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A Radical Intermediate in Tyrosine Scission to the CO and CN Ligands of FeFe Hydrogenase

Science  25 Oct 2013:
Vol. 342, Issue 6157, pp. 472-475
DOI: 10.1126/science.1241859

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Piecing Together Hydrogenase

Microbial hydrogenase enzymes generally use iron to catalyze the reversible formation of hydrogen from protons and electrons. Key to their efficiency is a set of iron-coordinating ligands, including CO and cyanide. Kuchenreuther et al. (p. 472) examined how the HydG maturase enzyme breaks down the amino acid tyrosine to derive these diatomic ligands for assembly of the diiron class of hydrogenases. The first step involves abstraction of an H atom from the phenolic OH substituent of the side chain. Electron paramagnetic resonance spectroscopy revealed a radical intermediate that subsequently results from heterolysis of the bond tethering the side chain to the α-carbon. With the side chain thus jettisoned, the residual dehydroglycine could be transformed into CO and CN.

Abstract

The radical S-adenosylmethionine (SAM) enzyme HydG lyses free l-tyrosine to produce CO and CN for the assembly of the catalytic H cluster of FeFe hydrogenase. We used electron paramagnetic resonance spectroscopy to detect and characterize HydG reaction intermediates generated with a set of 2H, 13C, and 15N nuclear spin-labeled tyrosine substrates. We propose a detailed reaction mechanism in which the radical SAM reaction, initiated at an N-terminal 4Fe-4S cluster, generates a tyrosine radical bound to a C-terminal 4Fe-4S cluster. Heterolytic cleavage of this tyrosine radical at the Cα-Cβ bond forms a transient 4-oxidobenzyl (4OB) radical and a dehydroglycine bound to the C-terminal 4Fe-4S cluster. Electron and proton transfer to this 4OB radical forms p-cresol, with the conversion of this dehydroglycine ligand to Fe-bound CO and CN, a key intermediate in the assembly of the 2Fe subunit of the H cluster.

  • These authors contributed equally to this work.

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