Nitrogenases crystal structure

In late 1992 the first crystal structures of the Fe and MoFe proteins of Mo nitrogenase frora Azotobacter vinelandii were published (1-3). 

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. 

The elucidation of the crystal structures of two high-spin EPR proteins has shown that the proposals for novel Fe-S clusters are not without substance. Two, rather than one novel Fe-S cluster, were shown to be present in nitrogenase, the key enzyme in the biotic fixation of molecular nitrogen 4, 5). Thus the FeMoco-cofactor comprises two metal clusters of composition [4Fe-3S] and [lMo-3Fe-3S] bridged by three inorganic sulfur atoms, and this is some 14 A distant from the P-cluster, which is essentially two [4Fe-4S] cubane moieties sharing a corner. The elucidation of the crystal structure of the Fepr protein (6) provides the second example of a high-spin EPR protein that contains yet another unprecedented Fe-S cluster. 

Cluster models of the polymetallic aggregates in nitrogenase should (1) contain structural features akin to those defined by the crystal structure (2) possess or approximate the observed stoichiometry (3) provide coordination sites at which substrate binding and activation may occur. A model need not mimic protein-bound cofactor reactivity, since isolated cofactors do not reduce sub-... 

The next two entries to Table 3 are cited for completeness. Nitrogenase is treated in Chapter 7 and CO dehydrogenase in Chapter 9. Nitrogenase contains a very complex iron-sulfur cluster that includes another metal, molybdenum or vanadium. The crystal structure of the Mo variant has been determined. There is a third variant, alternative nitrogenase [92], whose cluster apparently does not contain any heterometal. That cluster would thus be a perfect candidate for our definition of a redox-catalytic iron-sulfur cluster. Unfortunately, this third nitrogenase has thus far been characterized to a much lesser extent than the other two forms. For all nitrogenases holds that the binding of N2 to the cluster has not been established [53] therefore, formally these enzymes have not yet been positively identified as redox iron-sulfur catalysts. 

Strop P,Tatahara PM, Chiu H-J, Angove HC. Crystal structure of the all-ferrous [4Fe-4S]° form of the nitrogenase iron protein from Azotobacter vinelandii. Biochemistry 2001 40 651-6. 

The crystal structure of aldehyde ferredoxin oxidoreductase (AOR) from the hyperthermophile Pyrococcus furiosus was the first of any molybdenum or tungsten enzyme (excepting nitrogenase) (42). The AOR was adopted as the parent name for the family of tungsten enzymes. The structure of formaldehyde ferredoxin oxidoreductase (FOR) has recently been solved (44). 

With the availability of the crystal structures for both nitrogenase proteins, some general features of the complex between the two proteins may be addressed. Two residues of Fe-protein that have been identified as interacting with the MoFe-protein, Arg 100 (109) and Glu 112 (110),... 

Table 4.9 Selected data from XAS investigations of vanadium nitrogenases from Azofobacter. The number of scatterers (next neighbours to vanadium) n and the distances d are provided. For the model compound, data from single-crystal structure results (XRD) are added for comparison. Data from refs. 102 and 106a. See also text.

A cartoon representation of nitrogenase in operation, showing the component proteins. The active site of nitrogenase that resides in the Fe—Mo protein determined from a crystal structure analysis, and which contains two linked distorted M4S4 cubes, is also shown. 

A) Preliminary x-ray crystal structure of the Azotobacter vinelandii nitrogenase Fe protein. ... 

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