NEBRASKA REDOX BIOLOGY CENTER EDUCATIONAL PORTAL

Nebraska Redox Biology Center Educational Portal


Cobalt

Cobalt (Co) is a trace element essential for human health. Vitamin B12 is a major for of cobalt utilization in biological system. Cobalt can exist in oxidation states from Co1+ to Co4+, however, the major oxidation states for cobalt are Co2+ and Co3+ [ 1, 2, 3, 4, 5, 6 ].

Little is known about the role of dietary cobalt, other than as a component of vitamin B12. Humans obtain their vitamin B12 primarily from animal food sources the role of free cobalt is limited. Vitamin B12 produced by colon bacteria is not absorbed. Recommended Dietary Allowance for vitamin B12 is 2.4-2.8 μg/day. The median intake of vitamin B12 from food in the United States was estimated to be approximately 5 μg/day for men and 3.5 μg/day for women. Tolerable Upper Intake Level for vitamin B12 is not defined at this time. Normal daily cobalt intake is reported to be in the range 2.5-3.0 mg/day. Toxicity with cobalt has been reported to occur within the range of greater than 25-30 mg/day [ 7, 8 ].

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

The main form of cobalt in biological systems is vitamin B12, a cofactor involved in wide range of essential reaction. In addition cobalt may be used by narrow group of noncorrin cobalt-dependent enzymes such as methionine aminopeptidase, prolidase, nitrile hydratase, glucose isomerase, methylmalonyl-CoA carboxytransferase, aldehyde decarbonylase, lysine-2,3-aminomutase, and bromoperoxidase [ 1, 2, 3, 4, 5, 6 ]. Vitamin B12 enzymes includes adenosylcobalamin-dependentisomerases, methylmalonyl-CoA mutases, isobutyryl-CoA-mutases, ethylmalonyl-CoA mutaseglutamate mutases, methyleneglutarate mutases, d-lysine 5,6-aminomutases, diol dehydratases, glycerol dehydratases, ethanolamine ammonia lyases, B12-dependent ribonucleotide reductases, methylcobalamin-dependent methyltransferases, methionine synthases, methyltransferases (Mta, Mtm, Mtb, Mtt, Mts, and Mtv), B12-dependent reductive dehalogenases and B12-dependent reductive dehalogenases [ 2, 9, 10 ].


Bacteria uses several cobalt uptake systems to met cobalt requirements and avoit cobalt toxicity. Gram-negative bacteria have developed transport systems across both the outer and inner membranes [ 11 ]. Cobalt ions are transported across the outer membrane by high-affinity transporters called OM receptors. These receptors are energetically coupled to the proton motive force (PMF) of the IM by the TonB complex. This complex consist of three proteins, TonB-ExbB-ExbD, which span the periplasm and transduces the PMF to induce a conformational change in the TonB-dependent transporter. Then cobalt is transported into the cytosol through one of three types of transporters: an ABC transporter, an energy-coupling factor (ECF) transporter, or a nickel-cobalt (NiCoT) permease [ 3, 12, 13 ]. The BtuBCDF system of gram-negative bacteria takes up cobalamin similarly to the TonB-dependent iron-siderophore complex uptake systems. In this system, BtuB is an OM receptor which takes up cobalamin in a TonB-dependent fashion. Then, cobalamin is bound by BtuF, a periplasmic binding protein which is a component of ABC transporters. The cobalamin transported into the cytosol by BtuCD, the transmembrane permease and ATPase. There are no any high-affinity Co uptake systems have been reported to date for eukaryotes. [ 3, 14, 15 ].

RcnA is an efflux pump responsible for Ni and Co detoxification in E. coli. The expression of rcnA is regulated by RcnR, a Co2+/Ni2+ specific DNA-binding protein. RcnR binds to the rcnA promoter, blocking transcription of rcnA. When intracellular concentrations of Co2+ or Ni2+ increase, RcnR binds to these ions, dissociating from the DNA and triggering the transcription of rcnA [ 16, 17, 18 ]. CnrCBA is another type of cobalt antiporter belonging to the RND family of efflux pumps. CnrCBA spans the Gram-negative cell envelope and is responsible for the export of excess intracellular Co2+/Ni2+ [ 19, 20 ].

OM and IM indicate outer and inner membrane, respectively. Arrows in transporters indicate the direction of metal ion transport. Red crosses represent transporter inhibition or enzyme inactivation. Cobalt and iron ions are shown in pink and green spheres [ 3 ].

Vitamin B12 uptake is essential for B12-utilizing organisms that lack the ability to synthesize this cofactor. Vitamin B12 can produced oxygen-dependent (aerobic) and oxygen-independent (anaerobic) pathways in B12 synthesizing bacteria. These pathways are differ mainly in the early stages. The aerobic pathway incorporates molecular oxygen into the macrocycle as a prerequisite to ring contraction. The anaerobic pathway is based chelated cobalt ion in the absence of oxygen [ 21, 22, 23 ].

Vitamin B12 biosynthesis in bacteria [ 6 ].


Adenosylcobalamin-dependent isomerases are the largest family of B12-dependent enzymes. These enzymes are mainly found in bacteria. Adenosylcobalamin-dependent isomerases subfamilies include methylmalonyl-CoA mutase, isobutyryl-CoA mutase, ethylmalonyl-CoA mutase, glutamate mutase , methyleneglutarate mutase , d-lysine 5,6-aminomutase (5,6-LAM), diol/glycerol dehydratase, ethanolamine ammonia lyase and B12-dependent ribonucleotide reductase [ 2, 9, 10 ].

Methylmalonyl-CoA mutase catalyzes the isomerization of methylmalonyl-CoA to succinyl-CoA in the pathway that converts catabolites of odd-chain fatty acids, branched-chain amino acids, and cholesterol to a key intermediary metabolite. This is only B12-dependent enzyme found in bacteria and eukaryotes [ 24, 25, 26, 27 ].

Isobutyryl-CoA mutase catalyzes the reversible rearrangement of isobutyryl-CoA to n-butyryl-CoA. This enzyme is involved in play a key role in valine and fatty acid catabolism as well as in the production of fatty acid-CoA thioester compounds [ 28, 29 ].

Ethylmalonyl-CoA mutase catalyzes the transformation of ethylmalonyl-CoA to methylsuccinyl-CoA [ 30, 31 ].

Glutamate mutase catalyzes the reversible rearrangement of (2S)-glutamate to (2S,3S)-3-methylaspartate [ 32 ].

2-methyleneglutarate mutase catalyzes the equilibration of 2-methyleneglutarate with (R)-3-methylitaconate [ 33 ].

d-Lysine 5,6-aminomutase is adenosylcobalamin and pyridoxal-5'-phosphate-dependent enzyme that catalyzes a 1,2 rearrangement of the terminal amino group of DL-lysine and of L-β-lysine [ 34, 35 ].

Diol dehydratase and glycerol dehydratase are similar enzymes that catalyze the elimination of water from glycerol and 1,2-propanediol (1,2-PD) to the corresponding aldehyde via a B12-dependent radical mechanism [ 36, 37 ].

Ethanolamine ammonia lyase catalyzes the deamination of ethanolamine to the corresponding aldehyde [ 38, 39 ].

B12-dependent ribonucleotide reductase catalyzes the conversion of ribonucleotides to deoxyribonucleotides in all organisms and which is essential for DNA replication and repair [ 40, 41, 42, 43 ].

Methionine synthase is a B12-dependent methyltransferase, which catalyzes the transfer of a methyl group from N5-methyltetrahydrofolate to homocysteine resulting in production of tetrahydrofolate and methionine. This enzyme is typical for all domains of life [ 44, 45 ].

MtaB methyltransferase methylates methanol, MttB, MtbB, MtmB work with amines, MtsB is activ to methylated thiols and MtvB uses methoxylated aromatics as substrates. These methyltransferases are important for energy metabolism and in cell carbon cycle in anaerobic bacteria, acetogenic bacteria and methanogenic archaea [ 9, 46, 47, 48 ].

B12-dependent reductive dehalogenases are involved in detoxification of aromatic and aliphatic chlorinated organics in anaerobic bacteria [ 49, 50 ].

Non-corrin cobalt-containing enzymes (B12-independent) includes methionine aminopeptidase, prolidase, nitrile hydratase, glucose isomerase, methylmalonyl-CoA carboxytransferase, aldehyde decarbonylase, lysine-2,3-aminomutase, and bromoperoxidase. All of these enzymes are not strictly Co-dependant and may use iron, zinc, and manganese in place of Co [ 2, 5, 51, 52 ]. Methionine aminopeptidase cleaves the N-terminal methionine from newly translated polypeptide chains in both prokaryotes and eukaryotes [ 53 ]. Methylmalonyl-CoA carboxytransferase mediates the transfer of CO-2 from methylmalonyl-CoA to pyruvate for propionyl-CoA and oxaloacetate synthesis [ 54 ]. Nitrile hydratase catalyzes the hydrolysis of nitriles to the corresponding carboxylic acid and ammonia [ 55, 56 ].

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