Nitrite and Hydroxylamine as Nitrogenase Substrates: Mechanistic Implications for the Pathway of N2 Reduction

Investigations of reduction of nitrite (NO2 ) to ammonia (NH3) by nitrogenase indicate a limiting stoichiometry, NO2 + 6e + 12ATP + 7H(+) -> NH3 + 2H(2)O + 12ADP + 12P(i). Two intermediates freeze-trapped during NO2 turnover by nitrogenase variants and investigated by Q-band ENDOR/ESEEM are identical to states, denoted H and I, formed on the pathway of N-2 reduction. The proposed NO2 reduction intermediate hydroxylamine (NH2OH) is a nitrogenase substrate for which the H and I reduction intermediates also can be trapped. Viewing N-2 and NO2 reductions in light of their common reduction intermediates and of NO2 reduction by multiheme cytochrome c nitrite reductase (ccNIR) leads us to propose that NO2 reduction by nitrogenase begins with the generation of NO2H bound to a state in which the active-site FeMo-co (M) has accumulated two [e/H+] (E-2), stored as a (bridging) hydride and proton. Proton transfer to NO2H and H2O loss leaves M[NO+]; transfer of the E-2 hydride to the [NO+] directly to form HNO bound to FeMo-co is one of two alternative means for avoiding formation of a terminal M[NO] thermodynamic sink. The N-2 and NO2 reduction pathways converge upon reduction of NH2NH2 and NH2OH bound states to form state H with [-NH2] bound to M. Final reduction converts H to I, with NH3 bound to M. The results presented here, combined with the parallels with ccNIR, support a N2 fixation mechanism in which liberation of the first NH3 occurs upon delivery of five [e /H+] to N-2, but a total of seven [e /H+] to FeMo-co when obligate H-2 evolution is considered, and not earlier in the reduction process.