Inorganic-Electrochemistry Seminar
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 use electron paramagnetic resonance (EPR) 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 Calpha-Cbeta 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 (1). We apply stopped-flow Fourier transform infrared (SF-FTIR) and electron-nuclear double resonance (ENDOR) spectroscopies to explore in detail the formation of HydG-bound Fe-containing species bearing CO and CN- ligands, with spectroscopic signatures that evolve on the 1 to 1000 s timescale. Through study of the 13C, 15N, and 57Fe isotopologues of these intermediates and products, we identify the final HydG-bound species as an organometallic Fe(CO)2CN synthon that is ultimately transferred to apo-hydrogenase to form the [2Fe]H component of the H-cluster (2).
References
1. Jon M. Kuchenreuther, William K. Myers, Troy A. Stich, Simon J. George , Yaser NejatyJahromy, James R. Swartz, and R. David Britt. "A Radical Intermediate in Tyrosine Scission to the CO and CN- Ligands of [FeFe] Hydrogenase" Science (2013) 342:472-475
2. Jon M. Kuchenreuther, William K. Myers, Daniel L. M. Suess, Troy A. Stich, Vladimir Pelmenschikov, Stacey A. Shiigi, Stephen P. Cramer, James R. Swartz, R. David Britt, and Simon J. George "The HydG Enzyme Generates an Fe(CO)2(CN) Synthon in the Biosynthesis of the FeFe Hydrogenase H-Cluster" Science (2013) in press.