Nitrogenase substrates

As indicated in Fig. 1, nitrogenase can reduce substrates other than Na. In the absence of other reducible substrates it will reduce protons to dihydrogen, but it can also reduce a number of other small triple-bonded substrates, as indicated in Section V,E,1. Large substrates are not reduced efficiently, indicating physical limitations on access to the enzyme s active site. CO is a potent inhibitor of all nitrogenase substrate reductions except that of the proton to Ha. In the presence of CO the rate of electron transfer is generally not inhibited, but all electrons go toward the production of Ha. 

Certainly, all three of the bands observed with SF-FTIR must arise from different species, since they appear and disappear with different time courses. The peak at 1904 cm probably corresponds with that observed by ENDOR under low CO conditions, but the relationship of the other two bands to those observed under high CO is not clear, since the ENDOR technique will only detect CO molecules bound to paramagnetic species, whereas FTIR should detect all species. The SF-FTIR technique has the potential to observe the binding and reduction of a wide range of nitrogenase substrates, provided that the appropriate spectroscopic range can be accessed. This will be technically difficult, but well worth the effort. 

Figure 5 MoFe3S4 clusters with the nitrogenase substrates bonded to their Mo site...

Clasalcally, acetylene binds to metal centers by using its K and ir orbitals to form, respectively, a-donor and jT-acceptor bonds to the metal. This situation can hold when the metal is in a sulfur coordination 64-65) environment such as In Mo0(S2CNR2)2(Rinteracts directly with the metal center a mode of binding that must be considered for nitrogenase substrates. 

Table 1 Nitrogenase Substrates and Their Reduction Products...

So far the discussion on synthetic clusters illustrates the accumulating information that a variety of nitrogenase substrates can be transformed by simple hydronation reactions at reduced synthetic Fe-S-based clusters. The next level of detail must address the mechanisms of these transformations. Already we have indicated several cases where kinetic studies have been performed. The major problem with the approaches taken so far, looking directly at substrate transformation, is that they can lead to erroneous conclusions. This is because in this approach the experimenter relies on the kinetics to define the number of species essential to accomplish the transformation. For example, the order with respect to hydrons has been established in several of the catalyic systems discussed and invariably found to be one. It is tempting to jump to the conclusion that only one hydron is necessary to activate the cluster. However, studies on the hydronation of Fe-S clusters show that the kinetics of simple hydronation reactions is much more complicated. 

Combining Eqs. 62a and b demonstrates that the formation of HD from D2 and water protons (which are the hydrogen source for HD) can be mediated by a diazene species. Diazene, on the other hand, is the most plausible intermediate of a [2 H+/2e ] reduction of N2, and it is a reduction intermediate that can form only from N2 and not from any other nitrogenase substrate. 

Vanadium nitrogenase is produced by certain bacteria grown in molybdenum-deficient environments. It is effective in the reduction of N2 and other nitrogenase substrates, although with less activity than the Mo—Nase. The enzyme resembles the Mo analogue (see Sections 17-E-10 and 18-C-13) in the construction and structure of the prosthetic groups, as well as in its functions.101 It consists of a FeV protein, FeVco, and an iron protein (a 4Fe—4S ferredoxin). 

VFe3S4X3] (X = Cl, Br, and I), (Me4N)2[TpVFeS4Cl3], and (Me4N)[(NH3)-(bipy)Fe3S4Cl3]. A study of the catalytic reduction of hydrazine (a nitrogenase substrate) to ammonia in the presence of an external source of electrons and protons shows that the rate of reduction decreases as the number of labile solvent molecules coordinated to the V atom decreases but does not depend on the nature of the atom attached to the Fe atoms. 

FeMoco as Catalyst for the Reduction of Nitrogenase Substrates at Amalgam Surfaces... 

As we have already seen, FeMo cofactor of nitrogenase is a polynuclear complex of composition Fc7MoS9 (homocitrate), and all the available evidence implies it is the active site at which dinitrogen and other nitrogenase substrates are activated and reduced. Yet despite its first isolation in 1977 the catalytic activity of FeMoco was detected only in 1997, i.e. 20 years later. 

The direct interaction of acetylene with the metal center must be considered as a potential binding mode for nitrogenase substrates. 

The seleetive ineorporation of selenium into the central P2 S positions of the FeMo-eofaetor was very recently reported. Selenium substitution was aeeomplished through the use of [SeCN] as a non-native nitrogenase substrate under turnover eonditions, and the ehalcogen label was traeked by maeromoleeular erystallographic analysis. Selenium was found initially only at the speeifie p2"S position corresponding to the substitution site observed in the CO-inhibited cofaetor structure upon further catalytic turnover... 

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