Catalysis, by retaining

A. White and D. R. Rose, Mechanism of catalysis by retaining p-glycosyl hydrolases, Curr. Opin. 

White, A. and Rose, D.R. (1997) Mechanism of catalysis by retaining P-glycosyl hydrolases. Curr Opin Struct Biol, 7, 645-651... 

Fig. 7. Substrate distortion during catalysis by retaining glycosidases. The example shows the full conformational itinerary for a /3(l-4)-glucanase. Other itineraries are exploited by glycosidases, depending upon the substrate (66). (C, chair S, skew-boat H, half-chair. Numerals indicate the atoms above (superscript) and below (subscript) the four coplanar ring carbon atoms (see the text for a full description of the individual reaction steps).

Lactobacillus delbrueckii. In 1953, Rodwell suggested that the histidine decarboxylase of Lactobacillus 30a was not dependent upon pyridoxal phosphate (11). Rodwell based his suggestion upon the fact that the organism lost its ability to decarboxylate ornithine but retained high histidine decarboxylase activity when grown in media deficient in pyridoxine. It was not until 1965 that E. E. Snell and coworkers (12) isolated the enzyme and showed that it was, indeed, free of pyridoxal phosphate. Further advances in characterization of the enzyme were made by Riley and Snell (13) and Recsei and Snell (14) who demonstrated the existence of a pyruvoyl residue and the participation of the pyruvoyl residue in histidine catalysis by forming a Schiff base intermediate in a manner similar to pyridoxal phosphate dependent enzymes. Recent studies by Hackert et al. (15) established the subunit structure of the enzyme which is similar to the subunit structure of a pyruvoyl decarboxylase of a Micrococcus species (16). 

We also mentioned stereospecificity of metal-catalyzed reactions inside zeolite cavities. In acid catalysis by zeolites it is well known that shape selectivity can be imposed by (1) selective admission of reactants fitting into zeolite pores, (2) selective release of products able to diffuse through zeolite channels, while larger molecules are retained, and (3) transition state selectivity, favoring, e.g., a monomolecular transition state over a bimolecular state in a narrow cavity. New tools that have conceptually been added to this arsenal include the collimation of molecules diffusing through well-defined pores, which then hit an active site preferentially via one particular atom or group. 

For a large number of reactions of Cr(III) complexes with Cr++, a bridged activated complex is obviously also involved. Among these is a reaction of almost classical interest the catalysis by Cr++ of the dissolution of anhydrous CrCls (f). The product of the reaction has been shown to be CrCl++ [rather than Cr(OH2)e+ + as would be expected for ordinary dilute solutions if complete equilibrium were rapidly established], and the Cl retained has been proved not to have passed through the solution (129). The reaction can be formulated as... 

Experimental data on chemisorption and catalysis indicate that the different types of atoms in the surface of an alloy, such as nickel and copper, largely retain their chemical identities, although their bonding properties may be modified (6,7). At present the electronic factor in catalysis by metals generally is viewed in terms of localized chemical bonding effects similar to the "ligand effects of organometallic chemistry (6). 

Figure 13.1a depicts the classical double displacement mechanism proposed originally by Koshland as employed by retaining fi-exoglucosidases [3]. The enzyme active site contains two carboxylic acid (aspartate or glutamate) residues a general acid/base (carboxylic acid at the onset of enzyme catalysis) situated above the P-glucose substrate and a nucleophile (carboxylate) situated below. 

In summary, it is difficult to determine the catalytically active spedes in any supported catalyst, and there are still no well documented examples of catalysis by metal carbonyl clusters themselves in zeolites. There is, however, substantial indirect evidence that metal carbonyl clusters in zeolite cages may be either the catalysts or the catalyst precursors for a number of reactions involving CO. In some cases, these dusters are the only detectable organometallic or metallic spedes, and they are stable under the conditions of the catalytic reactions. Some of the catalysts retain the colors and the infrared spectra of the metal carbonyl dusters even after weeks of catalytic operation. In the few instances when EXAFS data were available, the presence of metal carbonyl clusters within the zeolites was indicated however, evidence for other spedes that are plaudble catalyst precursors was also obtained. 

Qualitative examples abound. Perfect crystals of sodium carbonate, sulfate, or phosphate may be kept for years without efflorescing, although if scratched, they begin to do so immediately. Too strongly heated or burned lime or plaster of Paris takes up the first traces of water only with difficulty. Reactions of this type tend to be autocat-alytic. The initial rate is slow, due to the absence of the necessary linear interface, but the rate accelerates as more and more product is formed. See Refs. 147-153 for other examples. Ruckenstein [154] has discussed a kinetic model based on nucleation theory. There is certainly evidence that patches of product may be present, as in the oxidation of Mo(lOO) surfaces [155], and that surface defects are important [156]. There may be catalysis thus reaction VII-27 is catalyzed by water vapor [157]. A topotactic reaction is one where the product or products retain the external crystalline shape of the reactant crystal [158]. More often, however, there is a complicated morphology with pitting, cracking, and pore formation, as with calcium carbonate [159]. 

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