Electron nuclear double resonance (ENDOR) studies of native and isotopically enriched MoFe proteins hold the promise of individually characterizing every atom of the catalytically active FeMo-co cluster of the nitrogenase MoFe protein. This report presents 1H, 57Fe, 95*97Mo, and 33S ENDOR measurements in a comparison of the MoFe protein isolated from the three titled organisms, Avl, Kpl, and Cpl. We have examined in detail single-crystal-like 57Fe resonances from at least Five distinct iron sites in each of the three enzymes, revising somewhat our earlier assignments. The analysis incidentally gives the electron spin zero-field splitting parameters to high precision. 95Mo ENDOR measurements for Cpl and Kpl give 95Mo hyperfine and quadrupole coupling constants. They indicate that a single molybdenum is integrated into the MoFe spin system and that the molybdenum is most plausibly viewed as being in an even-electron state, which may be assigned provisionally as unsymmetrically coordinated The observation of an exchangeable proton or protons from each protein source suggests a site on the cluster accessible to solvent and perhaps containing H20 or Cpl enriched in 33S gives the first observed ENDOR signals from this nucleus. The resonances from 33S are assignable to the inorganic sulfur because, it is argued, the FeMo-co cluster must be bound to the protein primarily, if not exclusively, by residues other than cysteinyl.
ASJC Scopus subject areas
- Colloid and Surface Chemistry