Connecting nitrogenase intermediates with the kinetic scheme for N ₂ reduction by a relaxation protocol and identification of the N ₂ binding state

A major obstacle to understanding the reduction of N₂ to NH ₃ by nitrogenase has been the impossibility of synchronizing electron delivery to the MoFe protein for generation of specific enzymatic intermediates. When an intermediate is trapped without synchronous electron delivery, the number of electrons, n, it has accumulated is unknown. Consequently, the intermediate is untethered from kinetic schemes for reduction, which are indexed by n. We show that a trapped intermediate itself provides a "synchronously prepared" initial state, and its relaxation to the resting state at 253 K, conditions that prevent electron delivery to MoFe protein, can be analyzed to reveal n and the nature of the relaxation reactions. The approach is applied to the "H⁺/H^- intermediate" (A) that appears during turnover both in the presence and absence of N₂ substrate. A exhibits an S = 1/2 EPR signal from the active-site iron-molybdenum cofactor (FeMo-co) to which are bound at least two hydrides/protons. A undergoes two-step relaxation to the resting state (C): A → B → C, where B has an S = 3/2 FeMo-co. Both steps show large solvent kinetic isotope effects: KIE ≈ 3-4 (85% D₂O). In the context of the Lowe-Thorneley kinetic scheme for N₂ reduction, these results provide powerful evidence that H₂ is formed in both relaxation steps, that A is the catalytically central state that is activated for N₂ binding by the accumulation of n = 4 electrons, and that B has accumulated n = 2 electrons.