Amino acid esterase

Fig. 3.2 Classification of "metabolism ESTs. Tentative unique genes (TUGs) from each species that clustered into the metabolism category (see Fig. 3.1J were further subdivided. Here, carbohydrate incorporates all glycoside hydrolases, lipid includes lipases, B-oxidation and steroid metabolizing enzymes, protein includes predominantly proteases and their precursors, and amino acid includes all enzymes involved in the interconversion of amino acids. Esterase includes those esterase-like enzymes with unknown substrates, and other contains those TUGs that do not sort into other categories. These include predominantly oxidoreductases and purine/pyrimidine metabolizing enzymes.

Amino acid esterases were generated from RNase by using a variety of indole derivatives as modifiers and, perturbing the conformation by titration to an acid pH.23 29 Crosslinking with glutaraldehyde was used to stabilize this new conformation. When assayed with L-tryptophan ethyl ester, the modified RNase was found to possess two pH optima one at 6 and the other at 7.5. Purification of the crude reaction mixture demonstrated the presence of two types of amino acid esterases which account for the two pH optima. After the conformational modification process, the native activity of RNase is lower. After purification of the crude mixture no native RNase activity can be measured in the fractions containing amino acid esterase activity. 

The production of semisynthetic enzymes by conformational modification has been well documented in the literature. 23,39 utility and versatility of this process is exemplified by the numerous starting proteins that have been successfully modified to produce new catalysts. Examples include the preparation of amino acid esterases, fluorohydrolases, and glucose isomerases. In many cases these semisynthetic cata-... 

The authors wish to thank the Army Research Office (Contract No. DAAL03-86-C-0021) for their financial support of the fluorohydrolase project. We also express our thanks to the Department of Energy - Office of Basic Energy Science (Contract No. DE-AC02-81erl2003) for support of the earlier amino acid esterase studies. Additional support was given by Owens-Illinios, Inc. and Anatrace, Inc. 

Enzymatic hydrolysis of A/-acylamino acids by amino acylase and amino acid esters by Hpase or carboxy esterase (70) is one kind of kinetic resolution. Kinetic resolution is found in chemical synthesis such as by epoxidation of racemic allyl alcohol and asymmetric hydrogenation (71). New routes for amino acid manufacturing are anticipated. 

Preparation of PhAcOZ amino acids proceeds from the chloroformate, and cleavage is accomplished enzymatically with penicillin G acylase (pH 7 phosphate buffer, 25°, NaHS03, 40-88% yield). In a related approach, the 4-ace-toxy derivative is used, but in this case deprotection is achieved using the lipase, acetyl esterase, from oranges (pH 7, NaCl buffer, 45°, 57-70% yield). 

Several classes of enzymes have been used to separate stereoisomers of a-H-and a-disubstituted amino acids, eg amidases, nitrilases, hydantoinases, acylases and esterases. 

Hayama et al.132 discussed the catalytic effects of silver ion-polyacrylic add systems toward the hydrolyses of 2,4-dinitrophenylvinylacetate 84 (DNPVA) by using the weak nudeophilicity of carboxylic groups and the change-transfer interactions between olefinie esters and silver ions133Metal complexes of basic polyelectrolytes are also stimulating as esterase models. Hatano etal. 34, 13S) reported that some copper(II)-poly-L-lysine complexes were active for the hydrolyses of amino acid esters, such as D- and L-phenylalanine methyl ester 85 (PAM). They... 

In another study a hyperthermophilic esterase from Aeropyrum pemix K1 (APE1547) was used as a catalyst in the hydrolytic kinetic resolution of rac-3-octanol acetate [53]. Following a single round of epPCR, a mutant displaying a 2.6-fold increase in enantioselectivity was identified having five amino acid substitutions, which were shown to be spatially distal to the catalytic center. 

The field of synthetic enzyme models encompasses attempts to prepare enzymelike functional macromolecules by chemical synthesis [30]. One particularly relevant approach to such enzyme mimics concerns dendrimers, which are treelike synthetic macromolecules with a globular shape similar to a folded protein, and useful in a range of applications including catalysis [31]. Peptide dendrimers, which, like proteins, are composed of amino acids, are particularly well suited as mimics for proteins and enzymes [32]. These dendrimers can be prepared using combinatorial chemistry methods on solid support [33], similar to those used in the context of catalyst and ligand discovery programs in chemistry [34]. Peptide dendrimers used multivalency effects at the dendrimer surface to trigger cooperativity between amino acids, as has been observed in various esterase enzyme models [35]. 

The introduction of redox activity through a Co11 center in place of redox-inactive Zn11 can be revealing. Carboxypeptidase B (another Zn enzyme) and its Co-substituted derivative were oxidized by the active-site-selective m-chloroperbenzoic acid.1209 In the Co-substituted oxidized (Co111) enzyme there was a decrease in both the peptidase and the esterase activities, whereas in the zinc enzyme only the peptidase activity decreased. Oxidation of the native enzyme resulted in modification of a methionine residue instead. These studies indicate that the two metal ions impose different structural and functional properties on the active site, leading to differing reactivities of specific amino acid residues. Replacement of zinc(II) in the methyltransferase enzyme MT2-A by cobalt(II) yields an enzyme with enhanced activity, where spectroscopy also indicates coordination by two thiolates and two histidines, supported by EXAFS analysis of the zinc coordination sphere.1210... 

The i-poly(3HB) depolymerase of R. rubrum is the only i-poly(3HB) depolymerase that has been purified [174]. The enzyme consists of one polypeptide of 30-32 kDa and has a pH and temperature optimum of pH 9 and 55 °C, respectively. A specific activity of 4 mmol released 3-hydroxybutyrate/min x mg protein was determined (at 45 °C). The purified enzyme was inactive with denatured poly(3HB) and had no lipase-, protease-, or esterase activity with p-nitro-phenyl fatty acid esters (2-8 carbon atoms). Native poly(3HO) granules were not hydrolyzed by i-poly(3HB) depolymerase, indicating a high substrate specificity similar to extracellular poly(3HB) depolymerases. Recently, the DNA sequence of the i-poly(3HB) depolymerase of R. eutropha was published (AB07612). Surprisingly, the DNA-deduced amino acid sequence (47.3 kDa) did not contain a lipase box fingerprint. A more detailed investigation of the structure and function of bacterial i-poly(HA) depolymerases will be necessary in future. 

A particular interest for clinical applications was a possibility for detection of dopamine by its oxidation on nickel [19], cobalt [65], and osmium [66] hexacyanofer-ates. Except for oxidation of dopamine, cobalt and osmium hexacyanoferrates were active in oxidation of epinephrine and norepinephrine. For clinical analysis it is also important to carry out the detection of morphine on cobalt [67] and ferric [68] hexacyanoferrates, as well as the detection of oxidizable amino acids (cystein, methionine) by manganous [69] and ruthenium [70] hexacyanoferrate-modified electrodes. In general, oxidation of thiols was first shown for Prussian blue [71] and nickel hexacyanoferrate [72], This approach has been used for the detection of thiols in rat striatum microdialysate [73], Alternatively, the detection of thiocholine with Prussian blue was employed for pesticide determination in acetylcholine-esterase test [74],... 

Peptide hydrolases (peptidases or proteases, i.e., enzymes hydrolyzing peptide bonds in peptides and proteins, see Chapt. 2) have received particular attention among hydrolases. As already described in Chapt. 2, peptidases are divided into exopeptidases (EC 3.4.11 -19), which cleave one or a few amino acids from the N- or C-terminus, and endopeptidas-es (proteinases, EC 3.4.21-99), which act internally in polypeptide chains [2], The presentation of enzymatic mechanisms of hydrolysis in the following sections will begin with peptidases and continue with other hydrolases such as esterases. 

The first step in the esterase activity of carbonic anhydrase (Fig. 3.15, e) is analogous to the first step in C02 hydration (Fig. 3.15,a). The tetrahedral intermediate so formed (Fig. 3.15,f) necessitates the participation of a proton donor for the departure of the leaving alcohol group (Fig. 3.15,f and g). It is possible that the Thr200 residue plays an important role in the esterase activity of carbonic anhydrase. Indeed, its replacement by other amino acids enhances the esterase activity but has no significant effect on the rate of C02 hydration [106],... 

Y. Kurono, T. Furukawa, T. Tsuji, K. Ikeda, Esterase-Like Activity of Human Serum Albumin. VI. Reaction with p-Nitrophenyl Glycinate , Chem. Pharm. Bull. 1988, 36, 4068-4074 Y. Kurono, I. Kushida, H. Tanaka, K. Ikeda, Esterase-Like Activity of Human Serum Albumin. VIII. Reaction with Amino Acid p-Nitrophenyl Esters , Chem. Pharm. Bull. 1992, 40, 2169-2172. 

The indomethacin-hydrolyzing enzyme from pig liver microsomes was purified and partially characterized [60]. The enzyme was found to be different from known pig liver esterases, since it did not hydrolyze naphth-l-yl-acetate and (4-nitrophenyl)acetate, which are typical substrates for these car-boxy lesterases. The amino acid sequence of the enzyme showed high homology with the mouse carboxylesterase isoenzyme ES-male. Human liver car-... 

Y. Kurono, I. Kushida, H. Tanaka, K. Ikeda, Esterase-Like Activity of Human Serum Albumin. VIII. Reaction with Amino Acid p-Nitrophenyl Esters , Chem. Pharm. Bull. 1992, 40, 2169-2172. 

RIBULOSE-5-PHOSPHATE 3-EPIM ERASE Escherichia coll glutamine synthetase, ENZYME CASCADE KINETICS GLUTAMINE SYNTHETASE Essential amino acid residues in catalysis, AFFINITY LABELING ESTERASES... 

Ammonia lyases catalyze the enantioselective addition of ammonia to an activated double bond. A one-pot, three-step protocol was developed for the enantioselective synthesis of L-arylalanines 50 using phenylalanine ammonia lyase (PAL) in the key step (Scheme 2.20). After formation of the unsaturated esters 48 in situ via a Wittig reaction from the corresponding aldehydes, addition of porcine Ever esterase and basification of the reaction mixture resulted in hydrolysis to the carboxylic acids 49. Once this reaction had gone to completion, introduction of PAL and further addition of ammonia generated the amino acids 50 in good yield and excellent optical purity [22]. 

The use of enzymes and whole cells as catalysts in organic chemistry is described. Emphasis is put on the chemical reactions and the importance of providing enantiopure synthons. In particular kinetics of resolution is in focus. Among the topics covered are enzyme classification, structure and mechanism of action of enzymes. Examples are given on the use of hydrolytic enzymes such as esterases, proteases, lipases, epoxide hydrolases, acylases and amidases both in aqueous and low-water media. Reductions and oxidations are treated both using whole cells and pure enzymes. Moreover, use of enzymes in sngar chemistiy and to prodnce amino acids and peptides are discnssed. 

Formation of an amide bond (peptide bond) will take place if an amine and not an alcohol attacks the acyl enzyme. If an amino acid (acid protected) is used, reactions can be continued to form oligo peptides. If an ester is used the process will be a kinetically controlled aminolysis. If an amino acid (amino protected) is used it will be reversed hydrolysis and if it is a protected amide or peptide it will be transpeptidation. Both of the latter methods are thermodynamically controlled. However, synthesis of peptides using biocatalytic methods (esterase, lipase or protease) is only of limited importance for two reasons. Synthesis by either of the above mentioned biocatalytic methods will take place in low water media and low solubility of peptides with more than 2-3 amino acids limits their value. Secondly, there are well developed non-biocatalytic methods for peptide synthesis. For small quantities the automated Merrifield method works well. 

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