Molybdenum nitrogenase substrates

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). 

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). 

Figure 1 Schematic representation of the molybdenum nitrogenase (a) and the substrate-reduction reaction of the molybdenum nitrogenase (b) and alternative nitrogenases (c)...

So little is known about molybdenum enzymes other than milk xanthine oxidase that there is little to be said by way of general conclusions. In all cases where there is direct evidence (except possibly for xanthine dehydrogenase from Micrococcus lactilyticus) it seems that molybdenum in the enzymes does have a redox function in catalysis. For the xanthine oxidases and dehydrogenases and for aldehyde oxidase, the metal is concerned in interaction of the enzymes with reducing substrates. However, for nitrate reductase it is apparently in interaction with the oxidizing substrate that the metal is involved. In nitrogenase the role of molybdenum is still quite uncertain. 

Fig. 6.11 The distribution in periplasmic space of the major molybdenum (and copper) enzymes except nitrogenase. Note the types of substrate.

The prosthetic group associated with the molybdenum atom of the molybdenum cofactor found in most molybdenum-containing enzymes except nitrogenase (See Molybdenum Cofactor). Many of these enzymes catalyze two-electron redox reactions involving the net exchange of an oxygen atom between the substrate and water. In bacterial enzymes a nucleotide is linked to the phosphoryl group. 

MgATP hydrolysis and, 47 189-191 nitrogenase complex, 47 186-189 substrates, 47 192-202 molybdenum iron proteins, 47 161, 166-174, 176-183, 191-192 structure, 47 162-164, 166-170 nitrogen fixation role, 36 78 in nitrogen fixation systems, 27 265-266 noncomplementary reactions with Sn", 10 215... 

Molybdoenzymes other than the nitrogenases are usually termed oxomolybdoenzymes. This prefix relates to the nature of the catalysis effected, i.e. the net effect of the conversion (xanthine to uric acid, sulfite to sulfate, nitrate to nitrite, or aldehyde to carboxylate) corresponds to the transfer of one oxygen atom to or from the substrate. Furthermore, molybdenum X-edge EXAFS studies have established that this metal is coordinated to one or more terminal oxo groups in each enzyme studied by this technique.204... 

As noted earlier, nitrogenase is made up of two proteins, the iron protein, and the molybdenum-iron protein, and will be linked to an electron-transport chain. The iron protein accepts electrons from this chain (a ferredoxin or flavodoxin in vivo, or dithionite in vitro) and transfers them to the molybdenum-iron protein. The MoFe protein is then able to reduce a number of substrates in addition to dinitrogen. No replacement electron donor will function instead of the iron protein. 

The substrate half-reactions are displayed in Tables I and II. In each case, a two-electron process seems to be involved. Only in nitro-genase are greater numbers of electrons transferred, and the discussion earlier in this paper summarizes the evidence that these processes occur in two-electron steps. The two-electron reaction of the molybdenum site never appears to be simply an electron transfer reaction. In the case of nitrogenase, each substrate takes up an equal (or greater) number of protons to form the product. In the other molybdenum enzymes, proton transfer and addition or removal of H20 are also required. In each case, however, there is at least one proton transferred in the same direction as the pair of electrons. These data, taken in conjunction with the EPR evidence for proton transfer from the substrate to the active site in xanthine oxidase, suggest that the molybdenum site in all the enzymes... 

There are >40 distinct molybdenum enzymes that occur in all classes of living systems and are especially important in the biochemical cycles of carbon, nitrogen, and sulfur (24b). The majority of the molybdenum enzymes, with notable exceptions including the nitrogenases (25-28) and a 2-hydroxyglutaryl-CoA dehydratase (10), catalyze a conversion of the type [Eq. 1], that is, the net effect of the catalysis corresponds to the transfer of an oxygen atom to or from the substrate. 

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