Nickel, in urease

The nickel in urease is nonmagnetic and appears to be in the oxidation state Ni(II). The broad optical absorption spectrum is influenced by ligands to the metal (Fig. 1). The spectrum obtained in the presence of the competitive inhibitor mercaptoethanol, after correction for Rayleigh scattering by the protein (31), shows absorption peaks at 324,380, and 420 nm, with molar absorption coefficients of 1550,890, and 460 A/-1 cm-1, respectively. These were assigned to sulfur-to-nickel charge transfer transitions. The spectrum is changed by addition of other inhibitors, such as acetohydroxamic acid (Fig. IB). Similar... 

A wide range of metal ions is present in metalloenzymes as cofactors. Copper zinc snperoxide dismntase is a metalloenzyme that nses copper and zinc to help catalyze the conversion of snperoxide anion to molecnlar oxygen and hydrogen peroxide. Thermolysin is a protease that nses a tightly bonnd zinc ion to activate a water atom, which then attacks a peptide bond. Aconitase is one of the enzymes of the citric acid cycle it contains several iron atoms bonnd in the form of iron-sulfur clusters, which participate directly in the isomerization of citrate to isocitrate. Other metal ions fonnd as cofactors in metalloenzymes include molybdenum (in nitrate rednctase), seleninm (in glutathione peroxidase), nickel (in urease), and vanadinm (in fungal chloroperoxidase). see also Catalysis and Catalysts Coenzymes Denaturation Enzymes Krebs Cycle. 

Until the discovery in 1975 of nickel in jack bean urease (which, 50 years previously, had been the first enzyme to be isolated in crystalline form and was thought to be metal-free) no biological role for nickel was known. Ureases occur in a wide variety of bacteria and plants, catalyzing the hydrolysis of urea,... 

Kinetic evidence obtained for intramolecular proton transfer between nickel and coordinated thiolate, in a tetrahedral complex containing the bulky triphos ligand (Pl PCE CE PPh to prevent interference from binuclear p-thiolate species, is important with respect to the mechanisms of action of a number of metalloenzymes, of nickel (cf. urease, Section VII. B.4) and of other metals (289). 

In contrast to urease the nickel in other bacterial enzymes appears to have a redox function and to take up oxidation states Ni(I) and/or Ni(III). Fortunately these states have recently become better understood in inorganic systems (see the preceding review in this volume by... 

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. 

Furthermore, it should be noted that the same metal can often play different roles in different enzymes. For example, nickel(II) displays electrophilic catalysis in urease and redox catalysis in hydrogenase. 

In urease, the active site with a pair of nickel ions is unique. A large number of esterases and phosphatases contain dinuclear clusters of zinc and/or iron, and in these a metal-bound hydroxide or water molecule is a common feature [170], In urease the active site is rather rigid and designed to favor binding of urea over that of solvent water, and for this purpose nickel may be preferable to zinc, which has more flexible coordination geometry. 

Nickel, atomic number 28, is a transition metal with a variety of essential uses in alloys, catalysts, and other applications. It is strongly suspected of being an essential trace element for human nutrition, although definitive evidence has not yet established its essentiality to humans. A nickel-containing urease metalloenzyme has been found in the jack bean. 

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). 

Nickel is required for the synthesis of active urease in plant and other cells. The enzyme catalyzes the hydrolysis of urea to carbon dioxide and ammonia, via the intermediate formation of carbamate ion (equation 46). The molecular weight has been redetermined recently as 590 000 30 000, with six subunits. Each subunit has two nickel centres and binds one mole of substrate. The activity of the enzyme is directly proportional to the nickel content, suggesting an essential role for nickel in the enzyme. Several approaches, including EXAFS measurements, suggest that histidine residues provide some ligands to nickel, and that the geometry is distorted octahedral. There appears to be a role for a unique cysteine residue in each subunit out of the 15 groups present. Covalent modification of this residue blocks the activity of the enzyme. 

Ni Nickel None known in mammals. May be essential to plants. Found in urease enzyme MH (10-100) M... 

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