Sulfide release and rebinding in the mechanism for nitrogenase

Area of Science:

• Biochemistry
• Bioinorganic Chemistry
• Computational Chemistry

Background:

• Nitrogenases are crucial enzymes catalyzing nitrogen (N2) fixation, a vital process for life.
• The precise mechanism of N2 activation by nitrogenases remains a significant challenge in biochemistry.
• Previous models suggested a multi-step pre-activation phase (A0-A4) preceding catalysis.

Purpose of the Study:

• To model the detailed steps of nitrogenase pre-activation, focusing on the release of a sulfide ligand.
• To investigate the transition states involved in sulfide release during the catalytic cycle.
• To elucidate the complete catalytic cycle, including steps before and after nitrogen activation.

Main Methods:

• Computational modeling was employed to simulate the reaction mechanism.
• Density Functional Theory (DFT) calculations were likely used to explore potential energy surfaces.
• Analysis of transition states for sulfide release and subsequent steps.

Main Results:

• The study successfully modeled the pre-activation steps (A0-A4) and the crucial release of a sulfide.
• Several transition states for sulfide release were identified.
• A state (A4(E0)) was reached, closely resembling the experimentally observed protonation state (E4).
• The reverse catalytic steps, including sulfide insertion, were also modeled.

Conclusions:

• The detailed modeling provides a deeper understanding of the nitrogenase pre-activation mechanism.
• Sulfide release is a key event in preparing the active site for N2 activation.
• The findings align with experimental Electron Paramagnetic Resonance (EPR) data, validating the proposed mechanism.