Nitrogenases vanadium-dependent

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. 

Enzymes requiring vanadium for catalytic activity. Perhaps the best studied of these are the vanadium-dependent nitrogenases [EC 1.18.6.1]. Other vanadium-dependent enzymes include vanadium haloperoxidase, vanadium chloroperoxidase, and vanadium bromoper-oxidase. In the vanadium chloroperoxidase and bromo-peroxidase reactions, the vanadium(V) is coordinated in a trigonal bipyramidal site to a histidyl residue, three nonprotein oxygens, and, presumably, to a hydroxide. 

The potentially serious aspects of vanadium pollution, the function of biologically occurring enzyme systems, the role of vanadium on the function of numerous enzymes, and the associated role in the insulin-mimetic vanadium compounds are inextricably linked. The key to our understanding all such functionality relies on understanding the basic chemistry that underlies it. This chemistry is determined to a significant extent by the V(IV) and V(V) oxidation states but clearly is not restricted to these states. Indeed, the redox interplay between the vanadium oxidation states can be a critical aspect of the biological functionality of vanadium, particularly in enzymes such as the vanadium-dependent nitrogenases, where redox reactions are the basis of the enzyme functionality. 

Coupling the results from the Mo model complex [57] with the explanation for the presence and possible action of homocitrate, these results provide the best model to date for N2 reduction by conventional molybdenum nitrogenases. Vanadium-dependent nitrogenases probably undergo similar reactions, but iron-only nitrogenases [66] and the unique molybdenum-dependent nitrogenase from Streptomyces thermoautrophicus [67] present further mysteries. 

Seefeldt, L. C., Hoffman, B. M., Dean, D. R. (2009). Mechanism of Mo-dependent nitrogenase. The Annual Review ofBiochemistry, 78,701—722. Ueki, T, Furano, N., Michibata, H. (2011). A novel vanadium transporter of the Nramp family expressed at the vacuole of vanadium-accumulating cells of the ascidian Ascidia sydneiensis samea Biochimica et Biophysica Acta, 1810, 457—464. 

Only recently the isolation of a vanadium dependent peroxidase from algae (ref. 2), and a vanadium nitrogenase from Acotobacter (ref. 3) has changed this view. 

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. 

The most extensively studied nitrogenase enzyme contains two kinds of transition metals, that is, iron and molybdenum, and is called molybdenum nitrogenase. In growth conditions where molybdenum concentration is low, a nitrogenase depending on iron and vanadium is expressed. ... 

Applications to specific biological systems containing vanadium will be addressed in some detail in the context of the respective subsections of Chapter 4 on naturally occurring vanadium compounds vanadium in sea squirts (e.g. Figure 4.3), vanadate-dependent haloperoxidases (e.g. Table 4.5) and vanadium nitrogenases (e.g. Table 4.8). The central messages, including key references, are briefly summarised here. 

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