Nitrogenase inhibition

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. 

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. 

The hydrogenase and nitrogenase, which produce H2, are inhibited or irreversibly damaged by the oxygen produced. 

Growth inhibited Photosynthesis inhibited Nitrogenase activity inhibited... 

Michalski, W. P., and Nicholas, D. j. D. (1987). Inhibition of nitrogenase by nitrite and nitric oxide in Rhodopseudomonas sphaeroides f. sp. denitrificans. Arch. Microbiol. 147, 304-308. 

Nitrogenase catalyzes the reduction of a number of substrates in addition to N2 and H+. Acetylene is reduced to ethylene, and nitrous oxide to dinitrogen, while azide undergoes reduction to N2 and NH3. In the last two cases the product N2 is not reduced further, implying that it is not in the correct binding position for reduction. The presence of multiple sites on the MoFe protein suggested by this observation is also supported by the non-competitive nature of the inhibition shown by some of these compounds. The reduction of acetylene has been used as a marker reaction for nitrogenase activity. 

Figure 3. Nitrogenase—scheme of H2 inhibition and HD production reactions...

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