Nitrogenase binding sites

Early data on the substrate and inhibitor reactions of nitrogenase were interpreted in terms of five binding sites, with competitive, noncompetitive, unclassified, and negative inhibition being observed (127). This apparent complexity can be readily rationalized in terms of the Lowe—Thorneley scheme (Fig. 9) by assuming that different substrates bind at different oxidation states of the same site. 

Fig. 1. Schematic illustration of the enzyme nitrogenase being composed of the molybdenum-iron (MoFe) protein, an oc2p2 tetramer with two unique iron-sulfur clusters (P-cluster) and two iron-molybdenum cofactors (FeMoco), and the iron protein with one [4Fe-4S]-cluster and two ATP binding sites.

Much data point to an Intimate connection between H2 and the N2 binding site in nitrogenase. Simpson and Burrls(58) confirmed the important finding of Hadfield and Bulenf59) by showing that one H2 is evolved for each N2 "fixed" even at 50 ATM N2, which is well above the pressure of N2 at which saturation occurs. H2 evolution is therefore a mandatory part of the N2 fixation process( - ) whose stoichiometry must be written as ... 

One of the enzymes given in Table 23 is nitrogenase, which is responsible for the fixation of dinitrogen to give ammonia. Molybdenum probably serves as the binding site for N2, and is present in the iron-molybdenum cofactor, which is a molybdenum-iron sulfide cluster. Nitrogenase will be considered in Section 63.1.14, which deals with the nitrogen cycle. 

There are several circumstantial lines of evidence that indicate that N2 binds to the enzyme by displacing H2 (i.e., that the binding site of the enzyme is a metal hydride). The main evidence for a hydride site is (1) that H2 is a specific, competitive inhibitor for the reduction of N2 (2) the limiting stoichiometry of the MoFe nitrogenase in which we observe the apparent obligatory evolution of H2 during... 

The best-characterized molybdenum nitrogenase comprises two metallosulfur proteins, that is, the molybdenum-iron (MoFe) protein and the iron (Fe) protein, both of which are essential for the enzymatic activity. The Fe protein is an U2 homodimer (encoded by nifH) of Mr 60 kDa. The two subunits are bridged by a [4Fe-4S] cluster and each has a MgATP binding site. The MoFe protein is an U2P2 tetramer (encoded by niJD and nifK) of Mr 220 kDa. It contains the [8Fe-7S] cluster (P cluster) that is bridged between each ap subunit pair and the [Mo-7Fe-9S-homocitrate] cluster (FeMo cofactor or FeMoco) that is located within each a snbunit. 

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