Nitrogenase active sites

Barney, B.M., Lukoyanov, D., Yang, T.-C., Dean, D.R., Hoffman, B.M. and Seefeldt, L.C. (2006) A methyldiazene (NH=N-CH3)-derived species bound to the nitrogenase active-site FeMo cofactor implications for mechanism, Proc. Natl. Acad. Sci. U.S.A., 103, 17113-17118. 

Thus, the hybrid cluster is a putative iron-sulfur redox catalyst. It is, however, a very uncommon cluster (perhaps only comparable to the nitrogenase active site) in two aspects (1) it is a hybrid cluster i.e., it contains intrinsic building blocks that are distinctly strange to iron-sulfur clusters and (2) it can exist in more than two (in fact, four [63]) oxidation states. 

The X-ray structure of the nitrogenase active site is obviously a good starting point for investigations on mechanistic aspects of the... 

Since the time of Daniel Rutherford, who discovered molecular nitrogen about 200 years ago, this gas has served as an example of a very inert substance. Thus, the mechanism of the relatively fast reduction of N2 in the nitrogenase active site with turnover about 0.2 s"1 appears as a mysterious and challenging problem not only for biochemists but for chemists as well. 

STRUCTURE AND PHYSICO-CHEMICAL PROPERTIES OF THE NITROGENASE ACTIVE SITES. 

Multielectron substrate reductions may involve the stepwise execution by the enzyme of two-electron processes. Further, about as many protons as electrons are usually transferred to the substrate. One way of viewing the nitrogenase active site is that it can add the elementary particles (H and e ) of H2 to the substrate. This may have mechanistic implications. ... 

But how does the FeMo-cofactor reduce N2 to ammonia, and where does the substrate N2 bind Several reaction pathways are discussed which employ a wide range of oxidation states of the Mo center [57,59,60]. Very little of the reactivity observed to date is likely to occiu at a nitrogenase active site. New insights in the speculative synthetic chemistry and the nitrogenase problem were reviewed recently. Note that without N2 as substrate, the enzyme spontaneously reduces protons to H2 [32]. 

These studies of protein-bound heterometallic cubanes have amply demonstrated that the heterometal site is redox active and able to bind small molecules. Although they have yet to be identified as intrinsic components of any protein or enzyme (except as part of the nitrogenase FeMo cofactor cluster (254)), they are clearly attractive candidates for the active sites of redox enzymes. 

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. 

Figure 31 X-Ray structure of the active site of Fe-protein in the nitrogenase of Azotobacter vinelandii. Polypeptide chains surround the central Fe4S4 cluster...

Despite the availability of the molecular structures of the different active sites of the FeMo-nitrogenases, the mechanism of nitrogen fixation remains obscure. The interest of our discussion, however, is centred on the various modes proposed to describe how molecular nitrogen might coordinate the FeMoco. Some of the reported schemes are inspired by the type of coordination found in model compounds. 

Dinitrogenase has been crystallized and its tertiary structure determined by Kim and Rees. [44, 45,46] As indicated, an Fe-Mo unit serves at the active site. Electrons are furnished to this active unit by the Fe enzyme dinitrogenase reductase. The two units together constitute nitrogenase. 

At the same time, this redox lability makes Mo well suited as a cofactor in enzymes that catalyze redox reactions. An example is the prominence of Mo in nitrogen fixation. This prokaryotic metabolism, the dominant pathway for conversion of atmospheric Nj to biologically-useful NH, utilizes Mo (along with Fe) in the active site of the nitrogenase enzyme that catalyzes Nj reduction. Alternative nitrogenases that do not incorporate Mo have been identified, but are markedly less efficient (Miller and Eady 1988 Eady 1996). 

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