N2 binds to the active-site metal cluster in the nitrogenase MoFe protein, the FeMo-cofactor ([7Fe-9S-Mo-homocitrate-X]; FeMo-co) only after the MoFe protein has accumulated three or four electrons/protons (E3 or E4 states), with the E4 state being optimally activated. Here we study the FeMo-co 57Fe atoms of E4 trapped with the α-70Val→Ile MoFe protein variant through use of advanced ENDOR methods: ?random-hop? Davies pulsed 35 GHz ENDOR; difference triple resonance; the recently developed Pulse-Endor-SaTuration and REcovery (PESTRE) protocol for determining hyperfine-coupling signs; and Raw-DATA (RD)-PESTRE, a PESTRE variant that gives a continuous sign readout over a selected radiofrequency range. These methods have allowed experimental determination of the signed isotropic 57Fe hyperfine couplings for five of the seven iron sites of the reductively activated E4 FeMo-co, and given the magnitude of the coupling for a sixth. When supplemented by the use of sum-rules developed to describe electron-spin coupling in FeS proteins, these 57Fe measurements yield both the magnitude and signs of the isotropic couplings for the complete set of seven Fe sites of FeMo-co in E 4. In light of the previous findings that FeMo-co of E4 binds two hydrides in the form of (Fe-(μ-H-)-Fe) fragments, and that molybdenum has not become reduced, an ?electron inventory? analysis assigns the formal redox level of FeMo-co metal ions in E4 to that of the resting state (MN), with the four accumulated electrons residing on the two Fe-bound hydrides. Comparisons with earlier 57Fe ENDOR studies and electron inventory analyses of the bio-organometallic intermediate formed during the reduction of alkynes and the CO-inhibited forms of nitrogenase (hi-CO and lo-CO) inspire the conjecture that throughout the eight-electron reduction of N2 plus 2H+ to two NH3 plus H 2, the inorganic core of FeMo-co cycles through only a single redox couple connecting two formal redox levels: those associated with the resting state, MN, and with the one-electron reduced state, MR. We further note that this conjecture might apply to other complex FeS enzymes.
ASJC Scopus subject areas
- Colloid and Surface Chemistry