Carboxyl Catalytic activity

The shape of a large protein is influenced by many factors including of course Its primary and secondary structure The disulfide bond shown m Figure 27 18 links Cys 138 of carboxypeptidase A to Cys 161 and contributes to the tertiary structure Car boxypeptidase A contains a Zn " ion which is essential to the catalytic activity of the enzyme and its presence influences the tertiary structure The Zn ion lies near the cen ter of the enzyme where it is coordinated to the imidazole nitrogens of two histidine residues (His 69 His 196) and to the carboxylate side chain of Glu 72... 

The first hint that two active-site carboxyl groups—one proto-nated and one ionized—might be involved in the catalytic activity of the aspartic proteases came from studies of the pH dependence of enzymatic activity. If an ionizable group in an enzyme active site is essential for activity, a plot of enzyme activity versus pH may look like one of the plots at right. 

Acetone is the best solvent for NBR hydrogenation in the presence of palladium carboxylates. No hydrogenation is achieved when chloroform or chlorobenzene are the solvents. Since it is understood that palladium is reduced to colloidal metal in the presence of hydrogen, attempts have also been made to reduce the palladium by hydrazine [76], methylaluminoxane [84], and trialky] aluminum [85] to improve the catalytic activity. 

Shimidzu etal.111 studied the catalytic activity of poly (4(5)-vinylimidazole-co-acrylic add) 60 (PVIm AA) in hydrolyses of 3-acetoxy-N-trimethylanilinium iodide 61 (ANTI) and p-nitrophenylacetate 44 (PNPA). The hydrolyses of ANTI followed the Michaelis-Menten-type kinetics, and that of PNPA followed the second-order kinetics. Substrate-binding with the copolymer was strongest at an imidazole content of 30 mol%. The authors concluded that the carboxylic acid moiety not... 

It is proposed that the boric acid reacts with the carboxylic acid to form a mixed anhydride as the actual acylating agent.913 Upon reaction with an amine, this intermediate forms the desired carboxamide and regenerates the catalytically active boric acid. 

Another SBU with open metal sites is the tri-p-oxo carboxylate cluster (see Section 4.2.2 and Figure 4.2). The tri-p-oxo Fe " clusters in MIL-100 are able to catalyze Friedel-Crafts benzylation reactions [44]. The tri-p-oxo Cr " clusters of MIL-101 are active for the cyanosilylation of benzaldehyde. This reaction is a popular test reaction in the MOF Hterature as a probe for catalytic activity an example has already been given above for [Cu3(BTC)2] [15]. In fact, the very first demonstration of the catalytic potential of MOFs had aheady been given in 1994 for a two-dimensional Cd bipyridine lattice that catalyzes the cyanosilylation of aldehydes [56]. A continuation of this work in 2004 for reactions with imines showed that the hydrophobic surroundings of the framework enhance the reaction in comparison with homogeneous Cd(pyridine) complexes [57]. The activity of MIL-lOl(Cr) is much higher than that of the Cd lattices, but in subsequent reaction rans the activity decreases [58]. A MOF with two different types of open Mn sites with pores of 7 and 10 A catalyzes the cyanosilylation of aromatic aldehydes and ketones with a remarkable reactant shape selectivity. This MOF also catalyzes the more demanding Mukaiyama-aldol reaction [59]. 

To examine if the higher catalytic activity and selectivity of 47a as compared to the COP-X system 46 is mainly caused by the pentaphenyl ferrocenium or by the imidazoline moiety, oxazoline 53-Cl was prepared in diastereomerically pure form starting from carboxylic acid 51 and (5)-valinol via oxazoline 52 (Fig. 27) [73]. 

Besides allylic substitution reactions it was also shown that [Fe(CO)3(NO)] 76 is catalytically active in transesterification reactions under neutral conditions (Scheme 24) [70]. Various activated acyl donors 97 can be used to give rise to the corresponding carboxylic esters 100 in good to excellent yields. This reaction proceeds in the absence of additional ligands in nonpolar solvents, for example, hexane. Mechanistically, the reaction is assumed to proceed via a Fe-acyl-complex 98 (Scheme 24). 

In the aligned primary structures of class I decarboxylases, the conserved amino acid residues are scattered over their primary structures. There have been few reports to identify the amino acid residues essential for catalytic activity or substrate binding. Huang et al. reported the E-X-P motif in the alignment analysis for 4-hydroxybenzoate decarboxylase of C. hydroxybenzoicum and its homologous unidentified proteins. The E-X-P motif is also conserved in pyrrole-2-carboxylate decarboxylase and indole-3-carboxylate decarboxylase (unpublished data). However, the corresponding motif sequence is not observed in the primary structures of 3,4-dihydroxybenzoate decarboxylase of E. cloacae P241. ... 

The original catalyst was Rh2(02CCH3)4, but other carboxylates such as nonafluo-robutanoate and amide anions, such as those from acetamide and caprolactam, also have good catalytic activity.199... 

There are two catalytically active residues in pepsin Asp-32 and Asp-215. Their ionizations are seen in the pH-activity profile, which has an optimum at pH 2 to 3, and which depends upon the acidic form of a group of pKa 4.5 and the basic form of a group of pKa 1.1.160,161 The pKa values have been assigned from the reactions of irreversible inhibitors that are designed to react specifically with ionized or un-ionized carboxyl groups. Diazo compounds—such as A-diazoacetyl-L-phenylalanine methyl ester, which reacts with un-ionized carboxyls—react specifically with Asp-215 up to pH 5 or so (equation 16.28).162-164 Epoxides, which react specifically with ionized carboxyls, modify Asp-32 (equation 16.29). 

Poly(methyl 3-(l-oxypyridinyl)siloxane) was synthesized and shown to have catalytic activity in transacylation reactions of carboxylic and phosphoric acid derivatives. 3-(Methyldichlorosilyl)pyridine (1) was made by metallation of 3-bromopyridine with n-BuLi followed by reaction with excess MeSiCl3. 1 was hydrolyzed in aqueous ammonia to give hydroxyl terminated poly(methyl 3-pyridinylsiloxane) (2) which was end-blocked to polymer 3 with (Me3Si)2NH and Me3SiCl. Polymer 3 was N-oxidized with m-ClC6H4C03H to give 4. Species 1-4 were characterized by IR and H NMR spectra. MS of 1 and thermal analysis (DSC and TGA) of 2-4 are discussed. 3-(Trimethylsilyl)-pyridine 1-oxide (6), l,3-dimethyl-l,3-bis-3-(l-oxypyridinyl) disiloxane (7) and 4 were effective catalysts for conversion of benzoyl chloride to benzoic anhydride in CH2Cl2/aqueous NaHCC>3 suspensions and for hydrolysis of diphenyl phosphorochloridate in aqueous NaHCC>3. The latter had a ti/2 of less than 10 min at 23°C. 

Metalloenzymes with non-heme di-iron centers in which the two irons are bridged by an oxide (or a hydroxide) and carboxylate ligands (glutamate or aspartate) constitute an important class of enzymes. Two of these enzymes, methane monooxygenase (MMO) and ribonucleotide reductase (RNR) have very similar di-iron active sites, located in the subunits MMOH and R2 respectively. Despite their structural similarity, these metal centers catalyze very different chemical reactions. We have studied the enzymatic mechanisms of these enzymes to understand what determines their catalytic activity [24, 25, 39-41]. 

Four macrolides, 11-undecanolide (12-membered,UDL) [85,86], 12-dodeca-nolide (13-membered,DDL) [86,87], 15-pentadecanolide (16-membered, PDL) [85,86,88,89], and 16-hexadecanolide (17-membered, HDL) [90], were subjected to the lipase-catalyzed polymerization. For the polymerization of DDL, lipases CC, PC, PF, and PPL showed the high catalytic activity and the activity order in the bulk polymerization was as follows lipase PC > lipase PF > lipase CC> PPL. These enzymes were also active for the polymerization of other macrolides. NMR analysis showed that the terminal structure of the polymer was of carboxylic acid at one end and of alcohol at the other terminal. 

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