Nickel transport protein

Figure 6.1. Topological model for the HoxN nickel permease of R. eutropha that is like the HupN of B. japonicum (Eitinger et al. 1997). The residues (two histidines and one aspartic acid) in the second transmembrane segment were shown to be key residues for nickel transport, and this motif (His-Xaa4-Asp-His) is conserved in nickel-transport proteins. Bold, residues studied by site-directed mutagenesis.

Figure 4.1 Nickel transport in . co//.The lower part of the figure shows the nik gene cluster from coli which encodes a nickel specific ABC-type transporter and the repressor protein (NikR).The likely roles of the various nik gene products are indicated in the upper part of the figure.The transporter is encoded by the nikBCDE genes whilst a periplasmic nickel-binding protein is encoded by n/M.The system is expressed when nickel is low. High nickel represses expression via NikR and under these conditions nickel is transported via the Mg transporter (top left).

Very little is known about the transport of nickel, manganese and cobalt. Plasma transferrin, conalbumin and citrate have been suggested to serve as carrier ligands. Transferrin is the main transport protein for vanadium in humans, and will transfer vanadium to ferritin. 

Intracellular transport of metal ions has been a very well studied research area in the last few years. The most often-cited case of this transport involves copper transport in yeast by the copper metallochaperones 24, 25). Cu(I) enters the cytoplasm of yeast via copper transport receptors, and Cu(I) is bound by one of three transport proteins Lys7, Atxl, or Cox 17. Lys7 delivers copper to CuZn superoxide dismutase, Atxl delivers copper to ccc2 that activates an Fe(II) uptake system, and Cox 17 delivers copper to the mitochondria for the ultimate uptake into cytochrome c oxidase. A similar copper transport system has been reported in humans (26), and there may be a system in bacteria as well (27). Metal ion transport systems are known for iron, nickel, and manganese 24, 25, 28). However, no cytoplasmic Zn(II) transporters have been identified in... 

Toxic and carcinogenic effects of nickel compounds are associated with nickel-mediated oxidative damage to DNA and proteins and to inhibition of cellular antioxidant defenses. Most authorities agree that albumin is the main transport protein for nickel in humans and animals and that nickel is also found in nickeloplasmin - a nickel-containing alpha-macroglobulin - and in an ultrafilterable semm fraction similar to a nickel-histidine complex. Normal routes of nickel intake for humans and animals are ingestion, inhalation, and... 

Physiologically important metal ions include iron, copper, cobalt and nickel. Normally, metal ions are not present in free solution to any significant extent, but are bound to transport proteins (in plasma) or storage proteins and enzymes (in cells). Thus, iron is bound to transferrin (in plasma) and haemosiderin and ferritin in tissues, copper is bound to caeruloplasmin in plasma, and metallothionein in plasma binds a wide variety of metal ions. The adverse effects of iron overload (section 4.5.1) are the result of free iron, not bound to storage proteins, acting as a source of oxygen radicals. 

Navarro, C., Wu, L. F. and Mandrand-Berthelot, M. A. (1993) The nik operon oi Escherichia coli encodes a periplasmic binding-protein-dependent transport system for nickel. Mol. 

Electron-transferring subunit, nickel-containing hydrogenases, 38 409-410 Electron transport blue copper proteins, 36 378 NiFe hydrogenase, 47 16-17 Electron volt, 16 73... 

The role of the transition elements in living systems is equally important. Iron is present in biomolecules such as hemoglobin, which transports oxygen from our lungs to other parts of the body. Cobalt is an essential component of vitamin B12. Nickel, copper, and zinc are vital constituents of many enzymes, the large protein molecules that catalyze biochemical reactions. 

Iron (Fe) Iron is arguably the most important construction metal. In addition to buildings and cars, iron alloyed with chromium, molybdenum, nickel, and carbon form surgical stainless steel. Each hemoglobin molecule, the protein that transports oxygen in the blood, contains four ferrous (Fe+2) ions, each of which serve as a point of attachment for an oxygen molecule. 

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