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A study is conducted to demonstrate that wild-type nitrogenase reduces diazene to ammonia. The findings suggest that the reduction of [N.sub.2] and diazene is inhibited by [H.sub.2].

Research has been conducted on the nitrite reductases. The kinetic mechanism of the nitrite reduction and the pH dependence on the enzymatic activity and its activation energy have been investigated and the details are reported.

The intermediate generated at a high concentration during reduction of the natural nitrogenase substrate ([N.sub.2]) by wild-type MoFe protein has shown that it contains [N.sub.2] bound to the active-site FeMo cofactor. The single type of [super 15]N-coupled nucleus from the field dependence, along with the absence of an associated exchangeable [super 1]H ENDOR signal, is shown to be consistent with the [N.sub.2] molecule bound end-on to the FeMo cofactor.

Nitrogenase reduces dinitrogen (N2) by six electrons and six protons at an active-site metallocluster called FeMo cofactor, to yield two ammonia molecules. Insights into the mechanism of substrate reduction by nitrogenase have come from recent successes in trapping and characterizing intermediates generated during the reduction of protons as well as nitrogenous and alkyne substrates by MoFe proteins with amino acid substitutions. Here, we describe an intermediate generated at a high concentration during reduction of the natural nitrogenase substrate, N2, by wild-type MoFe protein, providing evidence that it contains N2 bound to the active-site FeMo cofactor. When MoFe protein was frozen at 77 K during steady- state turnover with N2, the S = 3⁄2 EPR signal (g [4.3, 3.64, 2.00]) arising from the resting state of FeMo cofactor was observed to convert to a rhombic, S = ½, signal (g [2.08, 1.99, 1.97]). The intensity of the N2-dependent EPR signal increased with increasing N2 partial pressure, reaching a maximum intensity of approximately 20% of that of the original FeMo cofactor signal at ≥ 0.2 atm N2. An almost complete loss of resting FeMo cofactor signal in this sample implies that the remainder of the enzyme has been reduced to an EPR-silent intermediate state. The N2-dependent EPR signal intensity also varied with the ratio of Fe protein to MoFe protein (electron flux through nitrogenase), with the maximum signal intensity observed with a ratio of 2:1 (1:1 Fe protein:FeMo cofactor) or higher. The pH optimum for the signal was 7.1. The N2-dependent EPR signal intensity exhibited a linear dependence on the square root of the EPR microwave power in contrast to the nonlinear response of signal intensity observed for hydrazine-, diazene-, and methyldiazene-trapped states. 15N ENDOR spectroscopic analysis of MoFe protein captured during turnover with 15N2 revealed a 15N nuclear spin coupled to the FeMo cofactor with a hyperfine tensor A [0.9, 1.4, 0.45] MHz establishing that an N2-derived species was trapped on the FeMo cofactor. The observation of a single type of 15N-coupled nucleus from the field dependence, along with the absence of an associated exchangeable ¹H ENDOR signal, is consistent with an N2 molecule bound end-on to the FeMo cofactor. [ABSTRACT FROM AUTHOR]

Focuses on the kinetic mechanism of nitrite reduction by electron nuclear double resonance of protons type 2 and 1. Role of type 1 center in the regulation of activation energy; Characteristics of electron transfer from type 1 to type 2 center; Implication of protons in improving water formation.

Three genetically distinct, but structurally similar, isozymes of nitrogenase catalyze biological N 2 reduction to 2NH 3 : Mo-, V-, and Fe-nitrogenase, named respectively for the metal ( M ) in their active site metallocofactors (metal-ion composition, M Fe 7 ). Studies of the Mo-enzyme have revealed key aspects of its mechanism for N 2 binding and reduction. Central to this mechanism is accumulation of four electrons and protons on its active site metallocofactor, called FeMo-co, as metal bound hydrides to generate the key E 4 (4H) ("Janus") state. N 2 binding/reduction in this state is coupled to reductive elimination ( re ) of the two hydrides as H 2 , the forward direction of a reductive-elimination/oxidative-addition ( re/oa ) equilibrium. A recent study demonstrated that Fe-nitrogenase follows the same re/oa mechanism, as particularly evidenced by HD formation during turnover under N 2 /D 2 . Kinetic analysis revealed that Mo- and Fe-nitrogenases show similar rate constants for hydrogenase-like H 2 formation by hydride protonolysis ( k HP ) but significant differences in the rate constant for H 2 re with N 2 binding/reduction ( k re ). We now report that V-nitrogenase also exhibits HD formation during N 2 /D 2 turnover (and H 2 inhibition of N 2 reduction), thereby establishing the re/oa equilibrium as a universal mechanism for N 2 binding and activation among the three nitrogenases. Kinetic analysis further reveals that differences in catalytic efficiencies do not stem from significant differences in the rate constant ( k HP ) for H 2 production by the hydrogenase-like side reaction but directly arise from the differences in the rate constant ( k re ) for the re of H 2 coupled to N 2 binding/reduction, which decreases in the order Mo > V > Fe.

Of the three forms of nitrogenase (Mo-nitrogenase, V-nitrogenase, and Fe-nitrogenase), Fe-nitrogenase has the poorest ratio of N 2 reduction relative to H 2 evolution. Recent work on the Mo-nitrogenase has revealed that reductive elimination of two bridging Fe-H-Fe hydrides on the active site FeMo-cofactor to yield H 2 is a key feature in the N 2 reduction mechanism. The N 2 reduction mechanism for the Fe-nitrogenase active site FeFe-cofactor was unknown. Here, we have purified both component proteins of the Fe-nitrogenase system, the electron-delivery Fe protein (AnfH) plus the catalytic FeFe protein (AnfDGK), and established its mechanism of N 2 reduction. Inductively coupled plasma optical emission spectroscopy and mass spectrometry show that the FeFe protein component does not contain significant amounts of Mo or V, thus ruling out a requirement of these metals for N 2 reduction. The fully functioning Fe-nitrogenase system was found to have specific activities for N 2 reduction (1 atm) of 181 ± 5 nmol NH 3 min -1 mg -1 FeFe protein, for proton reduction (in the absence of N 2 ) of 1085 ± 41 nmol H 2 min -1 mg -1 FeFe protein, and for acetylene reduction (0.3 atm) of 306 ± 3 nmol C 2 H 4 min -1 mg -1 FeFe protein. Under turnover conditions, N 2 reduction is inhibited by H 2 and the enzyme catalyzes the formation of HD when presented with N 2 and D 2 . These observations are explained by the accumulation of four reducing equivalents as two metal-bound hydrides and two protons at the FeFe-cofactor, with activation for N 2 reduction occurring by reductive elimination of H 2 .