Kinetic Understanding of N 2 Reduction versus H 2 Evolution at the E 4 (4H) Janus State in the Three Nitrogenases.

The enzyme nitrogenase catalyzes the reduction of N2 to ammonia but also that of protons to H2 . These reactions compete at the mechanistically central 'Janus' intermediate, denoted E4 (4H), which has accumulated 4e- /4H+ as two bridging Fe-H-Fe hydrides on the active-site cofactor. This state can lose e- /H+ by hydride protonolysis (HP) or become activated by reductive elimination ( re) of the two hydrides and bind N2 with H2 loss, yielding an E4 (2N2H) state that goes on to generate two NH3 molecules. Thus, E4 (4H) represents the key branch point for these competing reactions. Here, we present a steady-state kinetic analysis that precisely describes this competition. The analysis demonstrates that steady-state, high-electron flux turnover overwhelmingly populates the E4 states at the expense of less reduced states, quenching HP at those states. The ratio of rate constants for E4 (4H) hydride protonolysis ( kHP ) versus reductive elimination ( kre ) provides a sensitive measure of competition between these two processes and thus is a central parameter of nitrogenase catalysis. Analysis of measurements with the three nitrogenase variants (Mo-nitrogenase, V-nitrogenase, and Fe-nitrogenase) reveals that at a fixed N2 pressure their tendency to productively react with N2 to produce two NH3 molecules and an accompanying H2 , rather than diverting electrons to the side reaction, HP production of H2 , decreases with their ratio of rate constants, k re / kHP : Mo-nitrogenase, 5.1 atm-1 ; V-nitrogenase, 2 atm-1 ; and Fe-nitrogenase, 0.77 atm-1 (namely, in a 1:0.39:0.15 ratio). Moreover, the lower catalytic effectiveness of the alternative nitrogenases, with more H2 production side reaction, is not caused by a higher kHP but by a significantly lower k re .