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Evidence for Functionally Relevant Encounter Complexes in Nitrogenase Catalysis.

Cedric P Owens1, Faith E H Katz1, Cole H Carter1

  • 1Department of Chemistry and Biochemistry, University of California, San Diego , La Jolla, California 92039, United States.

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Summary
This summary is machine-generated.

Encounter complexes between iron-protein (FeP) and molybdenum-iron protein (MoFeP) are crucial for nitrogenase catalysis. Mutations disrupting electrostatic interactions on MoFeP reduce catalytic activity by hindering FeP-MoFeP association.

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Area of Science:

  • Biochemistry
  • Enzymology
  • Nitrogen Fixation

Background:

  • Nitrogenase is essential for converting atmospheric nitrogen (N2) to ammonia (NH3).
  • This process involves the MoFe-protein (MoFeP) and Fe-protein (FeP) interacting in an ATP-dependent manner for electron transfer.
  • Understanding the protein-protein interactions is key to optimizing nitrogenase function.

Purpose of the Study:

  • To investigate the functional role of encounter complexes in nitrogenase catalysis.
  • To identify the specific interactions stabilizing these complexes.
  • To elucidate the mechanism of electron transfer between FeP and MoFeP.

Main Methods:

  • Site-directed mutagenesis of Azotobacter vinelandii MoFeP to disrupt electrostatic interactions.
  • Assessing catalytic activity of MoFeP variants.
  • Kinetic analysis using the Thorneley-Lowe model to determine rate constants for association and electron transfer.

Main Results:

  • Encounter complexes are stabilized by electrostatic interactions on the MoFeP β-subunit.
  • Mutations in this region significantly decreased catalytic activity, with βLys400Glu showing the largest effect.
  • The βK400E mutation reduced the FeP-MoFeP association rate constant fivefold without affecting ATP hydrolysis-electron transfer coupling.

Conclusions:

  • Encounter complexes play a functional role in nitrogenase catalysis.
  • FeP initially forms encounter complexes on the MoFeP β-subunit surface.
  • This interaction is a prerequisite for the formation of the ATP-activated, electron transfer-competent complex at the αβ-interface.