Metal ions catalysis

Schultz and coworkers (Jackson et a ., 1988) have generated an antibody which exhibits behaviour similar to the enzyme chorismate mutase. The enzyme catalyses the conversion of chorismate [49] to prephenate [50] as part of the shikimate pathway for the biosynthesis of aromatic amino acids in plants and micro-organisms (Haslam, 1974 Dixon and Webb, 1979). It is unusual for an enzyme in that it does not seem to employ acid-base chemistry, nucleophilic or electrophilic catalysis, metal ions, or redox chemistry. Rather, it binds the substrate and forces it into the appropriate conformation for reaction and stabilizes the transition state, without using distinct catalytic groups. 

Acceleration of reaction rate as a result of the presence of one or more metal ions. In many cases, the general acid properties of the metal ion contribute to catalysis. Metal ions play organizing roles as templates for reactants. The metal ion may also stabilize the transition... 

A dynamic model of the catalytic reactions that lead to strand cleavage has been proposed based on structural considerations (22). Assembly is thought to involve coordinate binding of transposase, its DNA substrate(s), and two divalent metal ions. On transposase DNA binding, the two metal ions (H and T for hydrolysis and transfer, respectively Fig. lal) find then-appropriate positions in the active site and are poised for catalysis. Metal ion H orients and activates the water molecule (depicted as OH) for nucleophilic attack. Metal ions T (which... 

Most of the known ribozymes require divalent cations, usually Mg +, which is used for proper assembly of a complex 3D structure that provides an appropriate environment for catalysis. Metal ions can also act as cofactors in the chemical reaction enabling catalysis by activation of the nucleophile, stabilization of the transition state and protonation of the leaving group in general acid catalysis [424]. Accordingly, ribozymes could be considered as metalloenzymes. This fact has suggested the use of ribozymes as sensing elements for the detection of metal ions. 

Several properties of transition metals make them useful in catalysis. Metal ions provide a high concentration of positive charge that is especially useful in binding small molecules. Because transition metals act as Lewis acids (electron pair acceptors), they are effective electrophiles. (Amino acid side chains are poor electrophiles because they cannot accept unshared pairs of electrons.) Because their directed valences allow them to interact with two or more ligands, metal ions help orient the substrate within the active site. As a consequence, the substrate-metal ion complex polarizes the substrate and promotes catalysis. For... 

Metal ions can serve a variety of functions in the mechanisms of action of metalloenzymes. They may polarize functional groups both in the substrate and in amino acid side chains in the active site. As a result, the reaction being catalyzed can be facilitated. If the metal ion can undergo a change in oxidation number (such as is found for copper and iron), this may further aid in catalysis. Metal ions may also serve as a means of stiffening the geometry of the active site so that appropriate functional groups in it are lined up with respect to the substrate in a finely tuned manner dictated by the stereochemical requirements of the biochemical reaction to be catalyzed. Catalysis proceeds most efficiently in an enzyme when the transition state of the reaction is stabilized with respect to substrate and product. 

Metal ion catalysis Metal ions function in electrophihc catalysis stabilizing the negative charges that are formed. Metal-bonnd hydroxyl ions are potent nucleophiles (Strater et al 1996) that participate in reactions catalyzed by metalloen-zymes (Christianson and Cox, 1999) and ribozymes (Cech and Bass, 1986). 

Bcamples of metal-ion catalysed organic reactions in water where the catalyst acts exclusively as Lewis acid are the hromination of diketones" " and the decarboxylation of oxaloacetate. The latter reaction has been studied in detail. In 1941 it was demonstrated that magnesium(II) ions catalyse this reaction" Later also catalysis by other multivalent metal ions, such as Zn(II), Mn(II), Cu(II), Cd(ir), Fe(II), Pb(II), Fe(III)... 

First, the use of water limits the choice of Lewis-acid catalysts. The most active Lewis acids such as BFj, TiQ4 and AlClj react violently with water and cannot be used However, bivalent transition metal ions and trivalent lanthanide ions have proven to be active catalysts in aqueous solution for other organic reactions and are anticipated to be good candidates for the catalysis of aqueous Diels-Alder reactions. 

In the previous section efficient catalysis of the Diels-Alder reaction by copper(II)nitrate was encountered. Likewise, other bivalent metal ions that share the same row in the periodic system show catalytic activity. The effects of cobalt(II)nitrate, nickel(II)nitrate, copper(II)nitrate and zinc(ll)nitrate... 

A quantitative correlation between rate and equilibrium constants for the different metal ions is absent. The observed rate enhancements are a result of catalysis by the metal ions and are clearly not a result of protonation of the pyridyl group, since the pH s of all solutions were within the region where the rate constant is independent of the pH (Figure 2.1). 

Catalysis by the four metal ions was also compared with respect to their sensitivity towards substituents in the dienophile. To this end the equilibrium constants for complexation of2.4a-g to the four different ions were determined. The results are shown in Table 2.6. 

Copper is clearly the most selective metal-ion catalyst. Interestingly, proton catalysis also leads to high selectivities. This is a strong indication that selectivity in this catalysed Diels-Alder reaction does not result from steric interactions. 

Inspired by the many hydrolytically-active metallo enzymes encountered in nature, extensive studies have been performed on so-called metallo micelles. These investigations usually focus on mixed micelles of a common surfactant together with a special chelating surfactant that exhibits a high affinity for transition-metal ions. These aggregates can have remarkable catalytic effects on the hydrolysis of activated carboxylic acid esters, phosphate esters and amides. In these reactions the exact role of the metal ion is not clear and may vary from one system to another. However, there are strong indications that the major function of the metal ion is the coordination of hydroxide anion in the Stem region of the micelle where it is in the proximity of the micelle-bound substrate. The first report of catalysis of a hydrolysis reaction by me tall omi cell es stems from 1978. In the years that... 

The mechanism and rate of hydrogen peroxide decomposition depend on many factors, including temperature, pH, presence or absence of a catalyst (7—10), such as metal ions, oxides, and hydroxides etc. Some common metal ions that actively support homogeneous catalysis of the decomposition include ferrous, ferric, cuprous, cupric, chromate, dichromate, molybdate, tungstate, and vanadate. For combinations, such as iron and... 

Metal Deactivators. The abiUty of metal ions to catalyse oxidation can be inhibited by metal deactivators (19). These additives chelate metal ions and increase the potential difference between the oxidised and reduced states of the metal ions. This decreases the abiUty of the metal to produce radicals from hydroperoxides by oxidation and reduction (eqs. 15 and 16). Complexation of the metal by the metal deactivator also blocks its abiUty to associate with a hydroperoxide, a requirement for catalysis (20). 

Many reactions catalyzed by the addition of simple metal ions involve chelation of the metal. The familiar autocatalysis of the oxidation of oxalate by permanganate results from the chelation of the oxalate and Mn (III) from the permanganate. Oxidation of ascorbic acid [50-81-7] C HgO, is catalyzed by copper (12). The stabilization of preparations containing ascorbic acid by the addition of a chelant appears to be negative catalysis of the oxidation but results from the sequestration of the copper. Many such inhibitions are the result of sequestration. Catalysis by chelation of metal ions with a reactant is usually accomphshed by polarization of the molecule, faciUtation of electron transfer by the metal, or orientation of reactants. 

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