Cytochrome and ubiquinone patterns during growth of Azotobacter vinelandii

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

The biological reduction of dinitrogen (N₂) to ammonia (NH₃) 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 Δrnf 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 chemical cross-linking in conjunction with homology modeling, revealed plausible electron transfer pathways to both high- and low-potential acceptors. Overall, this study informs a mechanism for electron bifurcation, offering insight into a unique method for delivery of low-potential electrons required for energy-intensive biochemical conversions.

Effect of ligands on cytochrome d from Azotobacter vinelandii

Spectra of oxidized and reduced cytochrome d in particles of A. vinelandii were studied in the presence of the ligands CO, azide, and NH2OH under oxidizing, reducing, and turnover conditions. Under oxidizing conditions, spectral changes were observed on oxidized cytochrome d (absorption maximum at 648 nm) in the presence of CO and NH2OH showing a shift of the maximum to shorter wavelengths (639 and 645 nm, respectively) and a broadening of the half-band width. Under reducing conditions, spectral changes were observed on reduced cytochrome d (absorption maximum at 631 nm) in the presence of CO (absorption maximum at 636 nm), NO, NO2-, and NH2OH (absorption maximum at 642 nm in the presence of dithionite). The spectral changes of cytochrome d in the presence of NH2OH or with dithionite and NO2- were ascribed to the formation of the NO-cytochrome d compound. Under turnover conditions CO, NH2OH, and azide cause a spectral shift of the absorption maximum of cytochrome d from 648 nm to 636, 645, and 655 nm, respectively. With NH2OH and azide a broadening of the half-band width of 7 and 6 nm, respectively, was also observed. The spectral changes caused by CO and NH2OH were interpreted as a binding of the ligands to cytochrome d changing its conformation from the oxidized state absorbing at 648 nm into a more stable liganded form. Since azide does not affect the spectral bands of oxidized and reduced cytochrome d, the spectral change during turnover in the presence of azide were ascribed to a preferential binding of azide to enzymically active conformation of cytochrome d (cytochrome dx).