Hydrolytic catalytic activity

Table I. Hydrolytic catalytic activity, Kd (sec ), of selected enzymes toward some nerve agents...

The Pectolyase Y-23 catalytic activity was studied on several solutions of pectin at pH 3.0 and 6.0 and of polygalacturonic acid at pH 4.1. During pectin depolymerisation, the PL activity is about 20 times lower than the one of the PG and reaches its saturation value after about 30 min whereas the PG activity increases regularly with time. This dual behaviour is probably connected to the competitive hydrolytic action of PE that quickly transforms the pectin into the polygalacturonic acid. Accordingly, the reducing group formation is mainly due to the PG action and the pectin depolymerisation derives from the sequential action of PE and PG. 

Jacobsen et al. reported enhanced catalytic activity by cooperative effects in the asymmetric ring opening (ARO) of epoxides.[38] Chiral Co-salen complexes (Figure 4.27) were used, which were bound to different generations of commercial PAMAM dendrimers. As a direct consequence of the second-order kinetic dependence on the [Co(salen)] complex concentration of the hydrolytic kinetic resolution (HKR), reduction of the catalyst loading using monomeric catalyst leads to a sharp decrease in overall reaction rate. 

A very successful example for the use of dendritic polymeric supports in asymmetric synthesis was recently described by Breinbauer and Jacobsen [76]. PA-MAM-dendrimers with [Co(salen)]complexes were used for the hydrolytic kinetic resolution (HKR) of terminal epoxides. For such asymmetric ring opening reactions catalyzed by [Co(salen)]complexes, the proposed mechanism involves cooperative, bimetallic catalysis. For the study of this hypothesis, PAMAM dendrimers of different generation [G1-G3] were derivatized with a covalent salen Hgand through an amide bond (Fig. 7.22). The separation was achieved by precipitation and SEC. The catalytically active [Co "(salen)]dendrimer was subsequently obtained by quantitative oxidation with elemental iodine (Fig. 7.22). 

Lysosomes are membrane-bound organelles that contain hydrolytic enzymes to break down macromolecules and other organelles taken up by the lysosomes. The pH within this organelle is very low (about 5.0) and the catalytic activities of the enzymes, within it, are highest at this pH. The pH in the cytosol is about 7.1, so that any enzymes released from the lysosome are not catalyticaUy active in the cytosol. 

By rect comparison of structural and functional characteristics of different, naturally-occurring enzymes and modified biocatalysts, researchers hope to delineate the structural features controlling the hydrolytic and S3mthetic catalytic activities of this family of glucosylases. This understanchng will eventually lead to the ability to "engineer" glucan biocatalyst capability. 

The secondary structure of the polypeptide chain in hydrolytic enzymes ensures the spatial proximity of the necessary functional groups, which are responsible for the observed catalytic effect. In synthetic enzyme mimics, it is possible to bring the requisite functionalities into close juxtaposition only if there is a rigid framework to which these groups are attached. It was thus logical to examine cyclic peptides, especially cyclodipeptides, which bear the necessary functional groups for their catalytic activity in ester hydrolysis. 

To investigate the catalytic activity of the materials prepared, hydrolytic kinetic resolution of terminal epoxides such as styrene oxide, 1,2-epoxyhexane, and epi-chlorohydrin was carried out using only water, which acts as a nucleophilic agent... 

The third investigation track demonstrated the immobilization of metal-salen complexes in mesoporous materials and their use in the hydrolytic kinetic resolution of meso and terminal epoxides. The best results were obtained over cobalt-Ja-cobsen catalysts. The catalytic activity of the (S,S)-Co(II)-Jacobsen complex immobilized on Al-MCM-41 was comparable with that of the homogeneous counterpart. Several other immobilization methods are still under investigation. 

A host of enzymes, which are described elsewhere in the book, act on DNA and RNA. They include hydrolytic nucleases, methyltransferases, polymerases, topoisomerases, and enzymes involved in repair of damaged DNA and in modifications of either DNA or RNA. While most of these enzymes are apparently proteins, a surprising number are ribozymes, which consist of RNA or are RNA-protein complexes in which the RNA has catalytic activity. 

Ribonuclease-S can be separated into S-peptide [residues 1-20 (21)] and S-protein [residues 21 (22)-124] by precipitation with trichloroacetic acid 73) or better, Sephadex chromatography in 5% formic acid 83). The best preparations of these components show no detectable hydrolytic enzymic activity and little if any transphosphorylation activity (see Section VI). Isolated S-peptide appears to have no regular secondary structure 83, 84) or 10-20% helicity 85, 86). (These slightly different interpretations are based on almost identical CD data.) When equimolar amounts of S-protein and S-peptide are mixed at neutral pH and room temperature or below, essentially full catalytic activity is recovered 73, 87). A schematic diagram is shown in Fig. 7. For a detailed summary of the preparative procedures see Doscher 88). 

The strong catalytic activity of bases in the caprolactam polymerization was recognized about as soon (44—47) as the "normal hydrolytic process was, but nevertheless, the fundamental informations about the basic process were disclosed only during the last few years. It was shown quite recently, that the base induced polymerization consists of an extremely complex action of the strong base (75, 31, 66—68) which is connected with the presence and/or formation of certain additional components. The base plays not only a role in the initiation and propagation reaction but it also involves important side reactions which in turn affect the active centers of the polymerization (67, 96). 

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