Nitrogenase Mossbauer spectroscopy

One large class of non-heme iron-containing biomolecules involves proteins and enzymes containing iron-sulfur clusters. Iron-sulfur clusters are described in Sections 1.7 (Bioorganometallic Chemistry) and 1.8 (Electron Transfer) as well as in Section 3.6 (Mossbauer Spectroscopy). See especially Table 3.2 and the descriptive examples discussed in Section 3.6.4. Iron-sulfur proteins include rubredoxins, ferrodoxins, and the enzymes aconitase and nitrogenase. The nitrogenase enzyme was the subject of Chapter 6 in the hrst edition of this text—see especially Section 6.3 for a discussion of iron-sulfur clusters. In this... 

Several spectroscopic techniques have been used to study nitrogenase proteins both in the isolated forms and during enzymatic activity. These include electron paramagnetic resonance (epr) spectroscopy, Mossbauer spectroscopy, uv-visible spectrophotometry, circular dichroism. X-ray absorption edge and fine structure spectroscopy (EXAFS), and linear electric field effects (LEFE). 

The evidence in the foregoing sections indicates that the Fe protein accepts electrons from the electron donor and that the MoFe protein binds the reducible substrate. Evidence from several sources and techniques, principally epr and Mossbauer spectroscopy and stopped-flow spectrophotometry, shows that electrons pass from the Fe protein to the MoFe protein with the concomitant hydrolysis of ATP. The earliest evidence came from steady-state epr studies on the nitrogenase of K. pneumoniae (Smith et al., 1972), A. vinelandii and C. pasteurianum (Orme-Johnson et al., 1972 Palmer et al., 1972 Zumft et al., 1972). In the presence of sodium dithionite and without MgATP, the epr spectra of the two proteins are additive. When ATP is added both spectra are largely bleached within (it is now agreed) the turnover time (185 ms/electron pair at 23 C) of the enzyme and remain so imtil the dithio-... 

B.E. Smith, G. Lang, Mossbauer spectroscopy of nitrogenase proteins from Klebsiella pneumoniae—structural assignments and mechanistic conclusions, Biochem. J. 1974, 137(2), 169-180. 

EPR, ENDOR, Mossbauer, EXAFS, and MCD spectroscopies to further elucidate intimate details of electron and proton transfer within nitrogenase are difficult... 

Extensive Mossbauer investigations of nitrogenase and FeMoco have been reported. Unlike EPR and EPR-based spectroscopies, which can be... 

Progress in the biochemistry and spectroscopy of iron-only nitrogenase has occurred, with the most active, pure preparations reported in 1997. Mossbauer and EXAFS results support electronic and structural analogy between the eight-iron cluster in this enzyme (the FeFeco ) and the FeMoco. One notable difference is that the FeFeco is diamagnetic in its dithionite-reduced form, and thus corresponds to M° in the MoFe protein. Therefore, the oxidation states are thought to be Fe(II)4Fe(III)4 or Fe(II)6Fe(III)2, in analogy to the two most likely oxidation... 

E. Munck, et al., Nitrogenase 8. Mossbauer and EPR spectroscopy—MoFe protein component from Azotobacter vinelandii OP,... 

---