Nickel enzyme

The oxidation states present in the natural systems can be determined by comparison of the spectral properties of the natural system in its oxidized and reduced states with those of the synthetic models the latter can be prepared in almost any desired oxidation state by electrochenucal means. The results show that the monoiron systems indeed shuttle between Fe(II) and Fe(in) as expected. The diiron enzymes are Fe(III), Fe(III) in the oxidized state, and Fe(II), Fe(III) in the reduced state. The mixed-valence species are fiilly delocalized in all cases. There is also a superreduced state, Fe(II), Fe(II), which is probably not important in vivo. The 4-iron proteins shuttle between 3Fe(II), Fe(III) and 2Fe(II), 2Fe(in), such as in the fenedoxins (Fd). One class of 4-iron proteins have an unusually high oxidation potential (HIPIP, or high potential iron fnotein), because the system shuttles between 2Fe(II), 2Fe(in) and Fe(II), 3Fe(III). 

The Naase crystal structure, apart from showing FeMo-co, also revealed the structure of the P clusters (16.16), which consist of a pair of Fc4S4 cubanes bridged by an S-S group. 

Nitrogen fixation is vital to life on Earth but is a very hard reaction to bring about. 

Urease is famous in enzymology for being the first enzyme to be purified and crystallized (1926). At the time enzymes were widely viewed as being too ill-defined for detailed chemical study. Sumner argued that its crystalline character 

The archaea are veiy rich in nickel-containing enzymes and coenzymes, and Nature has clearly chosen this element to bring about the initial steps in the biochemical utilization of H2, CO, CH4, and other C compounds, at least in an anaerobic environment. These steps almost certainly involve (Mganonickel chemistry, although how this happens in detail is only just beginning to be understood. 

Biological Inorganic Chemistry, 2nd Edition. DOI 10.1016/B978-0-444-53782-9.00015-2. Copyright 2012 Elsevier B.V. All rights reserved. 

CO dehydrogenase (CODH) interconverts CO and CO2, acetyl-CoA synthetase (ACS) in concert with CODH converts CO2 and a methyl group to acetyl-CoA and methyl-CoM reductase (MCR) generates methane. 

The remaining Ni enzyme, glyoxylase (Glxl), catalyses the conversion of toxic methylglyoxal, (it can readily form covalent adducts with DNA) to lactate. Its single octahedrally coordinated Ni acts as a Lewis acid, without changing valency, which presumably explains why it can be replaced by Zn, for example, in man. 

Ni sites in enzymes show considerable adaptability, both in terms of Ni coordination and redox chemistry. The Ni centre in SOD must be able to span redox potentials from +890 to —160 mV, whereas in MCR and CODH, it must be able to reach potentials as low as —600 mV. This implies that Ni centres in proteins can carry out redox chemistry over a potential range of 1.5 V. The low levels of available Ni in natural environments has necessitated the development of high-affinity Ni uptake systems, together with metallochaperones and regulators of Ni homeostasis. 

Historically, the earliest Ni-containing enzyme to be described was urease from jack bean meal, which was crystallised by James Sumner in 1926. However, analytical techniques did not allow urease to be recognised as a Ni-containing enzyme until 50 years later. Urease catalyses the hydrolysis of urea to ammonia and carbamate, which spontaneously hydrolyses to give carbonic acid and a second molecule of ammonia. It plays a key role in nitrogen metabolism in plants and microbes whereas land-dwelling animals excrete urea 

CODH can bring about two reactions (e.g., Eq. 16.26 and Eq. 16.28) of particular organometallic interest the reduction of atmospheric CO2 to CO (CODH reaction, Eq. 16.26) and acetyl coenzyme A synthesis (ACS reaction, Eq. 16.28) from CO, a CH3 group possibly taken from a corrinoid iron-sulfur protein (denoted CoFeSP in the equation), and coenzyme A, a thiol. Tliese are analogous to reactions we have seen earlier the water-gas shift reaction (Eq. 16.25) and the Monsanto acetic acid process (Eq. 16.27). 

FIGURE 16.5 The A cluster of ACS/CODH from Moorella thermoacetica. 

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Superoxide dismutases may contain a range of metals Mn, Fe, or both Cu and Zn, and representatives of all these are found in prokaryotes. The nickel enzyme is noted later. 

Drake HL. S-1 Hu, HG Wood (1980) Purification of carbon monoxide dehydrogenase, a nickel enzyme from Clostridium thermoaceticum. J Biol Chem 255 7174-7180. 

No nickel-requiring enzymes or proteins are known in vertebrates, although biological roles of nickel enzymes and cofactors have been found in plants and bacteria. Although the role of nickel in human physiology has not been confirmed directly, the evidence strongly suggests that nickel is required by humans [263],... 

Hausinger R. 1994. Nickel enzymes in microbes. Sci Total Environ 148 157-66. 

If E. coli is grown in a cadmium-containing, zinc-deficient medium, the enzyme is found to be active, but to contain six Cd2+ per molecule. The presence of cysteinyl ligands is confirmed by the observation of the characteristic charge-transfer bands. The binding of substrate perturbs the absorption and CD spectra of the zinc and cadmium enzymes, and the d-d spectrum of the nickel enzyme, showing that "the conformation of the R subunit is affected by the binding of substrate to the C subunit.530,531... 

For CO dehydrogenase no crystal structure has been published. On the basis of a combination of spectroscopies an active-center structure has been proposed that features a [4Fe-4S] cubane bridged through an unknown ligand to a protein-bound nickel ion. This would seem to be a situation analogous to that discussed for sulfite reductase. On the basis of resonance Raman spectroscopic experiments it has been concluded that the substrate CO binds directly to one of the Fe ions of the cubane [54] however, this claim has recently been retracted [55], The reader is referred to Chapter 9 on nickel enzymes. 

Nickel enzymes are particularly prominent in the metabolism of anaerobic bacteria. For example, the methanogenic bacteria, which are classified as Arch-aea, an ancient division of living organisms, can grow on a mixture of H2 and C02 to produce methane [9-11], The metabolism of methanogens involves three... 

A newly discovered nickel enzyme is superoxide dismutase (SOD) from the acti-nomycete Streptomyces sp [164,165]. In its protein properties, this enzyme is... 

It has not been possible so far to establish that Cr is an essential element required by plants, however, addition of Cr to soils deficient in the element has been shown to increase growth rates and yields of potatoes, maize, rye, wheat or oats (Scharrer and Schropp, 1935 Huffman and Allaway, 1973 Bertrand and De Wolf, 1986). Nickel appears to be an essential element for plants (Farago and Cole, 1988). Zerner and coworkers (Dixon et al., 1975) demonstrated that urease isolated from jack bean (Canavalia ensiformis) was a nickel enzyme. Eskew et al. (1983) have shown that Ni is an essential micronutrient for legumes. Most plants contain nickel in the range 1 - 6 mg kg-1 (Vanselow, 1966 Hutchinson, 1981). The uptake of Ni is enhanced by low pH values, and available nickel increases at pH less than 6.5 as a consequence of the breakdown of Ni complexes in the soil with Fe and Mn oxides. Uptake of nickel by plants and questions of toxicity and tolerance have been reviewed by Farago and Cole (1988). Nickel toxicity toward plants has been reviewed by Vanselow (1966) and Hutchinson (1981). 

Cobalt B Enzymes Coenzymes Cytochrome Oxidase Iron Heme Proteins Electron Transport Iron Proteins with Dinuclear Active Sites Iron Proteins with Mononuclear Active Sites Iron-Sulfur Models of Protein Active Sites Metallocenter Biosynthesis Assembly. Metalloregulation Molybdenum MPT-containing Enzymes Nickel Enzymes Cofactors Nitrogenase Catalysis Assembly Photosynthesis Tungsten Proteins Vanadium in Biology Zinc DNA-binding Proteins. 

Superoxide dismutase (SOD) catalyzes the disproportionation of superoxide to peroxide and oxygen according to equation (2). Four different types of SOD are known, containing either Cu and Zn see Copper Proteins with Type 2 Sites), Fe, Mn, or Ni see Nickel Enzymes Cofactors). The Fe and Mn containing SODs have very similar structures and can be further subdivided into metal-specific (i.e. functioning only when the correct metal is bound) and cambialistic (functioning with either Fe or Mn bound to the active site). 

The nickel enzymes covered in this article can be divided into two groups redox enzymes and hydrolases. The five Ni redox enzymes are hydrogenase, CO dehydrogenase (CODH), acetyl-CoA synthase (ACS), methyl-Coenzyme M reductase (MCR), and superoxide dismutase (SOD). Glyoxalase-I and urease are Ni hydrolases. Ni proteins that are not enzymes are not covered, because they have been recently reviewed. These include regulatory proteins (NikR) and chaperonins and metal uptake proteins (CooJ, CooE, UreE, and ABC transporters). A recent crystal structure of NikR, shown in Figure l(i), is a notable recent achievement in this area. ... 

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