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The HydG Enzyme Generates an Fe(CO)2(CN) Synthon in Assembly of the FeFe Hydrogenase H-Cluster

Science  24 Jan 2014:
Vol. 343, Issue 6169, pp. 424-427
DOI: 10.1126/science.1246572

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Abstract

Three iron-sulfur proteins–HydE, HydF, and HydG–play a key role in the synthesis of the [2Fe]H component of the catalytic H-cluster of FeFe hydrogenase. The radical S-adenosyl-l-methionine enzyme HydG lyses free tyrosine to produce p-cresol and the CO and CN ligands of the [2Fe]H cluster. Here, we applied stopped-flow Fourier transform infrared and electron-nuclear double resonance spectroscopies to probe the formation of HydG-bound Fe-containing species bearing CO and CN ligands with spectroscopic signatures that evolve on the 1- to 1000-second time scale. Through study of the 13C, 15N, and 57Fe isotopologs of these intermediates and products, we identify the final HydG-bound species as an organometallic Fe(CO)2(CN) synthon that is ultimately transferred to apohydrogenase to form the [2Fe]H component of the H-cluster.

Sourcing CO and Cyanide

Hydrogenase enzymes derive their activity in part from the coordination of CO and cyanide ligands to metals in their active sites. Recent work elucidated the jettisoning of a tyrosine side chain at the outset of the biosynthetic pathway toward these ligands in the di-iron class of hydrogenase. Kuchenreuther et al. (p. 424; see the Perspective by Pickett) now apply stopped-flow infrared spectroscopy to uncover the next portion of the pathway, during which the residual tyrosine fragment is further broken down into CO and CN ligands at a single iron center in an iron sulfur cluster associated with the HydG enzyme.

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