[Frontier Leaders] Nitrogen Fixation

Title: Nitrogen Fixation

Speaker: Markus Ribbe and Yilin Hu

Affiliation: University of California, Irvine, USA

Host: Inês Cardoso Pereira

Nitrogenase M-Cluster Assembly: Tracing the ‘9th Sulfur’ of the Nitrogenase Cofactor via a Semi-Synthetic Approach
Authors: Kazuki Tanifuji,1 Chi Chung Lee,1 Kazuyuki Tatsumi,2 Yasuhiro Ohki,2 Yilin Hu1 and & Markus W. Ribbe1,3
1Department of Molecular Biology and Biochemistry, University of California, Irvine, California 92697-3900; 2Department of Chemistry, Graduate School of Science and Research Center for Materials Science, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8602, Japan; 3Department of Chemistry, University of California, Irvine, California 92697-2025

Abstract:

The M-cluster is the active site of nitrogenase that contains an 8Fe-core assembled via coupling and rearrangement of two [Fe4S4] clusters concomitant with the insertion of an interstitial carbon and a ‘9th sulfur’. Combining synthetic [Fe4S4] clusters with an assembly protein template, we show that sulfite gives rise to the ‘9th sulfur’ that is incorporated in the catalytically important belt region of the cofactor after the radical SAM-dependent carbide insertion and the concurrent 8Fe-core rearrangement have already taken place. This work provides a semi-synthetic tool for strategically labeling the cofactor—including its ‘9th S’ in the belt region—formechanistic investigations of nitrogenase while suggesting an interesting link between nitrogen fixation and sulfite detoxification in diazotrophic organisms.

CO2 Activation and Reduction by Nitrogenase Fe Proteins and Synthetic [Fe4S4] Clusters
Martin T. Stiebritz,1 Caleb J. Hiller,1,2 Nathaniel S. Sickerman,1 Chi Chung Lee,1 Kazuki Tanifuji,1 Yasuhiro Ohki3 & Yilin Hu1
1Department of Molecular Biology and Biochemistry, University of California, Irvine, CA 92697-3900; 2Department of Chemistry, University of California, Irvine, CA 92697-2025; 3Department of Chemistry, Graduate School of Science, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, 464-8602, Japan.

Abstract:

The Fe protein of nitrogenase contains a redox active [Fe4S4] cluster that plays a key role in electron transfer and substrate reduction. We show that the Fe protein of Methanosarcina acetivorans can reduce CO2 and CO to hydrocarbons under ambient conditions. Further, we demonstrate that this reactivity is inherent to [Fe4S4] clusters, showing the ability of a synthetic [Fe4S4] compound to catalyze the same ambient reaction in solutions. Theoretical calculations suggest a reactionmechanisminvolving an aldehyde-like intermediate that gives rise to hydrocarbon products upon proton-coupled electron transfer and concomitant removal of water molecules. These results provide a framework for mechanistic investigations of FeS-based activation and reduction of CO2 and CO while facilitating potential development of FeS catalysts capable of ambient conversion of CO2 and CO into fuel products.