Molybdenum nitrogenases

Molybdenum nitrogenase has been the subject of intensive study for more than 30 years, but much less work has been done on the vanadium and iron-only nitrogenases. Consequently, we first review the properties of Mo nitrogenase, and then in later sections outline what is known of the other two enzymes. 

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 ... 

Fig. 8. The Fe protein cycle of molybdenum nitrogenase. This cycle describes the transfer of one electron from the Fe protein (F) to one afi half of the MoFe protein (M) with the accompEmying hydrolysis of 2MgATP to 2MgADP + 2Pf. The rate-determining step is the dissociation of F (MgADP)2 from M,rf. Subscript red = reduced and ox = oxidized.

Fig. 9. The MoFe protein cycle of molybdenum nitrogenase. This cycle depicts a plausible sequence of events in the reduction of N2 to 2NH3 + H2. The scheme is based on well-characterized model chemistry (15, 105) and on the pre-steady-state kinetics of product formation by nitrogenase (102). The enzymic process has not been chsiracter-ized beyond M5 because the chemicals used to quench the reactions hydrolyze metal nitrides. As in Fig. 8, M represents an aji half of the MoFe protein. Subscripts 0-7 indicate the number of electrons trsmsferred to M from the Fe protein via the cycle of Fig. 8.

Substrate reduction by vanadium nitrogenase has not been investigated as extensively as has molybdenum nitrogenase, but there are clear differences. Acetylene is a poor substrate and N2 does not compete as effectively with protons for the electrons available during turnover. Therefore, high rates of H2 evolution are observed in the presence of these substrates. Furthermore, acetylene is reduced to both ethylene and a minor product, ethane (172). Equation (2) summarizes the most efficient N2 reduction data yet observed for vanadium nitrogenase. 

Substrate reduction by the iron nitrogenase is very similar to that observed with vanadium nitrogenases. Acetylene is a relatively poor substrate, and N2 reduction is accompanied by considerable H2 evolution. Acetylene reduction leads to the production of some ethane as well as ethylene. Beyond this, little has been investigated. Under optimal conditions for N2 reduction, the ratio of N2 reduced to H2 produced was 1 7.5 compared with 1 1 for molybdenum nitrogenase 192). 

Information, particularly structural, concerning vanadium-dependent nitrogenases, is relatively limited. The consensus is that they resemble the molybdenum nitrogenase in most aspects except for the presence of a FeV cofactor, and they will not be discussed further. 

Elsen, S., Dischert, W., Colbeau, A., Bauer C. E. (2000) Expression of uptake hydrogenase and molybdenum nitrogenase in Rhodobacter capsulatus is coregulated by the RegB-RegA two-component regulatory system./. Bacteriol., 182, 2831-7. 

Schneider, K., Gollan, U., Drottboom, M. et al. Selsemaier-Voight, S. Plass, W. and Muller, A. (1997) Comparative biochemical characterization of the iron-only nitrogenase and the molybdenum nitrogenase from Rhodobacter capsulatus. Eur. J. Biochem., 244, 789-800. 

The electrosynthesis of hydride complexes directly from molecular hydrogen at atmospheric pressure by reduction of Mo(II) and W(II) tertiary phosphine precursors in moderate yield has been described as also the electrosynthesis of trihydride complexes of these metals by reduction of M(IV) dihydride precursors [101,102]. Hydrogen evolution at the active site of molybdenum nitrogenases [103] is intimately linked with biological nitrogen fixation and the electrochemistry of certain well-defined mononuclear molybdenum and tungsten hydrido species has been discussed in this context [104,105]. 

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

Hu YL, Lee CC, Ribbe MW. Extending the carbon chain hydrocarbon formation catalyzed by vanadium/molybdenum nitrogenases. Science. 2011 333 753-5. 

The most extensively studied nitrogenase enzyme contains two kinds of transition metals, that is, iron and molybdenum, and is called molybdenum nitrogenase. In growth conditions where molybdenum concentration is low, a nitrogenase depending on iron and vanadium is expressed. ... 

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