Kinetics of Elementary Steps of Electron Transfer in Nitrogenase in the Presence of a Photodonor

L. A. Syrtsova^*, V. A. Nadtochenko, N. N. Denisov, E. A. Timofeeva, N. I. Shkondina, and V. Yu. Gak

Institute of Problems of Chemical Physics, Russian Academy of Sciences, Chernogolovka, Moscow Region, 142432 Russia; E-mail: syrtsova@icp.ac.ru

^* To whom correspondence should be addressed.

Received May 31, 1999; Revision received March 30, 2000

---

The kinetics of transfer of two electrons from a photodonor (a system containing eosin and NADH or 4´,5´-dibromofluorescein and NADH) to Fe-protein (Av2) and the kinetics of transfer of the first and second electrons from Av2 to Mo-Fe-protein (Av1) were studied by kinetic laser spectroscopy of nitrogenase from Azotobacter vinelandii. The effects of the substrates of nitrogenase (nitrogen, acetylene, and protons) on the intramolecular electron transfer in nitrogenase were studied. Analysis of the effect of photodonor excitation radiation intensity on the rate of electron transfer was used to determine the transfer rate constants for the first (k₁) and second (k₂) electrons from Av2 to Av1. In the presence of MgATP, two electrons are sequentially transferred from Av2 to Av1, and no delay between these reactions was detected. The first electron transferred from Av2 to Av1 is not targeted to the substrate; k₁ = 154 ± 15 sec^-1 at 23°C for the system 4´,5´-dibromofluorescein-NADH; k₂ = 53 ± 5 sec^-1, 95 ± 9 sec^-1, and 24 ± 2 sec^-1 at 23°C in the presence of nitrogen, acetylene, and argon, respectively. An unidentified slow step (k₃ = 18 ± 2 sec^-1 at 23°C) may be associated with electron transfer within Av1.

---

KEY WORDS: nitrogenase, Fe-protein, Mo-Fe-protein, kinetic laser spectroscopy, pulse photolysis

---