Enzymes containing nickel

In a nickel-containing enzyme various groups of atoms in the enzyme form a complex with the metal, which was found to be in the +2 oxidation state and to have no unpaired electrons. What is the most probable geometry of the Ni2+ complex (a) octahedral (b) tetrahedral (c) square planar (see Exercise 16.96) Justify your answer by drawing the orbital energy-level diagram of the ion. 

In contrast to the abundance of Fe-proteins, there are only six known nickel-containing enzymes hydrogenase, CO dehydrogenase (CODA), acetyl-CoA synthase (ACS), superoxide dismutase, urease, and S-methyl-CoM methylreductase. Among these enzymes, it exists in very diverse environments, including a dinickel site (urease), a Ni-Fe heterobinuclear site (hydrogenase), a Ni-Fe4S4 heterometallic... 

Nickel exists in the tunicate Trididemnum solidum as the nickel complex of a modified chlorin (Bible et al. 1988) and is a component of a number of enzymes. Urease is the classic example of a nickel-containing enzyme, and several enzymes contain both nickel and iron. Details of enzymes that contain nickel have been provided in a review (Mulrooney and Hausinger 2003), and only brief summaries are provided ... 

The use of six equivalents of dihydrogen peroxide leads to a clean conversion of the dithiolate complex to the disulfonate compound. Earlier studies on oxidation of nickel thiolates showed that oxidations with dioxygen stop at monosulfinates. Our observation and the characterization of the first chelating bis-sulfonato nickel complex formed from the direct oxidation of a mononuclear nickel dithiolate, may also provide new insight into the chemistry of sulfur-rich nickel-containing enzymes in the presence of oxygen. 

Graf, E.-G. and Thauer, R. K. (1981) Hydrogenase from Methanobacterium thermoau-totrophicum, a nickel-containing enzyme. FEBS Lett., 36, 165-9. 

Hydrogenases are not the only nickel-containing enzyme, and researchers must therefore compare the maturation of different nickel proteins to obtain an integrated picture of nickel metabolism. Indeed, similarities between some of the hydrogenase-related nickel-processing proteins with urease and carbon monoxide dehydrogenase maturation factors have been noted, and this has facilitated interpretations of the results for... 

Figure 6.3. Alignments of regions identified to be important in nickel accessory proteins for maturation of the final nickel sink, the nickel-containing enzyme. A, The His-rich regions of known or putative HypB proteins and the UreE and CooJ proteins from various organisms. Reprinted with permission from Olson and Maier (2000). B, Alignment of the G motifs from known or putative HypB proteins and the related nucleotide-binding P-loop residues (in the G1 region) of CooC and UreG. (Adapted from Olson and Maier (2000).

Hausinger RP. 1997. Metallocenter assembly in nickel-containing enzymes. J Biol Inorg Chem 2 279-86. 

Corphin is the F-430 cofactor found in methyl-coenzyme M reductase, a nickel-containing enzyme that participates in the conversion of carbon dioxide to methane in methanogenic bacteria. The nickel ion in F-430 is coordinated by the tetrahydrocorphin ligand, which contains structural elements of both porphyrins and corrins. 

EPR spectra of CO oxidoreductase under non reducing conditions showed a spectrum at g = 2.21, 2.11, and 2.02, which, by analogy with spectra observed in nickel-containing hydrogenases, was attributed to Ni(III) (96). The spectrum was of low intensity and it was not established whether it represents an active state of the enzyme. 

The four types of nickel-containing enzymes are quite distinct in the coordination sites and catalytic function of the nickel centers. In urease, the nickel appears to be bound to oxygen and nitrogen ligands and appears to remain as Ni(II), a state which favors octahedral or square-planar coordination. The function of nickel in this unique case may be analogous to that of zinc in other hydrolases such as carboxypeptidase. 

It is noteworthy that the substrates or products are dissolved gases hydrogen, oxygen, carbon monoxide, carbon dioxide, methane, ammonia. However, the enzymes show no common pattern, either in the chemical state of nickel or in the type of reaction catalyzed. Their nickel-containing sites are remarkably diverse (Table 1), and in four enzymes the active center comprises groups in addition to the nickel ion. 

Nickel-containing enzymes have been isolated from anaerobic bacteria, which catalyze the reversible oxidation of carbon monoxide to carbon dioxide ... 

Kolodziej, Andrew F., The Chemistry of Nickel-Containing Enzymes. 41 493... 

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