Nitrogenase MoFe protein crystal structure

A famous example is the structure of Nitrogenase MoFe-Protein, a protein that contains a Fe7MoS9 cluster. The inside of this cluster is about 4 A wide with six iron atoms closest to the centre, and older crystal stmctures had been determined at resolutions of about 2 A. Termination of the Fourier summation at that resolution creates an artefactual miiumum in the electron density of about —0.2 electrons about 2 A away from each iron atom. These spurious minima from all heavy atoms in the... 

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. 

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),... 

The FeMoco has been observed in three redox states. When the enzyme is isolated in the presence of dithionite, the FeMoco is in the M ( native ) form. Most c stallographic studies have taken place on enzyme in which the FeMoco is in the M state. M° can be generated by one-electron oxidation of M with dyes, and can be reactivated by reduction. The M° /M redox potential is dependent on the organism from which the nitrogenase is derived, lying in the range OmV to — 180mV. " The X-ray crystal structure of MoFe protein with the FeMoco in the M° state shows no major differences from M is the turnover state of the enzyme,... 

Iron-vanadium nitrogenase was isolated in 1986. Its nitrogen-reducing activity is 1/3-1/2 of that for molybdenum nitrogenase, and the VFe protein is spectroscopically different from the analogous MoFe protein. There is no X-ray crystal structure of any component of iron-vanadium nitrogenase. 

Finally, the FeMoeo is transferred to nifDK (apo-MoFe protein) to form the holo-MoFe protein with the help of ATP and several proteins nifH, 7, and GroEL. The X-ray crystal structure of nifDK shows a positively charged channel through which FeMoeo could enter. " The mf system for synthesis of the FeVco is thought to be similar to the nif system, with added assistance in the last step by vnfG, a third subunit of VFe nitrogenase that is necessary for activity. 

The molybdenum-containing nitrogenase was the first to be identified and is the most thoroughly studied enzyme of the three known nitrogenase types. The solid-state crystal structure of the MoFe protein, obtained from Azetohacler vine-landii, has been solved in both a dithionite-reduced and an oxidized form. The... 

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