Nitrogenase electron transfer scheme

The simplest level of mechanistic information for the nitrogenases is the electron-transfer sequence shown in Figure 11. This scheme has been discussed in several previous reviews [71,72], and herein we shall outline only the basic pattern as a foundation for the later discussion. 

The structure of FeMoCo (and the dinitrogen-binding protein) is unknown. The environment of the metals is constituted primarily of sulfur atoms, but the interpretation of the X-ray absorption fine structure (EXAFS) data on both the cofactor and protein (117-119) are contentious. It is not known whether the system in any specific part is aqueous or anhydrous, but oxygen destroys the activity. The electron-transfer pathway in the functioning nitrogenase is believed to be as shown in Scheme 22. 

Pre-steady-state stopped-flow and rapid quench techniques applied to Mo nitrogenase have provided powerful approaches to the study of this complex enzyme. These studies of Klebsiella pneumoniae Mo nitrogenase showed that a pre-steady-state burst in ATP hydrolysis accompanied electron transfer from the Fe protein to the MoFe protein, and that during the reduction of N2 an enzyme-bound dinitrogen hydride was formed, which under denaturing conditions could be trapped as hydrazine. A comprehensive model developed from a computer simulation of the kinetics of these reactions and the kinetics of the pre-steady-state rates of product formation (H2, NH3) led to the formulation of Scheme 1, the Thorneley and Lowe scheme (50) for nitrogenase function. 

Scheme 1. The catalytic cycle for the reduction of N2 by the Mo nitrogenase. Eq represents the resting state of the MoFe protein of K. pneumoniae and species E -E represent intermediate forms of this protein following sequential one-electron reduction steps. The arrows between each state represent complex formation between the Fe protein and MoFe protein, electron transfer, and protonation, followed by protein dissociation. N2 binds to species 3, accounting for the stoichiometry of Eq. (1) the displacement of N2 from this species accounts for the competitive inhibition of N2 reduction by H2 (see Ref. 50 for a detailed presentation of this scheme).

Scheme 2. MgATP-dependent electron transfer between components of V nitrogenase, The pre-steady-state electron transfer reactions between the Fe protein and the VFe protein of V nitrogenase of A, chroococcum have been analyzed in terms of this scheme (52). Ac2 represents the Fe protein and Acl represents the VFe protein of this system. This scheme is analogous to that used in the detailed study of Mo nitrogenase (see Hef. 50).

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