vanadium nitrogenase, and a Fe protein in iron-only nitrogenase. Component II is a Fe protein that contains the Fe-S cluster (Figure 3: top), which transfers electrons to Component I. Component I contains 2 key metal clusters: the P-cluster (Figure 3: middle), and the FeMo-cofactor (FeMo-co) (Figure 3: bottom). Mo is replaced by V or Fe in vanadium nitrogenase and iron-only nitrogenase respectively. During catalysis, electrons flow from a pair of ATP molecules within Component II to the Fe-S cluster, to the P-cluster, and finally to the FeMo-co, where reduction of N2 to NH3 takes place. Lowe-Thorneley kinetic model The reduction of nitrogen to two molecules of ammonia is carried out at the FeMo-co of Component I after the sequential addition of proton and electron equivalents from Component II. Steady state, freeze quench, and stopped-flow kinetics measurements carried out in the 70's and 80's by Lowe, Thorneley, and others provided a kinetic basis for this process. The Lowe-Thorneley (LT) kinetic model (depicted in Figure 4) was developed from these experiments and documents the eight correlated proton and electron transfers required throughout the reaction. Each intermediate stage is depicted as En where n = 0-8, corresponding to the number of equivalents transferred. The transfer of four equivalents are required before the productive addition of N2, although reaction of E3 with N2 is also possible. Notably, nitrogen reduction has been shown to require 8 equivalents of protons and electrons as opposed to the 6 equivalents predicted by the balanced chemical reaction. Intermediates E0 through E4 Spectroscopic characterization of these intermediates has allowed for greater understanding of nitrogen reduction by nitrogenase, however, the mechanism remains an active area of research and debate. Briefly listed below are spectroscopic experiments for the intermediates before the addition of nitrogen: E0 – This is the resting state of the enzyme before catalysis begins. EPR characterization shows that this species has a spin of 3/2. E1 – The one electron reduced intermediate has been trapped during turnover under N2. Mӧssbauer spectroscopy of the trapped intermediate indicates that the FeMo-co is integer spin greater than 1. E2 – This intermediate is proposed to contain the metal cluster in its resting oxidation state with the two added electrons stored in a bridging hydride and the additional proton bonded to a sulfur atom. Isolation of this intermediate in mutated enzymes shows that the FeMo-co is high spin and has a spin of