Enzyme catalyzed reaction esterase

Immobilization. The fixing property of PEIs has previously been discussed. Another appHcation of this property is enzyme immobilization (419). Enzymes can be bound by reactive compounds, eg, isothiocyanate (420) to the PEI skeleton, or immobilized on soHd supports, eg, cotton by adhesion with the aid of PEIs. In every case, fixing considerably simplifies the performance of enzyme-catalyzed reactions, thus faciHtating preparative work. This technique has been appHed to glutaraldehyde-sensitive enzymes (421), a-glucose transferase (422), and pectin lyase, pectin esterase, and endopolygalacturonase (423). 

Lipases are a special class of esterases that also catalyze the hydrolytic cleavage of ester bonds, but differ in their substrate spectrum. Lipases have the special capability to catalyze the hydrolysis of water-insoluble substrates such as fats and lipids. Like many other enzyme-catalyzed reactions, the ester hydrolysis is a reversible process, which allows using lipases and other esterases for the synthesis of esters. The use of lipases as catalysts in synthetic chemistry is described elsewhere in this chapter. 

From a practical point of view the improved solubility of nonpolar reactants is very important. Low reactant solubility in water is a frequently encountered problem in enzyme-catalyzed reactions, leading to low production capacity per vessel volume. Also, the possibility of using hydrolytic enzymes such as lipases, esterases, peptidases, and amylases to catalyze condensation reactions instead of bond cleavage is of considerable practical importance because it opens new ground for enzyme-catalyzed processes. 

The use of a lipase to carry out ester syndiesis is one of the earliest examples of enzyme-catalyzed reactions in organic media (1). A voluminous amount of papers has been published in the literature, iiKluding several reviews and books (2). In the polymer and biomaterials areas, lipases and esterases are... 

For desymmetrization of diesters 3 via their hydrolysis in water, pig Hver esterase [12], o -chymotrypsin [12, 13a], and Candida antarctica Hpase (CAL-B) [14] were successfully used. However, further studies showed that respective anhydrides 5 can be used as substrates for enzyme-catalyzed desymmetrization in organic solvents [15]. The desired monoesters 4 were obtained in high yield in this way, using immobilized enzymes Novozym 435 or Chirazyme L-2 (Scheme 5.3). After the reaction, enzymes were filtered off, organic solvents were evaporated, and the crude products were crystalHzed. This was a much simpler experimental procedure in which control of the reaction progress was not necessary, and aU problems associated with extraction of products from aqueous phase and their further purification were omitted [15]. 

One of the first fluorescence-based ee assays uses umbelliferone (14) as the built-in fluorophore and works for several different types of enzymatic reactions 70,86). In an initial investigation, the system was used to monitor the hydrolytic kinetic resolution of chiral acetates (e.g., rac-11) (Fig. 8). It is based on a sequence of two coupled enzymatic steps that converts a pair of enantiomeric alcohols formed by the asymmetric hydrolysis under study (e.g., R - and (5)-12) into a fluorescent product (e.g., 14). In the first step, (R)- and (5)-ll are subjected separately to hydrolysis in reactions catalyzed by a mutant enzyme (lipase or esterase). The goal of the assay is to measure the enantioselectivity of this kinetic resolution. The relative amount of R)- and ( S)-12 produced after a given reaction time is a measure of the enantioselectivity and can be ascertained rapidly, but not directly. 

Major enzymes catalyzing flavonoid reactions are oxidative enzymes (i.e., polyphenoloxi-dases and peroxidases) arising from grape but also from molds contaminating them. Various hydrolytic enzymes (glycosidases, esterases), excreted by the fermentation yeasts or fungi or present in preparations added for technological purposes (e.g., pectinases), are also encountered in wine. 

Pectinesterase and limonin D-ring lactonase are the only enzymes known to catalyze reactions that adversely affect the quality of citrus juices. Bruemmer et al. (64) listed other enzymes that have been detected in citrus juices and described some of the reactions that can occur in the juices. None of the reactions appear to noticeably affect the quality of commercial juices. Freshly extracted citrus juices contain esterase (EC 3.1.1.1) (65, 66) and phosphatase (EC 3.1.32) (66, 67) activities. Native substrates in orange juice for peroxidase... 

One of the reactions catalyzed by esterases and lipases is the reversible hydrolysis of esters (Figure 19.1, Reaction 2). These enzymes also catalyze transesterilications and the desymmetrization of mew-substrates (vide infra). Many esterases and lipases are commercially available, making them easy to use for screening desired biotransformations without the need for culture collections and/or fermentation capabilities.160 In addition, they have enhanced stability in organic solvents, require no co-factors, and have a broad substrate specificity, which make them some of the most ideal industrial biocatalysts. Alteration of reaction conditions with additives has enabled enhancement and control of enantioselectivity and reactivity with a wide variety of substrate structures.159161164... 

Suppose the hydrolysis is catalyzed by an enzyme (called an esterase) at some fixed pH where the HOAc produced ionizes. The reactions occurring are ... 

Figure 11.7. Enzymatic events leading to removal of DNA damage. The various types of DNA damage are removed by the action of several families of DNA repair enzymes. The result, in many cases, is a region where part of one strand of the double-stranded helix is missing. The empty region is then filled in with new nucleotides in a reaction catalyzed by DNA polymerase. The sequential activities of uracil DNA glycosylase, AP endonuclease, and 2 -deoxyribophosphodi-esterase catal)rze the total removal of uracil residues, as shown. Uracil DNA glycosylase hydrolyzes the uracil base from the DNA. The other two enzymes catalyze the hydrolysis of a phos-phodiester bond, and then the complete removal of the sugar residue. (From Teebor, 1995.)...

Hydrolases catalyze the addition of water to a substrate by means of a nucleophilic substitution reaction. Hydrolases (hydrolytic enzymes) are the biocatalysts most commonly used in organic synthesis. They have been used to produce intermediates for pharmaceuticals and pesticides, and chiral synthons for asymmetric synthesis. Of particular interest among hydrolases are amidases, proteases, esterases, and lipases. These enzymes catalyze the hydrolysis and formation of ester and amide bonds. 

To find a suitable enzyme to catalyze the transesterification reaction (methyl ester to glycerol ester) under mild conditions, we screened a number of lipases and esterases. We found two lipases (Candida cylindracea lipase and porcine pancreatic lipase) to exhibit catalytic activity for this reaction. The reaction conditions entailed elevated temperatures, with the use of glycerol as the solvent and an acyl acceptor. The enzyme catalyzed the esterification of the carboxyl group of the pectin to the Cl (primary) alcohol of the glycerol to form the monoester. No substantial glycerol... 

Age does not seem to alter the pharmacokinetics. Tests in patients with genetic deficiency of pseudocholinesterase also showed no significant difference in tm. The half-life of succinylcholine, on the other hand, increased from 2.6 minutes to 4 hours in patients devoid of this enzyme. It appears that the ester group activation of the Hofmann elimination reaction mutually promotes ester hydrolysis as well. The goal of chemical- over enzyme-catalyzed metabolism in vivo seems to have been achieved. The possibility that ester hydrolysis may be catalyzed by nonspecific esterases is likely that plasma cholinesterase is not utilized is established. 

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