Nitrogenase enzyme system

The process of nitrogen fixation is an essential part of the nitrogen cycle on the planet earth(l). It is estimated that greater than 60Z of the N2 that is ultimately converted to NH4+ is done so by the nitrogenase enzyme system. The availability of fixed nitrogen is often the limiting factor in plant growth( ). To... 

R. N. F. Thomeley and D. J. Lowe, Kinetics and mechanism of nitrogenase enzyme system, in Molybdemun Enzymes , ed. T. G. Spiro, John Wiley Sons, New York, 1985,... 

N2 fixers represent an important functional group that includes a variety of physiological types with unique nutrient requirements. Furthermore, N2 fixers have unique or elevated cell quotas for certain metals as a result of possessing the nitrogenase enzyme system which requires both molybdenum and iron. 

All N2-fixing organisms contain an enzyme system called nitrogenase, which is responsible for N2-reduction. For the performance of N2-reduction the nitrogenase enzyme system has to solve two problems a) Activation of the inert N2-molecule via distortion of the interatomic bond probably by a sufficient energy supply" and b) Reduction of activated N2 by the use of an appropriate electron source of sufficient reducing power. 

In this chapter we hrst review the chemistry of the Fe-S sites that occur in relatively simple rubredoxins and ferredoxins, and make note of the ubiquity of these sites in other metalloenzymes. We use these relatively simple systems to show the usefulness of spectroscopy and model-system studies for deducing bioinorganic structure and reactivity. We then direct our attention to the hydrogenase and nitrogenase enzyme systems, both of which use transition-metal-sulfur clusters to activate and evolve molecular hydrogen. 

The conventional nitrogenase enzyme system consists of two components, the iron (Fe)-protein and the molybdenum-iron (MoFe)-protein. The Fe-protein is an electron carrier that serves, with ATP hydrolysis, as the functionally obligate reductant for the MoFe-protein where substrate chemistry occurs. 

In 1930, Hermann Bortels (1902-1979) recognised that nitrogen fixation is a molybdenum-dependent process. Obviously, the nitrogenases from Rhizobium meliloti, Azotobacter vinelandii and Clostridium pasteurianum have a similar constitution. In 1966, Leonard E. Mortenson identified for the first time an Fe- and a MoFe-protein as parts of the nitrogenase enzyme system. The exact structure of the nitrogenase-molybdenum-iron protein from Azotobacter vinelandii [28] was clarified in 1992, and that from Clostridium pasteurianum [29] in 1993, both by Douglas C. Rees. [30] The Fe-protein is a y2-dimer with a molar mass of some 60,000 Daltons, and the MoFe-protein is an ca. 

Molybdenum complexes with sulfur-donor atoms play an important role in the nitrogenase enzyme system, and the study of their chemistry is of continuing interest. The present synthesis concerns cis-dinitrosylbis(lV,Af-diethyldithiocarbamato)molybdenum, originally described by Johnson et al. Their method of synthesis involves the conversion of molybdenum hexa-carbonyl to the unstable MoBr2(NO)2 using NOBr, followed by reaction... 

The enzyme systems responsible for fixing atmospheric N2 to form ammonia are known as the nitrogenases. These enzymes function at field temperatures and 0.8 atm N2 pressure, whereas the industrial Haber-Bosch process requires high temperatures (300-400°C) and high pressures (200-300 atm) in a capital-intensive process that relies on burning fossil fuel. Small wonder, then, that the chemistry of the nitrogenases has attracted considerable attention for many years. 

The biological nitrogen fixation process is Introduced. Discussion focusses on the Dominant Hypothesis of nitrogenase composition and functioning. The enzyme system catalyzes the six-electron reduction of N2 to 2 NH3 concomitant with the evolution of H2. ATP hydrolysis drives the process. The two protein components of the enzyme,... 

Extensive studies on nitrogenases, and in particular on the Fe-Mo protein (1) component of these complex enzymic systems, have revealed the presence of a unique Fe/Mo/S aggregate intimately involved in biological N2 fixation. 

The potentially serious aspects of vanadium pollution, the function of biologically occurring enzyme systems, the role of vanadium on the function of numerous enzymes, and the associated role in the insulin-mimetic vanadium compounds are inextricably linked. The key to our understanding all such functionality relies on understanding the basic chemistry that underlies it. This chemistry is determined to a significant extent by the V(IV) and V(V) oxidation states but clearly is not restricted to these states. Indeed, the redox interplay between the vanadium oxidation states can be a critical aspect of the biological functionality of vanadium, particularly in enzymes such as the vanadium-dependent nitrogenases, where redox reactions are the basis of the enzyme functionality. 

Most of the current knowledge concerning the mechanism of nitrogenases is based upon extensive studies of the molybdenum nitrogenase hence this enzyme will be the focus of discussion in this article. The general properties of the molybdenum nitrogenase are described in Section 2, and the catalytic and assembly mechanism of this enzyme system will be dealt with in Sections 3 and 4. The alternative nitrogenase systems will also be briefly discussed in Section 5. 

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