Molybdenum, nitrogenase site

The electrosynthesis of hydride complexes directly from molecular hydrogen at atmospheric pressure by reduction of Mo(II) and W(II) tertiary phosphine precursors in moderate yield has been described as also the electrosynthesis of trihydride complexes of these metals by reduction of M(IV) dihydride precursors [101,102]. Hydrogen evolution at the active site of molybdenum nitrogenases [103] is intimately linked with biological nitrogen fixation and the electrochemistry of certain well-defined mononuclear molybdenum and tungsten hydrido species has been discussed in this context [104,105]. 

Despite the obvious relevance of dinitrogen complexes to the chemistry of the molybdenum-sulfur site of nitrogenase,6 dinitrogen complexes of molybdenum with sulfur ligation have proved to be very elusive. They are confined to the compounds... 

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. 

The mechanism of the reduction of N2 to ammonia catalyzed by the nitrogenase complex is one of the continuing mysteries of chemistry and biology. Since the first N2 complex was discovered [55] and the basic structure of the active site of conventional molybdenum nitrogenases was unraveled [43,44,48,53], efforts have been made to combine chemistry and biology to explain the mechanism of biological nitrogen fixation at the atomic level [56,57]. 

However, when the X-ray crystal structure of the MoFe protein was examined, it was clear that homocitrate could not directly hydrogen bond to the histidine, since the carboxylate group and imidazole are stacked parallel to each other in the crystal. Nevertheless, as noted in the previous section, studies on model complexes have suggested that homocitrate can become monodentate during nitrogenase turnover, with the molybdenum carboxylate bond breaking to open up a vacant site at molybdenum suitable for binding N2. 

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.

Stmcmre Elucidation in Unknown Systems. Certainly one of the most exciting uses of XAS is as a tool for characterizing stmcturally unknown systems. A dramatic example of this ability is provided by studies of the molybdenum site in nitrogenase. 

A large number of studies devoted to metal-sulfur centers are motivated by the occurrence of such arrangements at the active site of various metalloenzymes [1-13]. Mononuclear complexes with Mo=0 func-tion(s) and possessing sulfur ligands in their coordination sphere have been extensively investigated since they can be seen as models of the active site of enzymes such as nitrate- and DM SO reductases or sulfite- and xanthine oxidases [1-4]. On the other hand, a large variety of mono-, di-, and polynuclear Mo—S centers have been synthesized in order to produce functional models of the Mo-nitrogenase since the exact nature (mono-, di- or polynuclear) of the metal center, where N2 interacts within the iron-molybdenum cofactor (FeMo—co) of the enzyme is still unknown [4-8]. 

A large part of the research involving metal-sulfur complexes (metal = molybdenum or iron) is aimed at designing functional models of the active site of nitrogenase, the iron-molybdenum cofactor, EeMo—CO [4-8, 12, 13]. Only a very... 

In the earlier examples, the binuclear complexes with a M02S3 core clearly cannot be seen as modeling the metal sites in FeMo—co since it is known to contain a single molybdenum center [74]. However, what is suggested by these studies is that several steps (if not all) of the reduction of dinitrogen to ammonia by Mo-nitrogenase could take place at a... 

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