Nitrogenase functional

These were relatively low-resolution structures, and with refinement some errors in the initial structural assignments have been detected (4-7). Since the structures were first reported the subject has been extensively reviewed in this series (8) and elsewhere 9-15). This review will focus on the structure, biosynthesis, and function of the met-allosulfur clusters found in nitrogenases. This will require a broader overview of some functional aspects, particularly the involvement of MgATP in the enzymic reaction, and also some reference will be made to the extensive literature (9, 15) on biomimetic chemistry that has helped to illuminate possible modes of nitrogenase function, although a detailed review of this chemistry will not be attempted here. This review cannot be fully comprehensive in the space available, but concentrates on recent advances and attempts to describe the current level of our understanding. 

It is becoming clear that the MgATP hydrolysis is not required to induce protein-protein electron transfer, but its role in nitrogenase function is still undefined. The most likely hypothesis at the moment is that its hydrolysis, on the Fe protein, induces important changes in the MoFe protein, presumably by altering the conformation of the enzyme complex. Nevertheless, the nature of the changes in the MoFe protein remain obscure. 

As the names of the component proteins imply, iron-containing redox groups are essential to nitrogenase function. These clusters are of the Fe S type, but they exhibit many unique features that are not present in simpler protein and model systems. A brief summary of the properdes of the nitrogenase proteins, with emphasis on the metal clusters, follows. 

The evolution of dihydrogen on binding of dinitrogen has been attributed to the displacement of hydrides from a trihydridomolybdenum site (equation 85). If nitrogenase functions under an atmosphere of 50 50 D2 H2, then HD is produced. This is also consistent with a scheme involving molybdenum-bound hydrides (equation 86). 

This mechanism is not the only model for nitrogenase function. For example, an analysis of ]H2H formation during turnover of the enzyme has led to a rather different explanation of the reduction pathway, as shown in Eq. (88) (54). 

These two models are the principal attempts to explain nitrogenase function in atomic and molecular terms. There are other suggestions in the biological literature, of course, involving bridging dinitrogen, pro-... 

The postulation of the N2H2-level intermediate clearly implies that nitrogenase functions in a stepwise manner, with hydrazine therefore implicated as the second enzyme-bound intermediate. The question then arises as to whether hydrazine is a reducible substrate for nitrogenase. 

These genetic data support the suggestion of a parallel route for the synthesis of cofactors of Mo-independent nitrogenase function involving some early steps in common with FeMoco biosynthesis. 

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. 

Both in vivo and in vitro studies of nitrogenase functioning have shown that... 

---