Probing the All-Ferrous States of Methanogen Nitrogenase Iron Proteins

Joseph B Solomon; Mahtab F Rasekh; Caleb J Hiller; Chi Chung Lee; Kazuki Tanifuji; Markus W Ribbe; Yilin Hu

doi:10.1021/jacsau.0c00072

Probing the All-Ferrous States of Methanogen Nitrogenase Iron Proteins

JACS Au. 2020 Dec 29;1(2):119-123. doi: 10.1021/jacsau.0c00072. eCollection 2021 Feb 22.

Authors

Joseph B Solomon^{1

2}, Mahtab F Rasekh^{1

2}, Caleb J Hiller³, Chi Chung Lee¹, Kazuki Tanifuji¹, Markus W Ribbe^{1

2}, Yilin Hu¹

Affiliations

¹ Department of Molecular Biology and Biochemistry, University of California, Irvine, California 92697-3900, United States.
² Department of Chemistry, University of California, Irvine, California 92697-2025, United States.
³ Department of Physical Science, Southern Utah University, Cedar City, Utah 84720, United States.

Abstract

The Fe protein of nitrogenase reduces two C1 substrates, CO₂ and CO, under ambient conditions when its [Fe₄S₄] cluster adopts the all-ferrous [Fe₄S₄]⁰ state. Here, we show disparate reactivities of the nifH- and vnf-encoded Fe proteins from Methanosarcina acetivorans (designated MaNifH and MaVnfH) toward C1 substrates in the all-ferrous state, with the former capable of reducing both CO₂ and CO to hydrocarbons, and the latter only capable of reducing CO to hydrocarbons at substantially reduced yields. EPR experiments conducted at varying solution potentials reveal that MaVnfH adopts the all-ferrous state at a more positive reduction potential than MaNifH, which could account for the weaker reactivity of the MaVnfH toward C1 substrates than MaNifH. More importantly, MaVnfH already displays the g = 16.4 parallel-mode EPR signal that is characteristic of the all-ferrous [Fe₄S₄]⁰ cluster at a reduction potential of -0.44 V, and the signal reaches 50% maximum intensity at a reduction potential of -0.59 V, suggesting the possibility of this Fe protein to access the all-ferrous [Fe₄S₄]⁰ state under physiological conditions. These results bear significant relevance to the long-lasting debate of whether the Fe protein can utilize the [Fe₄S₄]^0/2+ redox couple to support a two-electron transfer during substrate turnover which, therefore, is crucial for expanding our knowledge of the reaction mechanism of nitrogenase and the cellular energetics of nitrogenase-based processes.