreuther, JM, et al. “A Radical Intermediate in Tyrosine Scission to the CO and CN Ligands of FeFe Hydrogenase.” Science 342, 472 (2013). [DOI: 10.1126/science.1241859]

Related Links

University of California, Davis Press Release

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