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  Section: Plant Nutrition » Micronutrients » Nickel
 
 
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Nickel Containing Enzymes and Proteins

 
     
 
Content
Introduction
Discovery of the Essentiality of Nickel
Physical and Chemical Properties of Nickel and Its Role in Animal and Bacterial Systems
  Nickel-Containing Enzymes and Proteins
  Essentiality and Function of Nickel in Plants
  Influence of Nickel on Crop Growth
Diagnosis of Nickel Status
  Symptoms of Deficiency and Toxicity
Concentration of Nickel in Plants
Uptake and Transport
Nickel in Soils
  Nickel Concentration in Soils
  Nickel Analysis in Soils
Nickel Fertilizers
Conclusion
References

The field of nickel metallobiochemistry has seen tremendous growth over the preceding 10 years, and nickel is clearly a biologically important element in a diverse range of organisms. Indeed, it is highly likely that with the advent of molecular techniques to search for genetic and functional homology rapidly, the diversity of known functions of nickel in biology will increase substantially in the coming years. Advances in the field of bacterial and animal biology will rapidly flow to the plant sciences.

To date, seven nickel-dependent enzymes have been identified. Two of these enzymes have nonredox function (urease and glyoxylase), and the remaining five involve oxidation–reduction reactions (Ni-superoxide dismutase, methyl coenzyme M reductase, carbon monoxide dehydrogenase, acetyl coenzyme A synthase, and hydrogenase).

In all microorganisms that produce nickel-dependent metalloenzymes, there exist a number of proteins involved in nickel uptake, transport storage, and incorporation into the metalloenzyme. In bacteria, the transport of nickel into the cell involves two high-affinity transport systems, an ATPdependent Nik family (Nik a–e) in Escherichia coli and a variety of nickel permeases (NixA, HoxN, etc.) in diverse species (17). Incorporation of nickel into the metalloenzyme involves a number of accessory proteins including metallo-chaperones (UreE, HypB, and CooJ) involved in nickel storage and in protein assembly (17).

Of the established nickel enzymes and proteins, urease is the sole nickel-specific enzyme known to function in plants; however, nickel-dependent hydrogenase also indirectly influences plant productivity through its role in nitrogen-fixing symbionts (20) and in leaf commensal bacteria (21). Currently, none of the bacterial proteins involved in nickel uptake and assimilation (NikA, NixA, UreE, etc.) is known to be present in plants. Interestingly, the hydrogenase and urease activities of leaf-surface symbionts are clearly inhibited when they colonize urease-deficient soybean mutants (21). The mechanism by which this inhibition occurs is unknown but may suggest that the ureasedeficient mutants lack key nickel assimilatory proteins, thus preventing the transfer of nickel to the leaf-surface bacterial enzymes. This possibility would suggest that plants might contain nickeldependent assimilatory proteins.



Nielsen reported the first description of a dietary deficiency of nickel in animals in 1970 for chickens and later for rats (Rattus spp.), goats (Capra hircus), sheep (Ovis aries), cows (Bos taurus), and mini pigs (Sus scrofa) (7). Nickel deficiency in these animals results in growth depression, physiological and anatomical disruption of liver function, and disruption of iron, copper, and zinc metabolism resulting in reduced levels of these enzymes in blood and various organs (22). Nickel deficiency also markedly reduces the activity of a number of hepatic enzymes, including several hydrogenases, urease, and glyoxylase, though a specific functional role for nickel in these enzymes in animals has not been determined.


One of the important and consistent findings from animal studies is that nickel deficiency induces iron deficiency, an observation that is also made in plants (15). In rats (22), and in sheep (23), nickel deprivation resulted in decreased iron uptake and reduced tissue-iron concentrations. Nielsen et al. (24) have suggested several possible roles for nickel in iron metabolism and oxidation- reduction (redox) shifts that draw upon the observation that nickel and iron are associated in a number of bacterial redox-based enzymes (17).



The suggestion that additional nickel-dependent enzymes and proteins are present in higher plants is supported by the observation that several of the known bacterial nickel-containing enzymes have analogs in plants and animals (including superoxide dismutase, glyoxylase, acetyl coenzyme A synthase, and hydrogenase). Our current failure to identify additional nickel-dependent enzymes in plants is likely a result of the relatively primitive state of plant enzymology, in contrast to bacterial enzymology, and the difficulty involved in research on complex organisms involving ultra-trace elements. The similarity between the effects of nickel deficiency in animals and plants also provides evidence of a common biological role for nickel in all organisms.



 
     
 
 
     



     
 
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