Issue 35, 2023

The mechanism for N2 activation in the E4 – state of nitrogenase

Abstract

Nitrogenases take nitrogen from the air and reduce it to ammonia. It has long been known that N2 becomes activated after four reductions in the catalytic cycle, in the E4 state. Several mechanisms for the activation have been suggested. In the present study a previous mechanism has been revised based on recent experimental findings. In the present mechanism N2H2 is formed in E4. As in the previously suggested mechanism, there are four initial reductions before catalysis (the A-states), after which a sulfide is released and the first state in catalysis (E0) is formed. In E4, N2 becomes bound and protonated in the Fe1, Fe2, Fe4 region, in which the hydrides have left two electrons. The rate-limiting step is the formation of N2H by a hydrogen atom transfer from Cys275 to N2 bound to Fe4, concerted with an additional electron transfer from the cofactor. The mechanism fulfills all requirements set by experiments. The activation of N2 is preceded by a formation of H2 from two hydrides, the carbide is kinetically hindered from being protonated, the E4 state is reversible. An important aspect is the presence of a water molecule in the Fe2, Fe6 region. The non-allowed formations of H2 from a hydride and a proton have been investigated and found to have higher barriers than the allowed formation of H2 from two hydrides.

Graphical abstract: The mechanism for N2 activation in the E4 – state of nitrogenase

Supplementary files

Article information

Article type
Paper
Submitted
20 Jun 2023
Accepted
18 Aug 2023
First published
21 Aug 2023
This article is Open Access
Creative Commons BY license

Phys. Chem. Chem. Phys., 2023,25, 23602-23613

The mechanism for N2 activation in the E4 – state of nitrogenase

P. E. M. Siegbahn, Phys. Chem. Chem. Phys., 2023, 25, 23602 DOI: 10.1039/D3CP02851H

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