Esterases and Lipases

Various chromogenic and fiuorogenic esters for Upases and esterase are also available commercially in the form of simple esters of umbelUferone or 

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One approach called enzymatic resolution, involves treating a racemic mixture with an enzyme that catalyzes the reaction of only one of the enantiomers Some of the most commonly used ones are lipases and esterases enzymes that catalyze the hydrol ysis of esters In a typical procedure one enantiomer of the acetate ester of a racemic alcohol undergoes hydrolysis and the other is left unchanged when hydrolyzed m the presence of an esterase from hog liver... 

Both saturated (50) and unsaturated derivatives (51) are easily accepted by lipases and esterases. Lipase P from Amano resolves azide (52) or naphthyl (53) derivatives with good yields and excellent selectivity. PPL-catalyzed resolution of glycidyl esters (54) is of great synthetic utiUty because it provides an alternative to the Sharpless epoxidation route for the synthesis of P-blockers. The optical purity of glycidyl esters strongly depends on the stmcture of the acyl moiety the hydrolysis of propyl and butyl derivatives of epoxy alcohols results ia esters with ee > 95% (30). 

Many substrates currently produced in the chemical industry are immiscible with water, but are readily miscible with organic solvents. Most enzymes, however, will not operate efficiently, or not operate at all, in non-aqueous media. Some exceptions do exist, such as lipases and esterases, which can operate in non-aqueous environments. Currently, there is considerable interest in extending the range of enzymes that do work in organic solvents. 

Cutinase is a hydrolytic enzyme that degrades cutin, the cuticular polymer of higher plants [4], Unlike the oflier lipolytic enzymes, such lipases and esterases, cutinase does not require interfacial activation for substrate binding and activity. Cutinases have been largely exploited for esterification and transesterification in chemical synthesis [5] and have also been applied in laundry or dishwashing detergent [6]. 

Syntheses of aliphatic polyesters by fermentation and chemical processes have been extensively studied from the viewpoint of biodegradable materials science. Recently, another approach to their production has been made by using an isolated lipase or esterase as catalyst via non-biosynthetic pathways under mild reaction conditions. Lipase and esterase are enzymes which catalyze hydrolysis of esters in an aqueous environment in living systems. Some of them can act as catalyst for the reverse reactions, esterifications and transesterifications, in organic media [1-5]. These catalytic actions have been expanded to... 

Yang, Y. Z., Babiak, P. and Reymond, J. L. (2006b). Low background FRET-substrates for lipases and esterases suitable for high-throughput screening under basic (pH 11) conditions. Org. Biomol. Chem. 4,1746-1754. 

Warded, J.M., Wright, A.J., Bardsley, W.G. and D Souza, S.W. (1984) Bile salt-stimulated lipase and esterase activity in human milk after collection, storage, and heating nutritional implications. Pediatr. Res. 18, 382-386. 

The resolution of a racemic substrate can be achieved with a range of hydrolases including lipases and esterases. Among them, two commercially available Upases, Candida antarctica lipase B (CALB trade name, Novozym-435) and Pseudomonas cepacia lipase (PCL trade name. Lipase PS-C), are particularly useful because they have broad substrate specificity and high enantioselectivity. They display satisfactory activity and good stability in organic media. In particular, CALB is highly thermostable so that it can be used at elevated temperature up to 100 °C. 

Although the hydrolysis of esters with lipases and esterases represents the most common process to obtain chiral intermediates for the synthesis of pharmaceuticals, proteases and other hydrolytic enzymes such as epoxide hydrolases and nitrilases have also been used for this purpose. We show here a few representative examples of the action of these biocatalysts that have been recently published. 

Figure 5.8 Fluorogenic substrates to screen for lipases and esterases (upper...

Kinetic optical resolution of racemic alcohols and carboxylic acids by enzymatic acyl transfer reactions has received enormous attention in recent years56. The enzymes generally employed are commercially available lipases and esterases, preferentially porcine liver esterase (PLE) or porcine pancreatic lipase (PPL). Lipases from microorganisms, such as Candida cylindracea, Rhizopus arrhizus or Chromobacterium viscosum, are also fairly common. A list of suitable enzymes is found in reference 57. Standard procedures are described in reference 58. Some examples of the resolution of racemic alcohols are given39. 

Irradiation by ionizing radiation and its effect on milk lipase activity have also been studied (Tsugo and Hayashi 1962). Irradiation doses of 6.6 x 104 rads destroyed 70% of the activity. The udders of lactating cows, when exposed to 60 Co gamma rays, gave milk with decreased lipase and esterase activity (Luick and Mazrimas 1966). 

Optically Active Alcahnls and Esters In addition to the hydrolysis nl esters formed by simple alcohols described above, lipases and esterases also catalyze the hydrolysis ot a wide range of esters based on more complex and synthetically useful cyclic and acyclic alcohols. Although the hydrolysis of acetates often gives the desirable resolution, to achieve maximum selectivity and reaction efficiency, comparison of various esters is recommended. Both saturated and unsamraicd derivatives are easily accepted by lipases and esterases. 

The stereospecificity may be carried, either by the carboxylic acid moiety, or by the alcohol part of the molecule. There is no rule up to now to predict if a given molecule will be a substrate, and if the enzyme will express its stereospecificity toward it. Screening of lipases and esterases is the only method ta select firstly the active enzymes, and secondly the specific ones that give the wanted isomer. 

The asymmetric acylation reaction has proven utility in the synthesis of biologically relevant targets. This is demonstrated by the plethora of applications of lipases and esterases in total syntheses [ 1 ]. While these enzymes often display superb selectivities, their application to a broad class of substrates may be difficult and unpredictable [2]. To increase access to these materials in optically pure form, over the past decade several groups have developed small molecule catalysts for the asymmetric acylation reaction [3,4], In addition, these catalysts... 

The topic of improving enantioselectivity, especially in lipases and esterases, has been reviewed recently (Bornscheuer, 2002). 

U. T. Bornscheuer, Methods to increase enantioselectivity of lipases and esterases, Curr. Opin. Biotechnol. 2002, 13, 543-547-... 

Applications of Lipases and Esterases in Pharma 13.4.3.1 LTD4 Antagonist MK-0571... 

A considerably simpler approach in the context of a biocatalytic pathway was reported by Sidler et al. (Scheme 4.16). Here, the methyl ester 45 could be hydrolyzed selectively by the protease subtilisin (lipases and esterases were unreactive), allowing hydrolysis of the unwanted (R)-enantiomer. The desired (S)-45 was recovered from the solution in 80-90% chemical yield (98% ee) and was further manipulated into (S) L-771,668 [191]. 

Hydrolases are widely used enzymes in organic synthesis, with most applications concentrating on lipases and esterases. This chapter discloses the possibilities of asymmetric accesses to chiral building blocks for the synthesis of natural products... 

Asymmetric synthesis with lipases and esterases can basically be performed by two different approaches - the desymmetrization of prochiral or meso compounds and the enzymatic kinetic resolution of racemic mixtures. The main bottleneck of kinetic resolutions, product yields of maximum 50%, can be overcome if an in situ racemization of the starting material is possible. In this case all starting material can theoretically be converted to the desired product [34],... 

Kinetic resolutions in general are regularly applied in organic synthesis. Since enzymes are highly attractive for asymmetric synthesis, various types of biocatalysts have been used in enzymatic (dynamic) kinetic resolutions, but the focus will remain on lipase- and esterase-mediated resolutions as the most common tools in early steps of natural product syntheses. 

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