Molybdenum nitrogenase mechanisms

A comprehensive description of the mechanism of molybdenum nitrogenase has been provided by the Lowe-Thorneley scheme 102) (Figs. 8 and 9). In this scheme the Fe protein (with MgATP) functions as a single electron donor to the MoFe protein in the Fe protein cycle (Fig. 8), which is broken down into four discrete steps, each of which may be a composite of several reactions ... 

Burgess BK, Lowe DL. Mechanism of Molybdenum Nitrogenase. Chem Rev. 1996 96 2983-3012. 

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. 

GJ. Leigh. 1995. The mechanism of dinitrogen reduction by molybdenum nitrogenases Eur. J. Biochem. 229 14-20. (PubMed)... 

Burgess BK and Lowe DJ (1996) Mechanism of molybdenum nitrogenase. Chem Rev 96 2983 -3011. 

There are notable reactivity differences between iron-vanadium and iron molybdenum nitrogenases. Iron-vanadium nitrogenase allocates electrons less effectively, producing at least three molecules of H2 for every molecule of N2 reduced. The mechanism may be similar, as evidenced by production of hydrazine under similar quenching conditions that are effective for Mo nitrogenase.Acetylene is reduced to mixtures of ethylene and ethane by iron-vanadium nitrogenase, whereas iron-molybdenum nitrogenase forms only ethylene. 

The mechanism of the reduction of N2 to ammonia catalyzed by the nitrogenase complex is one of the continuing mysteries of chemistry and biology. Since the first N2 complex was discovered [55] and the basic structure of the active site of conventional molybdenum nitrogenases was unraveled [43,44,48,53], efforts have been made to combine chemistry and biology to explain the mechanism of biological nitrogen fixation at the atomic level [56,57]. 

The conversion of dinitrogen to ammonia is one of the important processes of chemistry. Whereas the technical ammonia synthesis requires high temperature and pressure (1), this reaction proceeds at room temperature and ambient pressure in nature, mediated by the enzyme nitrogenase (2). There is evidence that N2 is bound and reduced at the iron-molybdenum cofactor (FeMoco), a unique Fe/Mo/S cluster present in the MoFe protein of nitrogenase. Although detailed structural information on nitrogenase has been available for some time (3), the mechanism of N2 reduction by this enzyme is still unclear at the molecular level. Nevertheless, it is possible to bind and reduce dinitrogen at simple mono- and binuclear transition-metal systems which allow to obtain mechanistic information on elemental steps involved... 

Scheme 1. Proposed mechanism for the catalytic reduction of dinitrogen at a molybdenum site in nitrogenase...

Mechanistic speculations about the molybdoenzymes must be considered to be in their infancy with the possible exception of those for xanthine oxidase. Although the detailed structural nature of the molybdenum site is unknown, there is sufficient information from biochemical and coordination chemistry studies to allow informed arguments to be drawn. Here we first discuss evidence for the nuclearity of the molybdenum site and then discuss both oxo-transfer and proton-electron transfer mechanisms for molybdenum enzymes. A final discussion considers the unique aspects of nitrogenase and the possible reasons for the use of molybdenum in enzymes. 

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