The Electron Bifurcating FixABCX Protein Complex from Azotobacter vinelandii: Generation of Low-Potential Reducing Equivalents for Nitrogenase Catalysis

作者：Rhesa N. Ledbetter, Amaya M. Garcia Costas, Carolyn E. Lubner, David W. Mulder, Monika Tokmina‐Lukaszewska, Jacob H. Artz, Angela Patterson, Timothy S. Magnuson, Zackary J. Jay, H. Diessel Duan, Jacquelyn B. Miller, Mary H. Plunkett, John P. Hoben, Brett M. Barney, Ross P. Carlson, Anne‐Frances Miller, Brian B. Bothner, Paul W. King, John W. Peters, Lance C. Seefeldt · 发表于：Biochemistry · 年份：2017 · DOI：10.1021/acs.biochem.7b00389 · 被引用次数：147 · 研究领域：Metalloenzymes and iron-sulfur proteins、Microbial Metabolic Engineering and Bioproduction、Enzyme Catalysis and Immobilization

The biological reduction of dinitrogen (N 2 ) to ammonia (NH 3 ) by nitrogenase is an energetically demanding reaction that requires low-potential electrons and ATP; however, pathways used to deliver the electrons from central metabolism to the reductants of nitrogenase, ferredoxin or flavodoxin, remain unknown for many diazotrophic microbes. The FixABCX protein complex has been proposed to reduce flavodoxin or ferredoxin using NADH as the electron donor in a process known as electron bifurcation. Herein, the FixABCX complex from Azotobacter vinelandii was purified and demonstrated to catalyze an electron bifurcation reaction: oxidation of NADH ( E m = −320 mV) coupled to reduction of flavodoxin semiquinone ( E m = −460 mV) and reduction of coenzyme Q ( E m = 10 mV). Knocking out fix genes rendered Δ rnf A. vinelandii cells unable to fix dinitrogen, confirming that the FixABCX system provides another route for delivery of electrons to nitrogenase. Characterization of the purified FixABCX complex revealed the presence of flavin and iron–sulfur cofactors confirmed by native mass spectrometry, electron paramagnetic resonance spectroscopy, and transient absorption spectroscopy. Transient absorption spectroscopy further established the presence of a short-lived flavin semiquinone radical, suggesting that a thermodynamically unstable flavin semiquinone may participate as an intermediate in the transfer of an electron to flavodoxin. A structural model of FixABCX, generated using che...