Nebraska Redox Biology Center Educational Portal


Nickel (Ni) is a trace element utilized by wide group of archaea, bacteria, plants, and primitive eukaryotes. Nickel may have oxidation states ranging from -1 to + 4. However, Ni2+ and Ni3+ oxidation state are the most common form of nickel in biosystems [ 1, 2, 3, 4 ].

No nickel utilizing enzymes has been found in mammalian species at this time and Ni is considered as toxic element for human organism. Based on the Food and Drug Administration Total Diet Study of 1984, the mean nickel consumption of infants and young children was 69 to 90 µg/day. On the basis of a national survey conducted in five Canadian cities from 1986 to 1988 the average nickel consumption for children was 190 to 251 µg/day; and for all adults, 207 to 406 µg/day. Nickel Recommended Dietary Allowance for all age groups is not determined yet. The Tolerable Upper Intake Level (UL) for adults is 1 mg/day and 0.2-0.3 mg/day for childrens [ 5, 6 ].

Map of soil iron content in the U.S. (red = high iron areas). Courtesy of U.S. Department of the Interior, U.S. Geological Survey, Mineral Resources.

Nickel is used as cofactor for several enzymes involved in wide range of biological processes. Most of these enzymes are involved in energy production pathways, ROS detoxification and nitrogen metabolism [ 1, 2, 3, 4, 7 ].

1. Urease is the first characterized Ni-containing protein that has been found in is found in plants, algae, fungi, and several microorganisms. It catalyzes the hydrolysis of urea to carbon dioxide and ammonia. The active site of enzyme is evolutionary concerved and binds two Ni atoms [ 3, 8, 9, 10 ].

Crystal structure of Sporosarcina pasteurii urease . Ni atoms are shown as blue spheres.

2. Ni-Fe hydrogenase are bacterial and archaeal enzymes that catalyzes reversible H-H bond cleavage and formation and primarily utilized for hydrogen oxidation as part of energy metabolism [ 11, 12, 13, 14, 15 ]. There are three differen types of hydrogenases classified on the metal and subunit composition: (I) Fe-Fe hydrogenase; (II) Ni-Fe hydrogenase (some organisms contain Ni-Fe-Se hydrogenase); and (III) metal-free hydrogenases. One class of Ni-Fe is composed of two subunits which are structurally conserved. The large subunit contains the Ni active site, and the small subunit that contains an Fe-S cluster appears to be used in electron transfer from the large subunit. Other Ni-Fe hydrogenases are tetramers and are integral membrane proteins. [ 16, 17, 18, 19, 20 ].

Proposed mechanism of hydrogen binding and heterolysis in Ni-Fe hydrogenases [ 12].

Crystal structure of membrane-bound respiratory [Ni-Fe] hydrogenase from Hydrogenovibrio marinus". Ni atoms are shown as blue spheres as Fe-S clusters are shown as pink lines.

3. Carbon monoxide dehydrogenases are involved in reversible oxidation of CO to CO2 and uses water as the source of oxygen. These type of enzymes is typical for bacteria and archaea. The active site of Carbon?monoxide dehydrogenase contains complex of Ni, Fe, and S. Carbon?monoxide dehydrogenases from acetogenic bacteria and methanogenic archaea are bifunctional enzymes that are involved in reversible CO oxidation and the synthesis or degradation of acetyl-coenzyme A. Nickel is essential for catalisis in all types of carbon?monoxide dehydrogenase [ 21, 22, 23, 24, 25, 26, 27 ].

Carbon dioxide activation at the Ni-Fe center of anaerobic carbon monoxide dehydrogenase [ 22 ].

Crystal structure of CO dehydrogenase hydrogenase from Carboxydothermus hydrogenoformans". Active siteNi-Fe-S cluster is shown as blue lines and Fe-S cluster is shown as pink lines.

4. Methyl-coenzyme M reductase catalyzes the final step in the biological synthesis of methane in methanogenic archaea. This enzyme contains Ni in context of coenzyme F430 which is typical in methanogens. Nickel in this enzyme traverse the Ni+, Ni2+ and Ni3+ oxidation states [ 28, 29, 30, 31, 32 ].

Reverse methanogenesis. In methanogenic archaea, Methyl-coenzyme M reductase with its prosthetic group coenzyme F430 converts a methyl thioether (methyl- coenzyme M) and a thiol (coenzyme B) into methane and the heterodisulphide of coenzymes M and B [ 29 ].

Crystal structure of Methyl-coenzyme M reductase from Methanothermobacter thermautotrophicus ". F430 is shown as blue lines.

5. NiSOD was discovered in the cytosol of Streptomyces and cyanobacteria as well as in a few green algae. NiSOD contains one Ni per monomer and may form tetramers or hexamers. Superoxide dismutase detoxify superoxyde by catalyzing its dismutation. One superoxide molecule is being oxidized to O2 and another is reduced to H2O2. In the first step of reaction the ion metal is reduced by superoxide which loses its electron and is converted to molecular oxygen. As result, metal cofactor changes oxidation state (Ni3+ → Ni2+. In second step of reaction the ion metal is oxidized by superoxide which accept electron from metal and is converted to hydrogen peroxyde (Ni2+ → Ni3+). As result, the enzyme is returned to its initial reactive state [ 33, 34 ].

Ni3+ + O2.- + H+ ↔ Ni2+ + O2 ;
Ni2+ + O2.- + H+ ↔ Ni3+ + H2O2 ;

Crystal structure of nickel superoxide dismutase (NiSOD). Ni atoms are shown as blue spheres.

Eubacteria and archaea utilizes an ATP-binding cassette NikABCDE system for nickel uptake, whereas the most common mechanism in eukaryotes (also widely distributed in bacteria) involves members of the nickel/cobalt transporter (NiCoT) family. NikABCDE was first characterized in E. coli. NikB and NikC are transmembrane proteins that form a nickel pore. NikD and NikE are proteins that bind and hydrolyze ATP. NikA is a periplasmic protein that binds one nickel per protein along with a naturally-occurring organic metallophore. The major representative of NiCoT transporter family is HoxN, a high-affinity nickel permease found in Cupriavidus necator. This family can be divided into those specific to nickel, those capable of transporting both nickel and cobalt, and those with a preference for cobalt. CorA is a nonspecific nickel transporter in E. coli [ 35, 36, 37, 38 ]. CorA and other nonspecific Ni transporters may serve as the route of nickel entry under high environmental nickel concentrations. Nickel efflux in E. coli is associated with RcnA efflux pump. RcnA is shown to pump nickel and cobalt out of the cytoplasm. Another exporters of nickel include a putative metal efflux P1 type ATPase (nmtA) and a putative metal efflux pump (cdf) [ 39, 40, 41, 42, 43 ].

Nickel trafficking pathways in bacteria [ 36 ].

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