Vanadium nitrogenases structure

A preparation of the third nitrogenase from A. vinelandii, isolated from a molybdenum-tolerant strain but lacking the structural genes for the molybdenum and vanadium nitrogenases, was discovered to contain FeMoco 194). The 8 subunit encoded by anfG was identified in this preparation, which contained 24 Fe atoms and 1 Mo atom per mol. EPR spectroscopy and extraction of the cofactor identified it as FeMoco. The hybrid enzyme could reduce N2 to ammonia and reduced acetylene to ethylene and ethane. The rate of formation of ethane was nonlinear and the ethane ethylene ratio was strongly dependent on the ratio of nitrogenase components. 

Eady RR (2003) Current status of structure function relationships of vanadium nitrogenase, Coord Chem Rev 237 23-30... 

Vanadium nitrogenase is produced by certain bacteria grown in molybdenum-deficient environments. It is effective in the reduction of N2 and other nitrogenase substrates, although with less activity than the Mo—Nase. The enzyme resembles the Mo analogue (see Sections 17-E-10 and 18-C-13) in the construction and structure of the prosthetic groups, as well as in its functions.101 It consists of a FeV protein, FeVco, and an iron protein (a 4Fe—4S ferredoxin). 

DUrre P, Andreesen JR (1989) Die biologische Bedeutung von Selen. Biologie unsererZeit 16 12-19 Eady RR (2003) Current status of structure function relationships of vanadium nitrogenases. Coord Chem Rev 237 23-30 Egami F (1974) Minor elements and evolution. J Mol Evol 4 113-120... 

The vanadium nitrogenase from A. vinelandii has the same a2 2 subunit structure as the Mo enzyme. Interestingly, a second VNase has been isolated from A. vinelandii, lacking one of the a subunits and consequently one of the M clusters and half of one of the P clusters. This incomplete a/ 2 variant is still active.I ] VNases in general are less stable than their Mo counterparts, leading to a larger variability and pronounced problems when it comes to crystallisation. 

Structure information on the FeVco of vanadium nitrogenases has been obtained from iron and vanadium K-edge X-ray absorption spectroscopy (XAS) (for background information, see Section 3.6.), in particular so from the extended (EXAFS) region. 

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.

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. 

Ye S, Neese F, Ozarowski A et al (2010) Family of V(ni)-tristhiolato complexes relevant to functional models of vanadium nitrogenase synthesis and electronic structure investigations by means of high-frequency and -field electron paramagnetic resonance coupled to quantum chemical computations. Inorg Chem 49 977-988... 

Fig. 5. Buildup of the vanadium-nitrogenase from A. chroococcum, and the proposed structure of the M cluster (FeVco). The light scatterer X has been proposed to be N.

A number of nitrogen-fixing bacteria contain vanadium and it has been shown that in one of these, Azotobacter, there are three distinct nitrogenase systems based in turn on Mo, V and Fe, each of which has an underlying functional and structural similarity.This discovery has prompted a search for models and the brown compound [Na(thf)]+[V(N2)2(dppe)2] (dppe = Pli2PCH2CH2PPh2) has recently been prepared by reduction of VCI3 by sodium naphthalenide... 

Information, particularly structural, concerning vanadium-dependent nitrogenases, is relatively limited. The consensus is that they resemble the molybdenum nitrogenase in most aspects except for the presence of a FeV cofactor, and they will not be discussed further. 

Eady RR (1996) Structure-function relationships of alternative nitrogenases. Chem Rev 96 3013-3030 Emerson SR, Huested SS (1991) Ocean anoxia and the concentrations of molybdenum and vanadium in seawater. Mar Chem 34 177-196... 

Recently, a second or alternative nitrogenase has been isolated from Azotobacter vinelandii (21) and Azotobacter chroococcum (22) that contains vanadium as opposed to molybdenum. The MoFe and VFe nitrogenase proteins from A. vinelandii (called Av and. 4vl , respectively) are known to have different polypeptide structures and it obviously of interest to know to what extent the cluster composition is conserved. Variable temperature MCD studies of the as isolated and thionine oxidized proteins provided a convenient means of addressing this question. 

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